Design and Manufacture of a Tensile Test Machine for in-situ Atomic
Force Microscope Mechanical Testing
by
Christian P. Grippo
B. S. Mechanical Engineering, B. S. Aeronautical Engineering
University of California, Davis, 2001
Licenciado en Administracion de Empresas
Universidad de Buenos Aires, 1996
Submitted to the Department of Mechanical Engineering
in Partial Fulfillment of the Requirements for the Degree of
Master of Science in Mechanical Engineering
at the
Massachusetts Institute of Technology
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
.JUL 0 8 2003
February 2003
LIBRARIES
D 2003 Massachusetts Institute of Technology
All Rights Reserved
Signature of Author ...................
-.--.- .-..
..*.
. . . . . ..
Department of Mechan af(ngineering
February 19, 2003
Certified by
Eberhard Bamberg
Research Scientist, Mechanical Engineering, MIT
Assistant Professor of Mechanical Engineering, University of Utah
_,_J~heis Supervisor
Certified by
7'Mary C. Boyce
Distinguished Alumnae Professor, Mechanical Engineering
Thesis Supervisor
Accepted by
. . . . . .. . . .. .
. k. . .. . .
Ain A. Sonin
Chairman, Department Committee on Graduate Students
BARKER
J7
Design and Manufacture of a Tensile Test Machine for in-situ Atomic
Force Microscope Mechanical Testing
by
CHRISTIAN P. GRIPPO
Submitted to the Department of Mechanical Engineering
on February 19, 2003 in Partial Fulfillment of the
Requirements for the Degree of Master of Science in
Mechanical Engineering
ABSTRACT
Thesis Supervisor:
Title:
Eberhard Bamberg
Research Scientist, Mechanical Engineering, MIT
Assistant Professor, University of Utah
Thesis Supervisor:
Title:
Mary C. Boyce
Distinguished Alumnae Professor of Mechanical Engineering, MIT
The microstructure and mechanical behavior of polymeric based materials can be controlled at the micrometer and nanometer lengthscales through blending, copolymerization,
and through the incorporation of micro- and nanometer scaled particles. The geometry and
properties of these phases determine the resultant macroscopic mechanical characteristics.
To facilitate the study of the connections among the morphology, deformation mechanisms, and mechanical properties of microcomposite and nanocomposite materials, an insitu set up with a tensile testing machine that can be used within an atomic force microscope (AFM) was designed and built. This in-situ test setup provides not only the macroscopic stress-strain behavior of materials under different controlled loading conditions,
but also simultaneously allows the microscopic structure changes to be observed with
nanometer resolution. The observations are being used to establish the correlation between
the macroscopic stress-strain behavior and the observed microstructure deformation.
This thesis deals mainly with the design and manufacture of the tensile testing machine
that has the capability of testing different types of polymeric-based micro- and nano-composites by applying a wide range of stresses and strains. This allows a wide range of materials to be tested ranging from rubbery to thermoplastic polymer. The tensile testing device
stretches the material specimen with a specified amount of displacement while recording
the resulting load and strain. The length of the sample is 25 mm or greater with a width of
up to 8 mm and a maximum thickness of 3 mm.
4
The challenge of this design was to build a machine capable of taking measurements with
a wide range of displacement and loads within a very restrained space while allowing the
normal operation of the AFM Dimension 3100.
The tensile testing machine can strain samples of up to 500% strain (for a 25 mm sample
length) with a maximum tension force of 4.4 kN. A special feature is that the center of the
sample remains stationary with respect to the AFM probe while the material is stretched.
This is achieved with left- and right handed precision ball screw assembly combined with
a stepper motor connected through a timing belt. The machine has an adjustable support
bridge that can be used to compensate for the decreasing thickness of the sample under
tension. This avoids sample vibration of thin films during the scanning operation, resulting in scanning images of great quality. To measure the displacement, the machine uses a
linear encoder with 5-micron resolution. To accurately measure the loads on a wide range
of materials, the machine is equipped with three interchangeable hardware coded load
cells (25 N, 250N and 4.4kN). The control software provides real-time feedback of the
measured load to characterize the state of the time-stress relaxation of the sample.
The thesis covers all steps in the design of the machine: Conceptualization, feasibility
study, error analysis and detailed engineering analysis. Finally, the thesis presents also
examples of images taken with this in-situ test setup and demonstrates the ability of this
testing capability to observe the evolution in structure with deformation of an example
nanocomposite polymer material system.
ACKNOWLEDGMENTS
A mis padres, Jorge Osvaldo Grippo y Barbel Zschau por haberme criado y apoyado en
todos mis planes y suenos y tambien a mis hermanos Axel D. M. Grippo y Tomas M.
Grippo. Sin la ayuda de ellos todos mis logros ubieran sido imposibles.
A Paola Cappellaro por su infinita paciencia en la correction de esta thesis asi como su
constante apoyo durante el anio y medio en MIT.
Special thanks to Professor Eberhard Bamberg and Professor Mary Boyce for their mentoring, support and guidance throughout the project.
Thanks to Alex Slocum for letting me work at his lab and for all grad student in Alex's
Slocum Lab for making it an great and fun place to work.
Thanks to Mauricio Diaz, Nader Farzaneh and Dhanushkodi D. Mariappan for all their
contribution on the earlier stages of the design.
Thanks to Yung-Hoon Ha and Panitarn Wanakamol for all their help and contribution in
obtaining the AFM images.
Thanks to Gerry Wentworth and Mark Belanger in the LMP machine shop for their help,
and for teaching me how to do machining.
Finally, thanks to GEM and DURINT group for their generous donation to my research
and tuition fees.
5
6
ACKNOWLEDGMENTS
7
TABLE OF CONTENTS
TABLE OF CONTENTS
1.1
. 11
..................................
LIST OF FIGURES
CHAPTER 1.
7
................................
TABLE OF CONTENTS ..........
LIST OF TABLES
5
..................................
ACKNOWLEDGMENTS ........
15
....................................
...........
INTRODUCTION
19
..
.............................
Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.2 AFM Background
. 20
..............................
1.3 Prior Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 27
1.4
Requirem ents
1.5
Thesis Overview ................................
CHAPTER 2.
................................
28
DESIGN STRATEGY: METHODS AND CONCEPT GENERATION
. . ..
2.1
Introduction
2.2
Design Framework
. . . . . ..
22
. . .
29
. . . . . . . . . . . . . . . . . . . . . . .
29
30
..............................
2.2.1
System Integration Perspective
. . . . . . . . . . . . . . . . . . . .
30
2.2.2
Design Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parameter Performance: Structural Loop and Error Analysis . . . . .
30
2.2.3
31
2.3 Preliminary Concept Generation - Motion Transmission System Selection-
33
2.4 Specification Iteration - Shaft Diameter and Drive Motor Calculations 2.4.1 Minimum Shaft Diameter Requirement . . . . . . . . . . . . . . . .
38
. . . . . . . . . . . . . . . . . . . . . .
40
2.4.2
Required Power Calculation
CHAPTER 3. DETAILED DESIGN .........
39
41
...........................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
3.2 Design Conceptualization
. . . . . . . . . . . . . . . . . . . . . . . . . . .
42
3.3 Error Reduction Analysis
. . . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.1
Introduction
3.3.1
3.3.2
3.3.3
3.3.4
Shaft Stiffness . . . . . . . . . . . . . . . . . .
N ut Stiffness . . . . . . . . . . . . . . . . . . .
Linear Motion Truck (LM-block) Stiffness . . .
Load Cell and Carriage Stiffness . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
47
48
48
50
TABLE OF CONTENTS
8
. . . . . . . . . .
50
3.4 Design Iteration- Other Design Aspects . . . . . . . . . .
3.4.1 Vibration: The Bridge Support - Flexure Mechanism
3.4.2 Gripper . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3 Wide Range of Operation: Load Cell Exchange . . .
3.4.4 Other Design Aspects . . . . . . . . . . . . . . . .
52
3.3.5 Overall stiffness and Error Result
CHAPTER 4.
SOFTWARE DESIGN .....
52
53
56
58
. . . 61
.................
. . . . .
. . .
61
. . . . . . . .
. . .
61
. . . . . . . . . . . . . . . . . . . . .
. . .
63
4.4 Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . .
64
4.1 Hardware Integration and Software Development
4.2 Components Strategy Layout and Selection
4.3 Software Overview
4.5 Software Architecture and Block Diagram
. . . . . . . . .
. . . 65
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . .
66
4.7 Software Design . . . . . . . . . . . . . . . . . . . . . . .
4.7.1 Front Panel and Operation Sequence . . . . . . . .
. . .
. . .
67
77
4.6 Safety
CHAPTER 5. AFM SCAN IMAGE QUALITY AND EXAMPLES
. . . . . . . . . . . . 81
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
5.2 AFM Scan Image Quality . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
5.3 AFM Images at Different Strain Percentage Values
. . . . . . . . . . . .
84
5.4 AFM Images with Load and Strain Measurements
. . . . . . . . . . . . 103
6. CONCLUSION AND FUTURE WORK
. . . . . . . . . . . . 113
5.1 Introduction
CHAPTER
6.1
Summ ary
6.2 Capabilities
6.3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3
Final Design Features
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 14
6.4 Future Design Improvements
REFERENCES
Appendix A.
. . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 1 16
. . . . . . . . . . . . . . . . . . . . . . . . .
Software Design
119
. . . . . . . . . . . . . . . . . . . . . . . . . . . 121
A.1 Software Limit Stop Subroutine Block Diagram . . . . . . . . . . . . . . . 121
A.2 Load Offset Calculation to Zero Load Cell Value Block Diagram
. . . . . . 122
A.3 Release Subroutine Block Diagram . . . . . . . . . . . . . . . . . . . . . . 122
9
TABLE OF CONTENTS
A.4 Stop Button and Sample Length Calculation Sub Routine Block Diagram . . 123
. . .
. . 123
A.6 Stop and Quit Program Subroutine Block Diagram
. . . . . . . . . . . . . 124
A.5 Machine Set Up Subroutine Block Diagram
A.7 Pre-record Data SubVI Block Diagram . . . . . . . . . . . . . . . . . . . . 124
A.8 Pre-record Data Front Panel Sub Vi . . . . . . . . . . . . . . . . . . . . . . 125
A.9 Set Up Machine Sub Vi Block Diagram
. . . . . . . . . . . . . . . . . . . 126
Appendix B.
Engineering Drawings . . . . . . . . . .
Appendix C.
Calculations
. . . 129
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1
C. 1 Minimum Shaft Diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1
C.1.1 Failure due to Yield . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1
. . . . . . . . . . . . . . . . . . . . 152
C.1.2 Failure due to Buckling .
C.2 Required Power Calculation
. . . . . . . . . . . . . . . . . . . . 152
. . . . . . . . . . . . . . . . . . . . 154
C.3.1 Design Strategy I: Using One Truck on Each Side of Carriage on the Linear
G uides: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
C.3.2 Design Strategy II: Using Two Trucks Next to Each Other . . . . . . 156
C.3.3 Design Strategy III: Using Two Trucks on Each Side of Carriage Separated
by a Gap: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
C.3 Truck Stiffness Calculation......
C.4 Socket Head Cap Screw Grade and Torque Estimation for the Clamping Force:
158
C.5 Height Estimation Between Bottom Surface of the Head Screw and the Top Surface of the Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
10
TABLE OF CONTENTS
LIST OF FIGURES
11
LIST OF FIGURES
Figure 1.1 AFM operation .
..................
...
21
Figure 2.1
Structural loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
Figure 2.2
System outline.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
Figure 2.3
AFM constrains
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
Figure 3.1
Concept I: Solid model of the tensile test machine. . . . . . . . . . . .
43
Figure 3.2
Concept II: Machine fully extended.
. . . . . . . . . . . . . . . . . .
44
Figure 3.3
Concept II: Machine fully contracted. . . . . . . . . . . . . . . . . . .
44
Figure 3.4
Concept III: Two ball screw shafts.
. . . . . . . . . . . . . . . . . . .
45
Figure 3.5
Delta displacement not capture by the encoder. . . . . . . . . . . . . .
49
Figure 3.6
Misalignment of axial forces.
. . . . . . . . . . . . . . . . . . . . . .
49
Figure 3.7
Displacement due to misalignment. . . . . . . . . . . . . . . . . . . .
49
Figure 3.8
Carriage 2 FEM analysis.
. . . . . . . . . . . . . . . . . . . . . . . .
50
Figure 3.9
Carriage 1 FEM analysis.
. . . . . . . . . . . . . . . . . . . . . . . .
50
Figure 3.10
Total error vs. shaft diameter.
. . . . . . . . . . . . . . . . . . . . . .
51
Figure 3.11
Flexure mechanism.
. . . . . . . . . . . . . . . . . . . . . . . . . . .
53
Figure 3.12
Extended support in vertical direction.
. . . . . . . . . . . . . . . . .
53
Figure 3.13
Stress cone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
54
Figure 3.14
Expanded view of load cell exchange.
. . . . . . . . . . . . . . . . .
57
Figure 3.15
Collapsed view.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
Figure 3.16
FEM load cell holder.
. . . . . . . . . . . . . . . . . . . . . . . . . .
57
Figure 3.17
FEM from modified carriage.
. . . . . . . . . . . . . . . . . . . . . .
58
Figure 3.18
Tensile test machine on AFM. . . . . . . . . . . . . . . . . . . . . . .
59
Figure 3.19
Side view of tensile test machine. . . . . . . . . . . . . . . . . . . . .
59
Figure 4.1
Hardware layout.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
63
Figure 4.2
High power & signal layout.
. . . . . . . . . . . . . . . . . . . . . .
63
Figure 4.3
Safety.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
Figure 4.4
Preliminary hierarchy design.
. . . . . . . . . . . . . . . . . . . . . .
68
Figure 4.5
Hierarchy diagram.
. . . . . . . . . . . . . . . . . . . . . . . . . . .
70
12
12
LIST OF FIGURES
LIST OF HGURES
Figure 4.6
Front panel of the configuration subVI. . . . . . . . . . . . . .
71
Figure 4.7
Block diagram of the digital input subroutine.
. . . . . . . . .
73
Figure 4.8
Block diagram of the load cell data acquisition subroutine . . .
74
Figure 4.9
Block diagram of the linear encoder data acquisition subroutine
75
Figure 4.10
Block diagram of the motor control subroutine . .
76
Figure 4.11
Front panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
Figure 5.1
Calibration grid on standard AFM chuck
. . . . . . . . . . . . . . . .
82
Figure 5.2
Calibration grid on fixed anti-vibration supp ort.
Figure 5.3
Calibration grid on the flexure mechanism s upport.
Figure 5.4
Microstrain vs. Macrostrain
. . . . . . . . . . . .
83
. . . . . . . . . .
83
. . . . . . . . . . . . . . . . . . . . . . .
86
Figure 5.5
SIS: 0% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . . .
88
Figure 5.6
SIS: 10% strain - scan area: 3 x 3 microns.
89
Figure 5.7
SIS: 20% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . . 90
Figure 5.8
SIS: 30% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . . 9 1
Figure 5.9
SIS: 40% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . . 92
Figure 5.10
SIS: 50% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . .
Figure 5.11
SIS: 60% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . . 94
Figure 5.12
SIS: 70% strain - scan area: 3 x 3 microns.
Figure 5.13
SIS: 80% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . . 96
Figure 5.14
SIS: 90% strain - scan area: 3 x 3 microns. . . . . . . . . . . . . . . . 97
Figure 5.15
SIS: 100% strain - scan area: 3 x 3 microns.
. . . . . . . . . . . . . . 98
Figure 5.16
SIS: 110% strain - scan area: 3 x 3 microns.
. . . . . . . . . . . . . . 99
Figure 5.17
SIS: 120% strain - scan area: 3 x 3 microns.
. . . . . . . . . . . . . . 100
Figure 5.18
SIS: 130% strain - scan area: 3 x 3 microns.
. . . . . . . . . . . . . . 10 1
Figure 5.19
SIS: 140% strain - scan area: 3 x 3 microns.
. . . . . . . . . . . . . . 102
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
93
95
Figure 5.20 All load-strain curves. . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Figure 5.21 All stress relaxation curves. . . . . . . . . . . . . . . . . . . . . . . . 104
Figure 5.24 SIS: Load vs. Strain (10%-20%).
. . . . . . . . . . . . . . . . . . . . 105
. . . . . . . . . . . . . . . . . . . . . . . . . 105
Figure 5.25
SIS: Stress Relaxation.
Figure 5.22
SIS: Load vs. Strain (0-10%).
Figure 5.23
SIS: Stress Relaxation.
. . . . . . . . . . . . . . . . . . . . . . 105
. . . . . . . . . . . . . . . . . . . . . . . . . 105
LIST OF FIGURES
13
. . . . . . . . . . . . . . . . . . . . 10 6
Figure 5.26
SIS: Load vs. Strain (20%-30%).
Figure 5.27
SIS: Stress Relaxation.
Figure 5.28
Load vs. Strain (30-50%).
Figure 5.29
Stress Relaxation.
Figure 5.30
SIS: 0% strain - scan area: 5 x 5 microns.
. . . . . . . . . . . . . . . 10 8
Figure 5.31
SIS: 10% strain - scan area: 5 x 5 microns.
. . . . . . . . . . . . . . . 109
Figure 5.32
SIS: 20% strain - scan area: 5 x 5 microns.
. . . . . . . . . . . . . . . 110
Figure 5.33
SIS: 30% strain - scan area: 5 x 5 microns.
. . . . . . . . . . . . . . . 111
Figure 5.34
SIS:50% strain - scan area: 5 x 5 microns.
. . . . . . . . . . . . . . . 112
Figure 6.1
AFM (left) & tensile test machine (right).
. . . . . . . . . . . . . . . 117
Figure A.1
Software limit stop subroutine.
Figure A.2
Load offset subroutine.
Figure A.3
Release subroutine.
Figure A.4
Stop button subroutine.
Figure A.5
Machine set up subroutine.
Figure A.6
Stop & Quit subroutine.
Figure A.7
Pre-record data subVI. . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Figure A.8
Front panel: pre-record data subVi.....
Figure A.9
Set up machine subVi.
. . . . . . . . . . . . . . . . . . . . . . . . . 10 6
. . . . . . . . . . . . . . . . . . . . . . . . 107
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
. . . . . . . . . . . . . . . . . . . . . 12 1
. . . . . . . . . . . . . . . . . . . . . . . . . 122
. . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2
. . . . . . . . . . . . . . . . . . . . . . . . . 123
. . . . . . . . . . . . . . . . . . . . . . . 123
. . . . . . . . . . . . . . . . . . . . . . . . . 124
. . . . . . . . . . . . . . . 125
. . . . . . . . . . . . . . . . . . . . . . . . . . 12 6
Figure C.1 Stiffness per unit lenght for one truck.
. . . . . . . . . . . . . . . . . 154
Figure C.2
Two adjacent trucks. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6
Figure C.3
Two truck separated by a gap. . . . . . . . . . . . . . . . . . . . . . . 157
Figure C.4 Integration limits.
Figure C.5
Stress cone.
. . . . . . . . . . . . .
. . . . . . . . . . . . . . . 157
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 0
14
LIST OF FIGURES
15
LIST OF TABLES
LIST OF TABLES
25
TABLE 1.1
. .
26
TABLE 2.1
Design process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
TABLE 2.2
Functional Requirements.
. .
36
TABLE 3.1
Pugh chart for concept selection process. ...............
. .
46
TABLE 3.2
Component and overall stiffness. ......................
TABLE 6.1
M ain design features.
TABLE 6.2
Hardware connection.
TABLE 1.2
Design process......................................
.....
.....................
51
. . . . . . . . . . . . . . . . . . . . . . . . . . 115
. . . . . . . . . . . . . . . . . . . . . . .
127
16
LIST OF TABLES
NOMENCLATURE
UPPER CASE:
A
C
Area [mm2]
E
F
H
J
K
L
Modulus of elasticity or Young's modulus
Force [N]
Height [mm]
Polar second moment of area
Stiffness
Length [mm], thread lead, truck length [mm], length distance between center of trucks
Moment
Normal force
Torque [N-m]
Watts, width [mm]
X coordinate
Y coordinate
Z coordinate
M
N
T
W
X
Y
Z
End condition constant
LOWER CASE:
d
h
p
Diameter [mm]
Vertical distance, height
Thread pitch
GREEK:
6
(0
a
Angle
Thread geometry parameter
Displacement [m/s]
Strain
Coefficient of friction
Angular velocity (rad/s)
Stress [N/m 2],[Pa]
Shear stress
SUBSCRIPTS:
As
dm
dp
dr
Tensile stress area
Mean diameter
Average pitch diameter
Root or minor diameter
17
18
Fc
LC
ns
Pb
SY
(TX
Critical axial load for buckling of columns [N]
Column or shaft length
Number of thread starts
Applied load
Shield strength
Axial normal stress
SUPERSCRIPTS:
F'
K'
6'
Vertical force
Stiffness per unit length
Vertical displacement
Equivalent Von Mises stress
Chapter 1
INTRODUCTION
This thesis presents the design and manufacturing of an in-situ testing machine with high
precision measured displacement resolution that provides the macroscopic stress-strain
behavior and the microscopic structure change observation for a wide range of micro- and
nano-composite materials that can withstand axial forces up to 4.4 kN. The real time
observation of microstructure changes during the test is achieved through the combination
of the tensile test machine with an AFM (Atomic Force Microscope). Section 1.1 briefly
describes the motivation for the design of the tensile testing device, section 1.2 provides
some background for the AFM operation explaining how vibration affects the quality
scanning imaging, section 1.3 contains a brief description of some of the prior art done in
the past and section 1.4 describes the requirement and goals of the design. Finally, section
1.5 outlines the framework structure of the thesis.
1.1 Motivation
Nanocomposites are polymeric-based materials that have significantly enhanced mechanical performance as well as other properties such as electrical conductivity, resistance to
permeability and abrasion resistance while maintaining the low inherent density of polymers. The microstructure and mechanical behavior of polymeric materials can be tailored
via the incorporation of second phase particles into the polymer, which can be done for
example through the blending of two or more polymers, and through copolymerization.
19
20
INTRODUCTION
These processes act to produce multi phase morphologies where the lengthscales of the
different underlying phases may range anywhere from nanometers to tens or hundreds of
microns. The microscopic geometry and properties of the constituent phases governs the
resulting macroscopic mechanical behavior. Therefore, to design and tailor polymericbased blends, microcomposites and nanocomposites, it is key to understand the connections between microstructure and mechanical behavior. In order to study the connection
among the morphology change, deformation mechanisms and mechanical properties, a
tensile testing machine that can be used in combination with an atomic force microscope
(AFM) is needed. The AFM is used to directly observe the microstructure at lengthscales
ranging from nanometer to tens of micrometers.
The aim of this thesis consists in the design and manufacture of a prototype for an in-situ
testing machine that in combination with an atomic force microscope (AFM) will provide
not only the macroscopic stress-strain behavior of polymeric-based micro- and nano-composite materials under different, controlled loading conditions but also will allow the
microscopic structure change to be observed with nanometer resolution. The observation
will be used to establish correlations between the macroscopic stress-strain behavior and
the observed microstructure deformation. For example, of particular interest is the chevron effect that can be observed in a uniaxial tensile test of triblock copolymers such as styrene-isoprene-styrene block copolymers.
1.2 AFM Background
For the microstructure analysis, the visualization and measurements of surface structure at
nanoscale resolution is done with the AFM. For the tensile testing design, it is important to
have a basic understanding of the AFM because the design and operation of the machine
can affect the resolution quality of the scanning imaging. The scanning of the surface is
realized in tapping mode. In this mode, the AFM measures the topography of the sample
by slightly tapping the surface with the tip of an vibrating cantilever beam at high frequency scanning in the X and Y direction [23 and 24]. The cantilever is made of silicon
AFM Background
21
and the length is 125 jim, the width is 45 ptm and the thickness is 4 ptm. The tip is also
made of silicon, the height is 17.5 ptm and a tip radius less than 10 nm. A piezo driver sets
the cantilever that holds the tip at resonance frequency (typically 100-300 kHz). A laser
beam hits the tip of the cantilever and bounces back to a photo detector called PSD (position sensitive detector). As the sample is scanned back and forth, the features on the sur-
Computer
AFM
-A
Figure 1.1 AFM operation.
face of the sample moves the cantilever tip up and down, moving the spot on the PSD. By
using feedback from the PSD, the controller tries to keep the laser spot on the PSD fixed
by adjusting the position of the stage that holds the cantilever. In this fashion, the local
height of the sample can be inferred. With the voltage reading variation on the PSD, the
software can measure the amplitude and lag of the piezo driver and cantilever resonance
oscillation. The change in oscillation amplitude due to the attractive forces above the surface describes the topography of the sample. Note that the phase lag of the piezo driver
frequency and the cantilever describes not only the topography but also the local sample
properties such as elasticity, adhesion, etc. Therefore, any vibration or oscillatory motion
that comes from the tensile testing machine or the sample will have an influence on the
22
INTRODUCTION
resolution quality of the images. Due to the above-described AFM operation, a support
device in the tensile testing machine is required to fit under the sample to avoid sample
vibration during scanning. The test, which reveals that the final design has no interference
in the quality imaging of the AFM is described in Chapter 5
1.3 Prior Art
The use of a tensile testing machine in combination with an AFM for in-situ testing for
certain types of materials (such as thin films) was done before. Most of the machines use
some type of lead screw, a load cell and a device to measure the displacement. Most do not
cover a wide range of materials to be tested because they were custom made for the particular type of experiment. There are a few companies, such as Deben and Kammrath &
Weiss GmbH 2 that have recently begun to build devices which cover a wider range of
applications.
J. Oderkerk et al [4] have used an AFM with a mini-stretching device for AFM measurement by stepwise increasing deformation to study the micromechanical deformation and
recovery processes of rubber Thermoplastic Vulcanizes. In his study, Oderkerk used the
mini stretcher designed by Dr. S Hild of the University of Ulm, Department of Experimental Physics. The advantage of this device is that it can easily fit under the AFM due to its
small dimension (17mm x 10mm x 10mm). Nevertheless, in this design, only one jaw was
movable, while the other remained fixed. Therefore, the center of the sample does not stay
stationary while the material is stretched. This device can only accommodate very small
films samples because the device has a maximum size of 17 mm x 10mm and a height of
10 mm. The maximum displacement was 7 mm and the elongation can be made in steps of
0.1 mm. Information about resolution and accuracy was not provided in the paper. They
used an additional support underneath the sample to avoid the vibration during the scanning in AFM tapping mode. In Oderkerk's work the samples were stretched to 50% and
1. Deben Ltd., Sheepcote Hall Stwupland, Stowmarket, Suffolk, IP 14 5 BS, UK
2. Kammrath & Weiss GmbH, Im Defdahl 5-10, 44141 Dortmund, Germany
Prior Art
23
100% strain at a constant rate of 20%/min. Dr. S. Hild stretcher cannot provide load measurements while the tensile test device can. Another advantage of the tensile tester device
in comparison with Dr. S. Hild mini stretcher is that the tensile tester device can hold
much bigger samples, up to 3 mm thick, 25 mm gage length and 8 mm width.
M.S. Bobji and B. Bhushan [2] also used a stretching device for in-situ testing of magnetic
tapes in the AFM. In their work "Atomic Force Microscopic Study of the Microcracking
of Magnetic Thin Films Under Tension", Bobji and Bhushan's stretcher used an opposite
left-right lead screw to stretch the sample while keeping the center of it stationary. They
used a stepper motor with a step length of 1.6 itm. They didn't use any type of encoder for
measuring the displacement or the strain, instead they used the number of steps of the
stepper motor rotation. In an application with stiffer materials, this methodology of measuring the displacement would induce significant error, because all these measurements
would not take into account all the deformations produced in the carriages under heavy
loads. For the force measurements, they used a beam type strain gauge sensor with a resolution of 10mN. In their experiment, the strain rate was 4x10- 3 % per sec. They also used a
support device with a polished metal surface to support the sample during the scanning
process to avoid vibration-induced noise in the AFM images. As explained in section 1.2,
this type of device is necessary when scanning thin films. Prof. H. D. Espinosa [22] also
uses a miniature stage for AFM/SEM in-situ testing that measures a load range of 100lbs2000lbs. His device uses an LVDT measurement device with a resolution of 1 micron.
Furthermore, A. Opdahl and G. A Somorjai [13] in his work for stretched polymer surfaces of polyethylene as well as Lepizzera at el [14] used a stretcher device in combination
with an AFM for in-situ testing; however, their paper didn't provide much technical information regarding to the stretcher.
Deben [21], a company based in England has developed micro test tensile stages for Scanning Electron Microscope (SEM), which apparently can be easily used on AFM's. Deben
has three different versions of tensile modules, with maximum forces of 300N, 2kN and 5
kN respectively. Deben uses an opposite threaded leadscrew that drives the jaws exactly in
24
INTRODUCTION
opposite direction keeping the center of the sample stationary. Deben tensile stretcher uses
a load cell and linear optical encoders for measuring the stress and strain respectively. The
tester can be used in tensile or compression mode and all standard models have a maximum travel of 10 mm and gearboxes for different speeds ranges. Apparently, a custom
version can be ordered with a greater travel range. The advantage of the tensile testing
device presented in this paper in comparison with the Deben modules is that the tensile
testing device integrates all three modules into one. The tensile testing device can support
maximum forces from 0.2N up to 4.4 kN. Also note that in the standard configuration,
Deben stages have a maximum travel of 10 mm, however, for samples other than thin
films, to fit under the AFM the stretching stages need to have an opening between the jaws
of 25 mm because the radius of the AFM probe is at least 20 mm long.
An interesting device was the one used by 0. Meissner, J. Schreiber and A. Schwab [10]
in their work "Formation of Mesostructures at the Surface of Ferritic Steel and a Nickel
Monocrystal under Increasing Load - an in situ AFM experiment". In this experiment, a
stretcher device built by Kammnrath & Weiss GmbH (Dortmund, Germany) [20] was utilized, which can be used in an AFM, SEM and X-ray diffractometers. The device can
accommodate a sample size of 50 mm length, 10 mm wide and 2 mm high. The specimen
can be stretched up to a load of 1,000 N; however Kammrath has other models: A 2000 N,
5,000 N and a reinforced version of 10,000 N. The maximum sizes for the specimen
dimensions are: 60 mm x 10 mm x 3 mm with two reamed holes of 4 mm diameter and 40
mm apart. The speed range is 0.1 to 100 Im /sec and the maximum displacement range is
up to 10 mm. In order to reduced space the motor was accommodated vertically, instead of
horizontally as many of the other stretcher devices. In the standard configuration, the
travel range is only 10 mm, while the tensile stretcher device is 125 mm. Table 1.1 on
page 25 and Table 1.2 on page 26 describe a comparison among five different tensile
stages:
Specification
Comparison
Type of Test
Tensile Testing
Device
Tensile and compression
Deban Module
Stage
Dr. S. Hild
Stretcher
Kammrath &
Weiss GmbH
Bobj's et al
Stretcher
Tensile and Compression
Not Specified
Tensile and Compression
Not Specified
Range of Forces
Integrated in one device:
0.2 N up to 4.4 kN
3 different modules: up
to 300N, 2 kN and 5kN
Not Specified
0 up to 5000 N
(option of 10000N)
Not Specified
Setup Mounting
Simple mounting on
AFM
In the std. config. simple
mounting on SEM or
optical microscope
AFM
SEM, some AFM'
and acoustic microscope
Not Specified
Type of Samples
From thin films up to
3 mm thick samples
From thin films up to
5 mm thick samples
Only thin
films
From thin films up to
3 mm thick samples
Thin Films
3 interchangeable hardware coded load cells
(25N, 250N and 4.4 kN)
accuracy
-1%
Integrated in 3 different
moduless:300N, 2kN and
5 kN. Only interchangeable within each module
(for 300N: 5, 150 N and for
2.5kN: 500N and 1kN)
Not Specified
Not specified
10 mN
Speed Range in
Std. Config.
0.0004 mm/s up to
0.83 mm/s
0.002 - 0.033 mm/s
Not specified
0.1 to 100 micron/sec
Capable of
4xl0A-3%/s
Strain Reading
Accuracy
Linear Encoder with 5
micron resolution
Linear or Optical
Encoder
Steps of
0.1 mm
Not specified
Sample Size
(gage length,
width and
height)
Max. Travel
Length/% Strain
Up to maximum travel:
std. size: 25 mm x 8mm
x 3 mm (L, W, H)
3 mm
Maximum Sizes:
60 mm x10mm x 3mm
Up to 125 mm /500%
- 300 N Module: up to
50 mm x 15 mm x 5 mm
- 2.5 kN Module: 60
mm x 15 mm x 5 mm
Up to 10 mm
Counting
motor steps
of 1.6 micron
Not Specified
3 - 7 mm
10 mm
Not Specified
Adjustable Support
Not Specified
Yes
Not Specified
Yes
Load Reading
and Accuracy
Anti-Vibration
Support
TABLE 1.1
W'.
Specification
Comparison/Software
Tensile Testing
Device
Deban Module
Stage
Dr. S. Hild
Stretcher
Kammrath &
Weiss GmbH
Bobj's et al
Stretcher
Software Application
Labview
Microtest
No Software
Manual /
Not Specified
Real Time Display
Yes
Yes
No Software
Not Specified
Not Specified
Data Export for
Excel Spreadsheet
Yes
Yes
No Software
Not Specified
Not Specified
Scaling of Horizon-
Manual
Automatic
No Software
Not Specified
Not Specified
Yes
Yes
No Software
Not Specified
Not Specified
Save sample details
Yes
Yes
No Software
Not Specified
Not Specified
Others
Control displacement
via% strain, increment
Graphical indication
on plot where motor
No Software
Choice between
manual contro-
Not Specified
or absolute distance
Preview of data to be
saved
was started, paused
or stopped. Cursor for
accurate measurement of force, extend
and time directly
form curve
Software
0
0
z
tal and Vertical
axis on graph display
Setup Menus for
selecting motor speed
and sample time
TABLE 1.2
ller and microprocessor with
interface and
software for PC
operation.
Requirements
27
1.4 Requirements
The requirements for the tensile testing machine are quite stringent since it should be able
to test different types of polymeric-based micro- and nano-composite materials with a
wide range of stresses and strains. The requirements may be satisfied with different design
approaches. The combination of the tensile testing machine and the AFM should allow the
surface characterization of the sample at any stage of the test allowing the observation of
the morphology changes of the material at different percentage strains. Because the materials to be tested can range from rubbery polymers to high-strength Aramids, it is desired a
machine able to achieve up to 500% strain (for a 25 mm sample length) with a range of
forces from 5 N up to 4.4 kN. Also the tensile testing machine should be able to measure
the displacement with a resolution of 0.2% strain (for a 25 mm sample).
A second requirement is that the tensile testing device should also stretch the material
specimen with a specified amount of displacement while recording the resulting load. The
control software needs to provide real-time feedback of the measured load to characterize
the time-stress relaxation of the sample as well as load and strain in order to characterize
the material samples. The control software should allow a friendly user interface for the
control and use of the machine as well as an easy manipulation of the data. Therefore, the
software should allow to easily exporting the recorded data to an Excel spreadsheet file.
The machine design should also take into account the combined operation with the AFM
Dimension 3100. In order to allow the scanning of the material with the AFM Dimension
3100, the minimum length of the sample should be 25 mm or greater with a width of up to
8 mm and a maximum thickness of 3 mm. The 25 mm gage length will allow the 20 mm
diameter AFM probe to contact the sample surface without touching the grippers that are
holding the sample. This distance will result in sufficient clearance to allow the probe of
the AFM to be in contact with the sample for the scanning process without interfering with
the grippers.
28
INTRODUCTION
Also the tensile machine design should consider a mechanism to compensate for the
decreasing thickness of the samples under tension and avoid the vibration of the sample
during scanning as described in section 1.2.
Finally, besides the performance parameters, the design should address the safety of the
user as well as the equipment, which includes the stretcher (in particularly the load cell)
and the AFM probe.
The complete list of the functional requirements can be found in Table 2.1 on page 31.
In summary, the challenge of this design is to build a machine that is able to take measurements with a wide range of displacements and loads within a restrained space allowing at
the same time the normal operation of the AFM Dimension 3100.
1.5 Thesis Overview
The rest of the thesis is outlined as follows.
Chapter 2 goes over the design methodology, alternative designs, structural loop, and the
quantification of certain parameters for the element selection done in Chapter 3. The idea
of this chapter is to select the best preliminary design approach.
Chapter 3 reviews the final design conceptualization, the detailed design and all the hardware element selection for the final assembly. It mainly analyses how well the machine
will perform before it is built.
Chapter 4 discusses the software development.
Chapter 5 briefly covers the verification of the quality of the AFM images with the new
tensile testing machine and describes a case study from a real test on a nanocomposite
sample material. The conclusion are given in Chapter 6. Finally the Appendix give greater
detail of the software code and engineering drawings.
Chapter 2
DESIGN STRATEGY: METHODS AND
CONCEPT GENERATION
2.1 Introduction
This chapter covers the framework for the design process, the preliminary concept generation and the quantification of necessary parameters for the preliminary design.
In design, there are often no clear solutions to a problem and the design process is a combination of analysis, synthesis, and creativity [7] rather than a recipe to follow. This chapter will not go in depth over the design methodology, but it will describe the framework
used to find the best solution.
For the concept generation, it is important to stress that the main problem that this design
faces is the geometrical constraints that make it difficult to achieve a broad load and displacement range of operation with high accuracy measurements.
Finally, the quantification of design parameters is performed for the preliminary design to
comply with the specifications provided in the functional requirements such as for example the calculation of the minimum motor power needed to achieve 4.4kN axial load.
29
30
DESIGN STRATEGY: METHODS AND CONCEPT GENERATION
2.2 Design Framework
2.2.1 System Integration Perspective
The design framework encompasses all the tools and aspects that should be taken into
account to achieve the best solution. For example, during the design process one can be
confronted with a choice among a power/ACME screw, a rack and pinion, a hydraulic piston or a ball screw that can all provide the necessary tensile axial force. The most advantageous mechanism should be chosen depending on factors such as accuracy, resolution,
maximum displacement, forces, geometrical constraints and costs. The design decision
also should take into consideration how the selection of one component will affect the rest
of the system. An ACME screw may be less expensive, but it produces more friction when
the carriage is displaced. Therefore the use of an ACME screw requires a more powerful
motor, which may require more space than is available. The advantage and disadvantage
of each system should be analyzed bearing in mind all the above-mentioned factors and a
system error budget for accuracy performance should be set up to optimize the final
design.
2.2.2 Design Process
In general, there are no specific rules for design but the following guidelines described in
Table 2.1 have been used in this project. The first three steps are crucial for the design and
it is important to iterate them as many times as necessary to cover all the details and to
maximize the performance of the machine. Once the prototype process has started, in most
cases, a modification of the original design will increase the cost significantly because in
general all the parts and components are coupled together. The coupling of the parts also
implies that once the prototype manufacturing process has started, there is much less freedom to perform design modifications while minimizing the cost at the same time.
Design Framework
31
- Recognition and assessment of customer needs.
Functional requirements
Literature search and technology assessment
e Idea generation, creativity and conceptualization
e Feasibility Study (preliminary sizing):
Range of displacement and forces and Error Analysis (resolution,
accuracy, repeatability).
- Detailed Engineering Analysis
Solid Model, design optimization, finite element analysis.
Prototype building.
Testing.
Modification and adjustment.
e Documentation (i.e. technical drawing, assembly drawings, etc).
TABLE 2.1 Design process.
Another important design principle is the use of symmetry since it tends to simplify analysis, balance stresses, eliminate possible errors and reduce cost of manufacturing [9].
To reduce cost and time it is also worthwhile to relay the design on standard machine elements such as linear motion (LM) guides or standard-pitch rolled ball screws because they
are offered in a highly competitive market that assures higher quality at lower cost.
2.2.3 Parameter Performance: Structural Loop and Error Analysis
To determine the performance, the importance of the main factors that are taken into
account depends mostly on what the customer needs. Since this design is for a high precision application, the most important aspects affecting the quality of the machine are the
accuracy and resolution. Therefore, the different sources of error that come from each
component and affect the performance of the machine are important to consider.
DESIGN STRATEGY: METHODS AND CONCEPT GENERATION
32
Structural Loop
After the literature search, ideas generation, basic component outline and basic system
selection, the design process continues with the establishment of the structural loop. In
any design, the loads and measurement loops are intrinsic to any machine [9]. All loads in
a static system are transmitted from body to body in a closed path producing elongation,
compression, bending or twisting. This strain produces different types of measurement
errors and the amount of error depends on the stiffness of each component. The structural
loop helps to identify the components through which the loads travel so that the errors can
be minimized.
Carriage 2
Carriage 1
p 1
Gripper 1
Ball Screw Nut 1
Linear Bearings 1
Gri pper 2
Ball Screw Nut 2
Screw Shaft
Linear Bearing 2
Rail
Figure 2.1 Structural
loop
The following structural loop optimization rules were taken into account [9]:
-
Keep forces and bending moments small.
" Keep loading as symmetric as possible.
-
Maintain a fixed temperature and avoid temperature gradient.
-
Keep resonance frequencies as high as possible.
*
Keep loop small.
Preliminary Concept Generation - Motion Transmission System Selection-
33
Figure 2.1 shows the smallest structural loop that can be achieved. It also shows the error
source components that need to be taken into account for the error analysis sometimes
called stiffness or error budget. The error analysis is developed in Chapter 3.
Error Analysis
Some of the sources of mechanical errors include geometric errors and dynamic errors.
The geometrical errors include parallelism, sine and cosine, Abbe (associated with measurements) and static deflection errors [7]. The static deflection error can be considerable
under heavy loads, where elastic deformation of the machine components will produce
errors that are not captured with the measurement devices. Bigger components would provide higher stiffness and allow greater specimen displacement with less displacement
measurement error, however in this application, that is not possible because the tensile
testing machine has to fit in an existing AFM machine. Consequently, the geometrical
constraints depend on the geometry of the AFM (for details see Figure 2.3 on page 35),
which allows a working space of 340 mm (L), 200 mm (W) and 96 mm (H).
The most important dynamics errors are those corresponding to the vibration induced on
the AFM probe as described in Section 1.2 on page 20. The error analysis will determine
the selection and size of the components to overcome the given restriction and achieve the
required performance. The error analysis is developed in Section 3.3 on page 47.
2.3 Preliminary Concept Generation - Motion Transmission
System SelectionThe first step is to define the functional requirements, which are provided by the customer
and were described in Chapter 1 and summarized in Table 2.1 on page 31. To assure the
tension/compression of the sample and load and strain measurement capability, a load cell
with an encoder, controller and a motion system is required. The outline of the major components of the overall system is described in Figure 2.2.
34
DESIGN STRATEGY: METHODS AND CONCEPT GENERATION
Data Adquisition Card
Laptop
Compi iter
Contile
Sample Specimen
Encoder
Motion
Load Cell
Support
Figure 2.2 System outline.
Notice that this design requires some asymmetric components because of space restrictions. The design has essentially two conflicting requirements. On the one hand, the
machine has to provide a wide range of displacements and forces with high measurement
accuracy while on the other hand the machine has to fit under the AFM. The use of the
AFM creates many space constraints, as shown in Figure 2.3 on page 35, that limit the
required range of displacements and the location of oversized components with inherent
large stiffness. The larger component stiffness would provide a broad range of forces and
larger displacements with greater accuracy. Since there is a camera on the right of the
AFM, there is only one position where the tensile testing machine can be located, that is
perpendicular to the camera in the y-direction of Figure 2.3. An optimal solution that
solves all these conflicting issues will be developed later, in Section 3.3 on page 47.
The next step is to define the type of motion and power system to assure the capability to
stretch the sample with the required accuracy and ranges of forces. The power motion
transmission element selection is made based on the functional requirements summarized
in Table 2.2.
35
Preliminary Concept Generation - Motion Transmission System Selection-
I
Figure 2.3 AFM constrains
1 - Camera obstructs the longitudinal tensile machine positioning in the x direction.
2 - The wall behind the probe obstructs the displacement in the y direction.
Each functional requirement can be satisfied with a number of different design
approaches. Each design approach considers different linear power transmission elements.
There are many power transmission elements [7] but for the sake of simplicity the main
power transmission elements described here are:
" Rack and Pinion
" Pneumatic Piston
- Hydraulic Piston
" Ball Screw
- Machine Screw
Each power element and the subsequent design was analyzed and compared with others to
identify the optimal solution. In precision applications, the main factors that will affect the
36
DESIGN STRATEGY: METHODS AND CONCEPT GENERATION
- Compatibility with a Digital Instrument Dimension 3100
Atomic Force Microscope.
" Wide range of displacements: up to 125 mm (500% strain for
a 25 mm sample).
* Wide range of forces: up to 4.4 kN.
- Resolution: 0.2% strain (for a 25 mm sample).
" Sample size: 20 x 8 x 2 (L x W x H).
- Center of sample remains fixed with respect to the AFM
probe while the sample is stretched.
- Easy specimen set up.
- Friendly user interface.
- Safe to operate.
TABLE 2.2 Functional Requirements.
accuracy and controllability of system such as Rack and Pinion and Screw based systems
are the errors present in their components, component misalignments, hysteresis, backlash
and friction. Fluid power elements on the contrary don't present these last two problems
since they have almost zero friction and almost no backlash [7].
Rack and Pinion is the least expensive method to generate linear motion and can achieve
great displacement distances [7]. Rack and Pinion can be very precise systems but they
have low force of transmission because they do not provide the mechanical advantage as
for example the Power Screw. Rack and Pinion systems normally run at low speed and
high torque. However, there is always the possibility to design them in such a manner to
overcome this limitation, for example using a hydrostatic design on a worm gear rack [7].
However, this solution makes the design more complex than using a standard power
screw. Therefore, a Rack and Pinion transmission system has not been chosen for the
design.
A Pneumatic Piston has a nonlinear behavior due to the air compressibility, which creates
difficulties in the control of the displacement and the application of precision positioning
Preliminary Concept Generation - Motion Transmission System Selection-
37
system. It is also a more complicated system than the Rack and Pinion and Power Screw
since it requires more specialized components such as a compressor station, regulator,
plumbing, flow control devices and valves. It is also more difficult to determine what
should be the airflow rate to the cylinder since the density of the air is not only a function
of the pressure but also of the temperature, which can vary due to friction produced during
operation. The flow of the fluid is important for precise positioning [7].
Hydraulic actuators can be used in precision design applications [7] and as mentioned previously, they can present the advantage of zero friction and exhibit essentially zero backlash. However, in this type of application, the hydraulic piston has the disadvantage that it
is dangerous to operate because of the sudden change of pressure in the occurrence of the
sample breaking after reaching the ultimate strength. Also the hydraulic system can easily
leak if it is not properly maintained. Moreover, the pump produces a significant amount of
vibration that if not adequately isolated, affects the AFM scanning. For this application,
hydraulic actuators present too many disadvantages when compared with the power screw
system.
Power Screws convert the input rotation of an applied torque into an output axial translation with an axial force [5]. The advantage of a power screw is that it can produce large
axial forces with small torques. This characteristic matches perfectly with the requirements of this type of application because space is constraint and large axial forces are
required.
There are two principal types of power screws: machine screws and ball screws [5]. A typical machine screw is composed of a threaded screw which can be driven by a worm shaft
and gear nut. The high thread friction presents both an advantage and a disadvantage. The
advantage is that the machine screw has the capability to self-lock without the need of any
type of brakes, a feature which is desired in a tensile testing machine when the test is
stopped. However, since the machine screw presents a higher sliding friction than its
counterpart, the ball screw, the machine screw needs more torque to overcome friction
38
DESIGN STRATEGY: METHODS AND CONCEPT GENERATION
than the ball screw and therefore a more powerful motor is needed, which in general
implies bigger motor size. Ball screws achieve low friction through recirculating balls
lying in the grooves between the screw-shaft and the nut. Ball screws have also the advantage among other type of elements that they are very standardized components in a highly
competitive market. A highly competitive market guaranties high quality and reliable
components at a very competitive price. They also provide high rigidity, excellent lifetime
and low maintenance [12].
As mentioned before, space is a limitation factor and because more powerful motor generally involves bigger sizes, the power machine screw approach was discarded.
Among all the systems analyzed, the ball screw system appears to be the most adequate
for this design because it can achieve high accuracy, low friction, high transmission of
forces, and easy control of displacement at a competitive price.
Once the ball screw system has been selected it is necessary to connect it with the rest of
the components such as the load cell, encoder, grippers, etc described in Figure 2.2 on
page 34. Therefore, two carriages on each side of the sample location will connect the
grippers, load cell and linear encoder to the ball screw. Two linear guides parallel to the
ball screw will support the carriages.
2.4 Specification Iteration - Shaft Diameter and Drive Motor
Calculations Once the motion transmission elements were selected and the structural loop was defined,
preliminary calculations for the specification were performed in order to establish the
main design features (i.e. shaft diameter, motor power). As mentioned previously, the
design process is an iterative method, hence the order in which the material is presented
does not mean that the calculations are done just once, but many times in a cyclic manner
until the optimal design is achieved.
Specification Iteration - Shaft Diameter and Drive Motor Calculations -
39
The maximum height is 96 mm, therefore it is important to use components as small as
possible. The smaller the shaft diameter, the smaller are the linear motion (LM) guides,
nut housing, LM blocks and all other system components. However, there is a critical minimum diameter size that would avoid yielding and buckling. The linear motion system is
overconstrained due to the combination of two linear guides and the shaft. The linear
guides are fixed, constraining two linear and three angular degrees of freedom. The driven
end shaft is fixed and constrains all three linear and two angular degrees of freedom. In
order not to overconstrain the system even further, the non-driven end of the shaft is left
unsupported. However, since the non-driven end is attached to the nut, and the nut is
attached to the linear guides through the carriage, another three linear constraints are
added to the shaft. Hence, during operation both ends of the shaft are now fixed and buckling can occur.
2.4.1 Minimum Shaft Diameter Requirement
For a first order approximation the stress analysis using the Von Mises criterion will provide the minimum shaft diameter requirement.
(2.1)
SY >!a'
=
+
3482
(2.2)
Using a very conservatively estimated yield strength of steel of 200 MPa (the value for the
actual screw is about 689 MPa) and with an axial force F = 4400 N and torque T = 6.2 Nm (preliminary assumed values), the estimated diameter has to be at least 6.9 mm. A second failure criteria would be buckling. For a commonly available shaft diameter of 16 mm
and assuming fixed-fixed boundary condition for the buckling equation (Appendix C.1)
and a Young's modulus of steel of 200 GPa, the critical force for buckling is found to be
1.20- 10 6N. Therefore, there is no risk of buckling under the required load condition. The
40
DESIGN STRATEGY: METHODS AND CONCEPT GENERATION
minimum diameter for the required stiffness is calculated in the error budget done in
Chapter 3.
2.4.2 Required Power Calculation
Assuming an outer diameter of 16 mm with a lead on a square type screw of 5.08 mm, a
linear speed of 25mm/min. and the maximum axial force of 4.4kN, the required torque is
3.858 Nm (Appendix C.2). The linear speed together with the lead provides 4.92 rpm.
Since power is torque (N m) times angular velocity (rad/s), the required motor power
ended up to be 2 W.
Chapter 3
DETAILED DESIGN
3.1 Introduction
The main goal of this chapter is to present the final design that comprises detailed modules
and parts. First it describes the concept generation and selection process. The selection of
the final concept is based on the expected performance of the machine. Therefore, for the
final concept selection process it is important to know how well the machine will perform
before it is built. In general, some of the most important factors that influence the performance of the machine are the accuracy, repeatability and resolution [7]. In this context, the
resolution is the smallest displacement increment that the machine can detect, which in
general is determined by the type of encoder. Note that the smallest motion that the
machine can deliver is not always the same as the resolution because of some inefficiency
of the components, such as backlash or hysteresis. The repeatability is the error between
all the reached points in successive trials, given a desired input. The accuracy is the
expected difference between the real and actual measurement [7]. The design is concerned
mainly with the accuracy of the measurement of the stretched sample. Accuracy, resolution and repeatability not only depend on the encoder, but also on the other component's
sources of errors described in the structural loop in Chapter 2. Given a load, the stiffness
of those components determine the amount of error in the accuracy of the machine. For
this purpose, an error analysis is performed. Finally, the remainder of the chapter describes
with greater detail the design characteristics of the remaining components.
41
42
DETAILED DESIGN
3.2 Design Conceptualization
Chapter 2 has illustrated the main configuration of the design. The sample has to be
stretched 125 mm, that is 62.5mm on each side. From the center of the AFM probe to one
end of the AFM there is only 113 mm of free space. Since only 52.5 mm in length are left
to place the carriage, trucks and gripper, an asymmetric carriage design is needed. Three
optimal concept designs using the ball screw motion system type described in Chapter 2
were created.
Concept I
Concept 1 has a centered ball screw with opposite thread and linear motion guides
attached on each side of the carriages. The solid model with all its elements can be
observed in Figure 3.1. In this design, the two carriages support the grippers, load cell and
encoder. The load cell is placed between the gripper and one of the carriages to assure
accurate load measurements. The stepper motor drives the ball screw through a timing
belt. The stepper motor is placed near the carriages so that its weight will not tilt the tensile test machine.
Specimen support, heightadjustable to compensate for
the effects of Poisson's ratio
ELHS-B1 load cell range up to 4.4kN
with 1 % FSO for high-strength
materials.
1/8" dowel pins that
can withstand
1490 lb shear load
Stepper
Motor.
1018 Steel
Plate
Linear encoder
200 mm scale
(5 microns
resolution).
Non-preloaded ball nut
r.j~
Thomson Precision Thread
Grippers with serrated
blades. Belleville springs
to maintain compression
while sample stretches.
Screw .004 in./ft accuracy
standard.
0
0
rt~
-o
0
Figure 3.1
Concept I: Solid model of the tensile test machine.
-15
44
DETAILED DESIGN
Concept II
Concept II has the same elements as Concept 1 but utilizes a different carriages design. In
concept II, an outside carriage passes through the inner carriage, resulting in a zero offset
between the forces transmitted to the carriage by the sample and the forces transmitted to
the carriages from the ball screw. The offset of the forces is explained in greater detail in
Section 3.3.3.
Figure 3.2 Concept II: Machine fully extended.
Figure 3.3 Concept II: Machine fully contracted.
The carriage design can be better appreciated in Figure 3.3 that shows Concept II machine
fully contracted and in Figure 3.2 that shows the machine fully extended.
Design Conceptualization
45
Concept III
Concept III is another version that uses two ball screw shafts rather than one and therefore
Figure 3.4 Concept III: Two ball screw shafts.
there is no misalignment of forces because they are all acting in the same plane. It uses a
gearbox to run both ball screws simultaneously. The rest of the setup is similar to Concept
I.
The advantages and disadvantage of each concept are laid out in a Pugh chart [12]. Pluses
means that they achieve the desired performance, double pluses perform significantly better and minus and double minus performs worse. The comparison among the total scores
will provide a good indication to select the design with the highest performance.
Concept III has the advantage that it can use a bigger shaft diameter, therefore with respect
to the maximum load range, Concept III can perform much better than the other two concept designs. However, for the optical AFM microscope to work properly, the bottom edge
of the camera (Figure 2.3 on page 35) has to be approximately at the same level as the top
surface of the sample. In Concept III, the camera on the AFM will obstruct the displacement of the carriages since the sample is at the center level of the ball screw. Concept III is
46
DETAILED DESIGN
Concept I
Concept II
Concept III
4.4kN Axial Load
+
+
++
%500 Strain
+
++
+
Fit on AFM
+
+
Safety
+
+
Support Specimen
+
+
+
Low Complexity
+
Low Cost
+
+
+
Total
7
6
2
TABLE 3.1 Pugh chart for concept selection process.
for this reason discarded. Concept II doesn't have the same misalignment problems as
Concept I but it has a more intricate shape which increases the complexity in manufacturing and assembly. An error analysis, explained in Section 3.3.3 on page 48, also has
shown that it cannot achieve an error lower than 50 microns. Concept I achieves all the
parameter performance and its much simpler as Concept II; therefore, Concept I is chosen
as the final prototype design.
In Chapter 2, it was mentioned that the second most important restriction is the error propagation. The error propagation becomes smaller when the structural loop together with the
bending moments becomes smaller while the structural elements become stiffer. Because
of the former described geometrical constrains, it is important to analyze how to make
these elements stiffer without using oversized components. One way to achieve higher
stiffness, as explained in Appendix C, is to locate a pair of trucks separated by a distance
rather than placing them next to each other.
47
Error Reduction Analysis
3.3 Error Reduction Analysis
The functional requirements stipulate a resolution of 0.2% of the sample length. With this
criterion, a gauge length of 25 mm corresponds to a resolution of 50 microns. One option
to achieve this resolution is to use a rotary encoder attached between the motor and the
shaft. Rotary encoders are very common measurement elements in the market and they
have great resolution at low cost. However, with this configuration, it is important to note
that the measurement error comes not only from the encoder reading but also from the
sum of the deflection of each component in the structural loop. In a static analysis, the displacement is related to the force by the stiffness:
(3.1)
F = K-8
If the system is approximated as a set of components in series, the overall stiffness comes
from the sum of the stiffness of each component identified in the structural loop.
1
Ktotal
_
=
1
+
KShaft
1
Knuts
1
+
+K
KTrucksonLinearGuides
1
Loadcell
1(32
(3.2)
Carriage
Therefore it is important to analyze and quantify the stiffness for each component.
3.3.1 Shaft Stiffness
Since stress is related to strain through the Young's modulus, the stiffness is given by:
= E E :-
F
A
Kshaft
6
E- L
-
L d
K=
E*-A
L
(3.3)
(3.4)
Assuming that the shaft is made of steel, an average value for the Modulus of elasticity is
200 GPa. Note that from equation 3.4 when comparing a shaft of the same materials, the
stiffness varies only with the diameter. Also note that an increase in diameter also requires
48
DETAILED DESIGN
bigger size nut and carriages. The size of all these components, however, is limited by the
96 mm free vertical space in the AFM.
3.3.2 Nut Stiffness
The stiffness of a non-preloaded nut for an axial load higher than 30% of the basic
dynamic load rating (Ca) is given by [15]:
Knu = K
(o.3FC 9
(3.5)
Where K is the nut rigidity under an axial load of 30% of the basic dynamic load rating
(Ca). The stiffness of the ball screw nut changes from model to model, Ca and K are
obtained from the THK'catalog [15]. Therefore, the stiffness is a function of the applied
axial load.
3.3.3 Linear Motion Truck (LM-block) Stiffness
A rotary encoder will also not capture the exact position of the gripper because it does not
take into account the bending of the LM block or truck due to the misalignment of axial
forces depicted in a simplified model in Figure 3.5.
The offset between the forces causes a deflection of the carriage-truck subsystem, which
in turn produces a small displacement that cannot be detected by the sensing elements
(Figure 3.6 and Figure 3.7). 8 is the difference between the position that the encoder is
reading and the real sample end position. Using a small angle approximation, 6 can be
defined as 6 = h - 0 and since M = Kmoment - 0, then replacing 0 gives:
h -= M
Kmoment
1. T H K CO., LTD, Tokyo. Japan
(3.6)
49
Error Reduction Analysis
Truck6
Applied
Force
Truck
Applied
Force
Samole
Real position of the
Position that the
sample
encoder is readingv+
Figure 3.5 Delta displacement not capture by the encoder.
I
Force exerte
from Samplk
A
Ii
Force exerted
from Ball Screw
Truck
Truck
Figure 3.7 Displacement due to misalignment.
Figure 3.6 Misalignment of axial forces.
Therefore the bigger the stiffness (Kmoment), the smaller the deflection and hence smaller
error.
When replacing the moment in equation 3.6 with the axial force and moment arm h,
defined in Figure 3.6, the deflection is obtained as a function of the axial force:
SFaxial'
K h
moment
hence
Kmoment
Kaxial
h
(3.7)
DETAILED DESIGN
50
Note that the height h has a great impact because it is squared. From equation 3.7, the error
becomes smaller, as the linear bearings become stiffer and the offset (h) of forces becomes
smaller.
3.3.4 Load Cell and Carriage Stiffness
N
The load cell stiffness provided by the supplier [15] is 800-gm . The carriages are designed
with a particular shape not only to support and connect the grippers, load cells, (and linear
encoder) to the rest of the system, but also to minimize the error propagation due to the
offset between the forces. The carriage stiffness was obtained using standard finite element analysis tool. The total deflection obtained with 4.4 kN was measured to be 5
microns (Figure 3.8 and Figure 3.9).
Figure 3.8 Carriage 2 FEM analysis.
Figure 3.9 Carriage 1 FEM analysis.
3.3.5 Overall stiffness and Error Result
Table 3.2 summarizes the stiffness and error for commercial off-the-shelf shaft with different standard diameters. The selection of the shaft diameter will affect not only the shaft
stiffness but also the nut and truck stiffness. The separation on the linear guides is 15 mm
and the effective length of the shaft is 125mm. As noted in Table 3.2, the error is above 50
microns. One solution to minimize the error is to use a linear encoder. In this case, the
51
Error Reduction Analysis
Axial
Nut
Nut
Stiffness
Stiffness
Bearing
Overall
Stiffness
Overall
Error
(N/(pm))
173.3
N/(gm)
N/(gm)
pm
592.1
49.7
103.5
331.4
189.3
534.4
53.8
96.7
150
517.7
210.9
404.1
56.8
92.5
50
180
809.0
243.9
342.0
60.5
87.8
60
200
1014.8
267.1
237.5
55.1
94.8
Shaft
Diam
Flange
Diameter
Nut
Rigidity
(mm)
14
(mm)
38
16
40
130
20
46
25
28
Shaft
Stiffness
N/(pm) N/(pm)
116
253.7
TABLE 3.2 Component and overall stiffness.
Axial Error Displacement vs. Shaft Diameter
106.0
------
104.0
102.0
100.0
98.0
E 96.0
594.0
92.0
w
90.0
88.0
86.0
13
18
23
Shaft diameter (mm)
28
33
Figure 3.10 Total error vs. shaft diameter.
error is reduced to 10.5 microns because the misalignment error (deflection of the linear
bearings), deflection of the nut, the lead error and the axial shaft error are avoided by placing the linear encoder on the carriage close to the point of measurement (Figure 3.19 on
page 59).
Since the linear encoder is placed as close as possible to the sample on the carriage, the
only errors left are the carriage deflection (5 microns), load cell error (5.5 microns) and
the linear encoder accuracy (5 microns). The total final error is in this case 15.5 microns,
that is, in compliance with the customer requirements.
52
DETAILED DESIGN
3.4 Design Iteration- Other Design Aspects
3.4.1 Vibration: The Bridge Support - Flexure Mechanism
A flexure mechanism for the bridge support with a polished metal surface was placed
under the sample to minimize the vibration that induce noise on the AFM as explained in
the AFM Background Section 1.2. The advantage of using a flexure mechanism for this
component is that they are simple and inexpensive to manufacture and assemble. As Smith
[9] pointed out, flexures can even produce predictable and repeatable motions at displacement of atomic resolution. These mechanisms are designed such that they are flexible in
the drive direction but stiff in all other directions. The design aims to obtain a precise vertical displacement upon the application of a specific applied force. To achieve a vertical
displacement, a standard M4 screw will produce the force that will actuate the flexure
mechanism to close the gap between the bottom of the sample and the top support surface.
This support will eliminate any vibration that may occur if the sample is let freely to oscillate. The straight edge cuts were made through the use of a waterjet. The mechanism consists of the commonly known crab legs, which basically consist of two folded beams
connected in series as shown in Figure 3.11. The bridge support is attached to the base of
the tensile test machine through a magnet.
A-
53
Design Iteration- Other Design Aspects
Figure 3.12 Extended support in vertical direction.
Figure 3.11 Flexure mechanism.
3.4.2 Gripper
Two types of fasteners are used for the grippers: Dowel pins and M4 socket head cap
screws. The dowel pins are used to withstand the shear force produced by the machine
while the socket head cap screws are used to provide the necessary clamping force.
Socket Head Cap Screw Grade and Torque Estimation for the Clamping Force:
Assuming that the coefficient of friction is approximately 0.15 [16].
(3.8)
Pb
(allow
-
As
9
(Fallow = 2.67 -10 Pa
Since two screws are used, then
(3.9)
(3.10)
Fallow = 1.335 - 10 9Pa.The recommended grade has to
be above SAE Grade 7 (H or I) [16]. For this design a SAE Grade 8 was chosen. More
details can be found in Section C.4 on page 158.
54
DETAILED DESIGN
The total torque necessary to develop the axial load Pb [16] is equal to:
(3.11)
T = P'b- [0.159 - L + 1.156. pi- d]
Pb
Where Pb was replaced by P'b =
because two screws are used.
L = 8 mm, g = 0.15, d=4 mm
(3.12)
T = 28.82 N-m
(3.13)
Height Estimation between Bottom of the Head Screws and Top of the Sample
Surface
force,
The total specimen sample area under the grippers has to be under the compression
otherwise one region of the specimen can slip and the test will lose its validity.
Assuming that a stress cone expands with a 30-degree angle as shown in Figure 3.13, the
to be:
height from the sample to the bottom of the head screw (Section C.5) is calculated
Stress Cone
7 mm diam.
Top Gripper
--
+
Plate
Sample
x
Bottom Gripper
Support
Cross Section View
Figure 3.13
x = 3.8mm
Stress cone
y = -
tan Oc
=
6.58mm
(3.14)
Design Iteration- Other Design Aspects
55
Hence the minimum distance between the top of the sample and the bottom of the screw
head has to be 6.58 mm in order to have all the sample surface bellow the gripper under
compression to avoid slip of the sample under axial load.
Dowel pin selection
Dowel pins are selected to withstand the shear force due to the tensile load. An 1/8 diameter pin can withstand 1490 lb. shear load [15]. Due to space limitation, two 4 mm pins
were chosen for the right hand gripper and one 6 mm pin for the left hand gripper. In order
to avoid subjecting the screw to shear force, the tolerances of the screw holes are higher
than the tolerances of the pin holes. The values of the tolerances and other details can be
obtained from the technical gripper drawings in Appendix B.
Other considerations:
A teeth design below the gripper top
plate was chosen in order to assure a
coefficient of friction above p=0.15. In
the teeth region, the segment from the
edge of the gripper up to 3 mm in an
axial direction is flat. This flat surface
avoids any crack formation and propagation in the sample due to stress concentration on the sample below the tip of the
teeth. Also the outermost edge of the clamping pressure region on the sample has to flush
the frontal surface of the gripper in order to accurately measure the strain of the sample.
Because of the Poisson's ratio effect, the sample becomes thinner as it is stretched.
Belleville washers that act like springs are used on the grippers in order to maintain a vertical compression force while the material is stretched horizontally. The Belleville washers
are placed in series vertically on top of each other.
56
DETAILED DESIGN
3.4.3 Wide Range of Operation: Load Cell Exchange
The design should allow the user to exchange different load cells because the measurements need to cover a wide range of loads. The load cell has a threaded portion, which
allows its location in the centerline of testing. A threaded hole on the carriage would support the load cell; however, in this configuration it is not possible to unscrew the load cell
from the carriage to exchange it with another one because the cable of the load cell interfere with the ball screw shaft. A greater vertical distance between the shaft and the load
cell would allow an easy exchange of load cells. Nevertheless, because the vertical distance between the axial shaft and gripper forces have to be kept at a minimum to reduce
the error, it is desired to maintain the actual design. One solution is to attach the load cell
to a cylindrical holder which is slipped and locked into the carriage through a square key
as in Figure 3.15 on page 57. The load cell holder has a stem and two cylindrical bodies
with a greater diameter at each end. One end has a threaded hole to attach the load cell
while the other end works as a pressing stem. There is a square key that fits on top of the
holder on the pressing stem like a key slot. When the sample is stretched, the holder
presses on the key which at the same time presses on the carriage. To avoid any slip the
load cell holder is axially preloaded with a spring. The weakest part of the whole machine
is precisely the load cell holder because of the stress concentration at the section where
there is an abrupt change in the cross section due to the diameter changes. This discontinuity works as a stress raiser [6]. To mitigate the stress a fillet with a radius of 0.2 mm is
placed at the corner of the diameter change. For greater detail refer to the technical drawings on Appendix B. Nevertheless, this new modification will introduce a new source of
error. To quantify the error a FEM analysis was done on each new component. The error
that will be produced at maximum axial load of 4.4 kN is 12.96 microns on the load cell
holder (Figure 3.16) and 5.4 microns on the modified carriage (Figure 3.17).
Therefore, the new overall error using this new configuration and a linear encoder is 30.9
microns.
Design Iteration- Other Design Aspects
Figure 3.14 Expanded view of load cell exchange.
Figure 3.15
Collapsed view.
UZ
15%-M
.25%-M
MG.M
7 ZM
4i4&=
405.-M
Figure 3.16 FEM load cell holder.
57
58
DETAILED DESIGN
Figure 3.17 FEM from modified carriage.
3.4.4 Other Design Aspects
The linear encoder was assembled in such a way that it can read the absolute displacement
of both carriages (Figure 3.19). For the linear guides assembly, a reference edge on the
base support was used to align the linear guides. Because a precision lead ball screw is
used for the motion system, the stepper motor was purposely design to be on the stage as a
mechanism to avoid the backdrive of the ball screw when the carriages are subjected to
high loads (such as 4400 N). To drive the power from the motor to the ball screw, a timing
belt drive system is used. The timing sheaves are attached into the screw and motor shafts
by means of taper lock bushings. Regarding to the motor mounting, the motor support is
constrained to rotate horizontally due to the belt tension by means of a key lock placed
between the base of the machine and the motor mounting. The motor support is adjustable
to allow belt tensioning. For user safety, a belt cover will prevent any hair, clothes and fingers from becoming entangled in the belt that drives the ball screw. There is also an emergency bottom that will shut down the power to the motor. The rest of the safety
considerations are addressed in Chapter 4.
Design Iteration- Other Design Aspects
Figure 3.18 Tensile test machine on AFM.
Figure 3.19 Side view of tensile test machine.
59
60
DETAILED DESIGN
Chapter 4
SOFTWARE DESIGN
4.1 Hardware Integration and Software Development
The preceding chapters were only concerned with the mechanical aspects of the design.
This chapter covers the hardware integration and software development needed to assure
the optimal performance of the tensile testing machine. The hardware integration and software design plays an important key in the achievement of the functional requirements. For
example, the resolution of the linear encoder may be 5 microns but if the analog to digital
converter in the DAQ system had not enough number of voltage level (ADC resolution) to
represent the analog signal then all the efforts done in the mechanical phase would be
wasted. Other aspects such as real time acquisition, noise, speed response, and safety may
affect the performance of the design.
4.2 Components Strategy Layout and Selection
In order to assure real time readings and fast transfer commands for controlling the operation of the motor, high-speed buses rather than slow serial connections are chosen for data
transfers. The plug-in board that contains the multifunction data acquisition components,
the signal conditioning that converts the signal into voltages and the hardware connection
for sensor are integrated in a NI-DAQ 1 card which is a PCMCIA 2 . The DAQ card (AI-
1. NI-DAQ is a registered trademark of National Instruments
61
62
SOFTWARE DESIGN
16XE-50) has 8 digital 1/0 lines, 16 analog inputs with 16-bit resolution and two 24-bit
counter timers. It seems that for the data acquisition two DAQ are required because the
software code needs to control only one analog channel and eleven digital channels. The
linear encoder requires two counters, one for each channel. However, two additional 32bit counters are required to generate the train pulse to drive the stepper motor at a wide
range of frequencies. To open the stretcher 125 mm, 500,000 pulses need to be generated.
The 24-bit counter cannot generate that amount of pulses at high frequencies (i.e.
1300Hz). On the other hand, even at low frequencies, the PCMCIA DAQ card won't work
because the DAQ card has a STC type chip that does not allow z indexing. Therefore, a
32-bit counter device such as the PCI 6601 NTIO base device with 4 counters is used. The
PCMCIA card is used for the remaining digital and analog channels. The 16-bit analog
resolution DAQ card and the 32-bit counter PCI 6601 card assure that the system provides
the necessary resolution for the data acquisition and motor control. For example, the 50 lb.
(222.4 N) load cell has a sensibility of 218.91 mV/FS (full scale) while the smallest
detectable change in signal voltage is 3.3 mV. That is with 16-bit:
Small detectable change: DC =
range _ = 3.3mV
(4.1)
gain - 2(16)
The software is supported on a laptop. For safety consideration, the rest of the power
sources for the load cell and linear encoder, the motor controller and all other high power
connections are physically separated from the signal circuitry as shown in Figure 4.2.
2. PCMCIA: Personal Computer Memory International Association Cards
Software Overview
Figure 4.1
Hardware layout.
63
Figure 4.2 High power & signal layout.
4.3 Software Overview
The application software was done using LabVIEW1 . LabVIEW is a graphical programming language (G-program) that instead of following a top to bottom sequential order of
text lines uses graphical icons. In this type of graphical language, the flow of data determines the order of execution. LabVIEW has a block diagram for the code and a user interface with tools and objects known as the front panel (Figure 4.11 on page 78). In this
application case, LabVIEW is integrated with hardware through the DAQ and PCI 6601
devices described previously. LabVIEW programs are called virtual instruments (VIs)
[11]. There are three main components: front panel, block diagram and connector pane.
The connector pane allows using a VI as a subVI in another VI. A subVI is like a subroutine or a function in a text-based programming language such as C. One of the key features
of LabVIEW is its hierarchical nature of the VI. Each subVI can be used as a subVI of a
higher level VI.
1. LabVIEW is a trade mark of National Instrument
64
SOFTWARE DESIGN
4.4 Signals
The tensile testing machine uses digital and analog signals to control the machine and to
acquire data. The digital input and output signals are for on-off control and monitoring,
pulse train generation and encoder measurement. The analog signals are only used for
continuous voltage measurements. An adequate signal manipulation assures the optimal
performance of the tensile testing device.
Linear Encoder Signal
The linear encoder is an absolute quadrature encoder, which means that the encoder uses
two TTL signals and one z indexing signal. As the tensile testing machine is opened and
closed, the encoder produces squared pulses on two channels, generally called channel A
and B. To indicate the direction, one channel leads the other by a phase shift of 90 degrees.
For absolute position measurement, the z indexing produces a pulse every time the reading
head passes a unique mark on the metal scale. To acquire the data, the encoder needs to be
connected to one of the clocks/counters of the PCI card and to a digital channel for zindexing.
Motor, Load Cell and Limit Switch Signals
Digital signals are used to identify the on-off state of these components, the hardcoded
load cell type identification and limit switches status as well as the automatic set up switch
status. The limit switch status function is explained in Section 4.7 on page 67. Furthermore, the frequency signal for driving the motor is also digital (TTL type). The motor is
connected to the second clock/counter of the PCI NTIO base device card. The load cell is
the only device that requires an analog signal. Both encoder and load cell signals are
amplified by their respective power sources of 15 and 5 V. To reduce the noise, all the
cables were shielded and grounded, and the power box circuitry modified to isolated the
cables from high-noise inducing devices such as the controller in the high-power box as
well as other unknown sources of electromagnetic fields and radio-frequency interference
Software Architecture and Block Diagram
65
(RFI). Also, a capacitor was placed at the power source of the load cell to eliminate the
high frequency noise originated from the wall outlet.
Real Time vs. Virtual Time Acquisition
There was a problem with the real time axis data, which did not match the recorded data
with the acquired time. The recorded time was distorted because the program is run in the
Microsoft Window environment, where the loop speed varies depending on the number of
simultaneously running programs. Therefore, depending on the number of open applications and memory usage, the software may run faster or slower, introducing small errors in
the readings which accumulate in a considerable manner after 10 or more minutes. In
order to synchronize the data acquisition, the code was modified. Section 4.7 addresses
this issue.
Finally, all channel configurations are specified in the configuration file and described in
the Connection Sheet in Appendix A.
4.5 Software Architecture and Block Diagram
The control of the machine is done with an open loop system because a stepper motor with
high resolution is used. There is no need of feedback for lead screw positioning because
the smallest increment that the stepper motor can achieve is within the 50 microns. The
hierarchy of the code is shown in Figure on page 70. The control software consists of
more than 50 independent programs or subVIs but only three were fully developed: configuration, main and setup subVI, the rest of the subVIs such as the pulse train generation,
port read, set attribute, etc. were used with few or none modifications from the National
Instrument subVI library.
GO
66
SOFTWARE DESIGN
4.6 Safety
Safety considerations were fundamental in the design. In particular the software development had to deal with safety issues. The machine has to be designed such that it will not
run unless all safety checks are completed. The design considers the safety of the user as
well as the equipment, including the stretcher (particularly the load cell), and the AFM
probe. The machine has to stop with a gap of 25 mm in between the grippers, allowing a
gap of 2.5 mm on each side of the AFM probe. This limit is imposed by the software: it
reads the position of the encoder and will not allow opening more than 125 mm (so that
the balls of the trucks and ball screw nuts will not fall out) or closing beyond 25 mm. If for
any reason the software limits fail, a second backup system should prevent the damage of
the equipment. This is addressed in Section 4.7 with the use of hardware limit switches.
Support
AFM
Probe
Bridge
Grippers arnd
Load Cell
Figure 4.3 Safety.
Safety Hazards: Sample may buckle and damage the AFM probe
Grippers may close beyond 25 mm and damage load cell and AFM
probe.
As will be explained in Section 4.7, the machine needs to go through a set up procedure
when it is powered on. The set up procedure basically opens and closes the carriage so that
the
the linear encoder passes through a reference mark. The software design should allow
Software Design
67
user to skip the setup procedure only in the event that the user accidentally turns off the
computer while the machine is loaded with a sample, since the operations done in the set
up could damage the equipment. The design contemplates this case with the SetUp subVI
where the physical limit switch will become activated and the machine will stop at the
position limit before causing any damage (for example the load cell hitting the support
bridge).
Also, the software design needs to take into account the exchange of load cells. If the user
forgets to change the low range load cell with the high load cell for thermoplastic materials, he/she could damage the equipment. To avoid that, the software needs to recognize
which load cell is in place and shut the motor down in case of overload.
4.7 Software Design
The advantage of LabVIEW is that it is a flexible programming language that allows different design approaches. The design follows a mixture of top-down and bottom-up structure design. The top down approach focuses on the big picture and starts with the final
user interface. It answers the question:"what needs to be controlled or measured?" Using
this approach, a main Vi with dummy variables was developed, which consists of the following important subroutines:
- Load cell data acquisition.
- Linear encoder data acquisition.
- Motor control.
- Limit switches/connector on-off state status and load cell identification.
During the development of the main Vi, the front panel objects, data types, connector pane
and terminal layout are defined. A preliminary Vi hierarchy was developed (Figure ). The
preliminary hierarchy shows the Vi or subroutines that accomplish the required tasks or
main application, flow of data and interaction of subVIs. Each node in the hierarchy graph
corresponds to a task that represents a subVI, a subroutine (like a while-loop), or a global
variable.
68
SOFTWARE DESIGN
Main
Program
o
ti
Ciios.n
Switches
e
Sampl
b
En
ode and
Motor setup
Control
Stop and
Sh utdown
Routine
becus
su~lor ubou
Linear
Motor Control
Eoer
Adqu stion
Motor Pulsemai
Train
Generation
V
Data Record
Load Cell
Load
Data T
Adquisition
Dt
inear cetdohalesEifcoetaskesuhiasfu
Cell
den-leve
Calibration
Te
ipa
ti onpr
Figure 4.4 Preliminary hierarchy design.
Once the preliminary hierarchy was completed, a bottom-up approach was developed. The
bottom-up approach followed more closely a lower level programming style, taking care
of each subroutine or subVI. It could be considered also as a modular design approach
because subVI or subroutines are created to handle specific task, such as function or operations. The combination of subVI and subroutine form an application. The lower-level
subVI's were written in complete and final form and tested as stand-alone programs. In
this phase, it is important to know how the I/0 hardware works. Finally, the stand-alone
programs were placed and integrated with the main VI developed using the top-down
approach design.
In the software architecture, independent parallel loops are used because many tasks are
needed to be done concurrently. The architecture of the general application follows three
main phases: startup, main application and shutdown. Each subVI and subroutine follows
the same architecture. The startup phase reads all the configuration information and initializes the hardware. The main application contains all the loops needed for the operation
of the tensile testing device and the shutdown phase consists of all subVis needed to close
Software Design
69
down the system properly. In the main subVI, the shutdown subroutine makes sure that all
the loops have been stopped before the user exits the program. As trivial as it may sound,
the last phase is important because if the software is improperly closed during the opening
of the carriages, the motor can still run with the last sent signal. In this event, a catastrophe
could occur because there is no active signal to stop the motor from continuously opening
the tensile tester. The final hierarchy can be seen in Figure on page 70. Some of the most
important subVI are explained below:
lConfiqf
L
Port Configuration: Reads which device will interface with the software. Fur-
thermore, it establishes the channel configuration for all the digital channels:
switches 1 and 2, load cell types, cables connection status, etc. The standard Port Config.
subVI provided by LabView was modified to establish the direction of each line in the
port, that is, to determine which is an input or output line. This configuration is read by all
the Port Read subVI. When using a standard subVI, it is important to verify that no subVI
will not configure a port that has already been configured by a different subVI. Otherwise,
ports will be configured incorrectly and the system will fail to operate properly.
Set Attribute: Sets attributes for the linear encoder subVI: in this case, it is a
L.90J quadrature encoder with z-indexing.
Motor Pulses: This is a modified subVI from the standard NI library. Basically, it
IM-T1
1
generates a finite pulse train with two counters. The initial delay, frequency, duty
cycle and number of pulses that are calculated in the subroutine 1 are the inputs for this
subVI.
PreRecorded SubVi: When called it previews the acquired data and prompt the
user to proceed or not to save the data.
A
5-SCAd
-6-
Scan SubVI: Returns the data from one scan from the analog board (DAQ PCMCIA card).
70
SOFTWARE DESIGN
y0
I-
S_
ip
Software Design
71
Global Variables: Are used to read and write values to be passed among different
subVI that are running in parallel.
Configuration and Start Up Subroutine: The main VI calls first the Configuration file where the entire configuration for the load cells, linear encoder, motor,
software setup, drive mode and digital input/output channels is stored. The Drive Configuration passes to the main VI the ball screw lead, the number of steps per revolution of the
stepper motor, the internal gear ratio and the pulley ratio values. The motor configuration
includes the channel and counter number identification as well as the duty cycle and its
initial delay values. The linear encoder as well as the load cell configuration consists of all
the channel identification, load cell ranges values, encoder type identification etc. The
Figure 4.6 Front panel of the configuration subVI.
72
SOFTWARE DESIGN
Digital Input/Output configuration takes care off all the communication channels for all
limit switches and the motor. The Configuration can be modified to adapt the program to
other tensile testing machines that use an encoder, stepper motor and some type of analog
load cell sensors. The whole program developed in this project can work as a VI or subVI
in the same manner as the VI's found in the National Instrument Library, can work for
other application. Moreover, the user can change or add some hardware by accessing the
configuration files.
Pre-recorded graph: this subVI shows the user the complete recorded data
graphs of load vs. time and load vs. strain. The user has also the option to scale the
x and y-axis. Finally, the subVI prompts the user to discard or save the data in text format-
ted file.
Main VI: The main VI has seven subroutines, that consist of while-loops (WL).
These subroutines control all the input/output signals and data acquisition. Each of
these subroutines has other subVIs and each subVI uses other subroutines with lower level
subVI. This continues until the code reaches the lowest level subVIs.
Digital Input Subroutine
Figure 4.7 shows the data flow for the digital input subroutine. This subroutine follows
the same architecture pattern described previously. In the figure, the lines coming from the
left and top of the loop represent the configuration information that is read and passed to
the individual subVI, such as the Port Read subVI when the hardware is initialized. The
configuration subVI tells each port which line to read. The data flows to the main application where it is processed. The last component of the architecture is an Error handling
subVI which determines if an error has occurred and displays a message. None of the main
application or subVI will run until they get all the input data. In the main application section, the subroutine reads all the digital channel and checks the status of the limit switches,
setup switch, motor state, and load cell type identification. When the limit switches are
activated, the subroutine will send a command to the pulse train generation subVI and stop
73
Software Design
o
Ta
arwe Limit Stop Progam
el2
pReadefo
tor
Te
rofad
A cCa
0
I
Lmit Swttch Connctonj
IACt
-
on
Umt
-mR R
mit Switchesand Load
8 Overoad
ed
Swith Conectlcn
Limit Switch
LImt
Swi7
True
Samople
et Uo
--
-
Lcod Ce Zero offel
oCdlTime Lop
Zoad Cdl
-
False
False
LodCs2
Load Celnme Loop
n Iteraton
Calrad
Load Cel Time Lo
r
I-
-----
,
-
m1000*z
Vol"
atated
(mV)
....
....
....
--
StoC Sta3u.
-
Figure 4.7 Block diagram of the digital input subroutine.
the motor. Furthermore a warning message to the user is prompted. The software will not
allow the user to run the motor until he/she presses the "release" button which activates
the release subroutine. The release subroutine will move the carriages in opposite direction just enough to free the switches to the off status. Once the switches are in the off state
the user regains the control of the machine. In this manner, the software avoids the user to
move accidentally the carriages beyond the limit switches position. The digital input subroutine also identifies which load cell is connected and adjusts the calibration and maximum range values according to the load cell type (25N, 250N or 4.4kN). Moreover, the
subroutine also controls if the load cell has reached the maximum range. In this case, the
subroutine will send a command to the pulse train generation subVI to stop the motor and
SOFTWARE DESIGN
74
proceed in the same manner described for the limit switches case. All subroutines in this
code use the same architecture.
Load Cell Subroutine
phanne(s
.
viceor~rphv
(1R)ern
*read newest data
Jsto
/0]u
-dest
Figure 4.8 Block diagram of the load cell data acquisition subroutine
In this subroutine, the voltage values from the load cell are acquired at a specified scan
rate and then converted to a calibrated value in Newtons. The Load Cell Data Acquisition
Subroutine is based on a Hardware-Timed subVI that uses a hardware-timed, non buffered
technique of data acquisition. In this way the scan rate is not affected by the speed at
which the software is running. Because the program runs in a Microsoft Windows platform, the software and consequently the while-loops may run at different speeds depending on how many applications are open. In order to avoid this time distortion the scan
clock on the Daq board controls the acquisition timing, and the acquired scans are read
directly from the device memory. Through local variables the absolute position from the
75
Software Design
linear encoder is passed and with these two coordinated sets of data the Load vs. Strain
curve is plotted and stored. The prerecorded graph is displayed when the user stops the
recording. It also counts and displays the time elapsed since the record has started.
Linear Encoder Subroutine
oam~
tye
hdex vakue
7 idex active
Loop:
True
ount
1~
__
True
Pposito
b stance to Zero P:.
eset
CJ
bson
iters
R;;N
ateeencoder X17281
Measured Pd iMon
j~esue~oo
j~j
_tep Posn
fosion meastrement 2tion Stepl
'ero Ref Sep= 25mm
Zero Ref
rt
I
I
Step= 25mm
ptrtatu
Figure 4.9 Block diagram of the linear encoder data acquisition subroutine
This subroutine is based on a subVI from the NI library. It basically measures the frequency of a continuous square wave through the use of the two counters from the PCI
6601 NI-TIO based device. The two counters count the rising edges of the square wave
sent from the linear encoder. In this way, the subroutine can measure the position of the
carriages that are interfaced to the quadrature encoder. The subroutine also allows the use
of a Z index pulse, so that the position measurement can be reset automatically to an initial
value upon the arrival of a Z index pulse. The pulses are calibrated to the accuracy of the
linear encoder and the data values are displayed and passed to the load cell subroutine
through local variables for the data coordination and final storage.
76
SOFTWARE DESIGN
Motor Control Subroutine
ITrue
btstance to open
700
-
False
bfC
oen
Lum t Sw
Eoftwe
Funct]on to
n
tu
tchl
mittatus
C~i
en
-
Lverodad
nsSK
thtn
S
opel
mit
F-
ste
Tran
(N1-TIO
modi5
-
EFsoftsweumt status
True
te :I add thisso that
True
m
True
Tue
Figure 4.10 Block diagram of the motor control subroutine
The motor control directly interfaces with the user and the finite pulse train subVI. The
inputs are the drive parameters, such as the ball screw lead, steps per revolution, gear ratio
and pulley ratio values obtained from the configuration file as well as the user input. The
pulse specs consists of the frequency, number of pulses, delay and duty cycle variables.
Before sending the pulse specs to the finite pulse train subVI, the motor control subroutine
not only checks the status of the hardware and software limit switches and the overload on
the load cell but also checks if the encoder set up process was performed. If any of these
checks are not successful then the subroutine will prompt a message to the user warning of
Software Design
77
the problem and will do nothing until the user resolves the problem. The Motor Control
subroutine also includes another subroutine to exchange the load cells. The carriage are
positioned with the required distance between the grippers to allow the physical interchange of load cells. Yet, before performing this action, the code prompts the user a warning message so that the user will not forget to take out the sample, because if the grippers
are too far open, the carriage may close and the sample may buckle. In this case the sample can hit the AFM tip and damage the probe. Therefore, in such a case as well as in general, the software will not perform any action until the user reacts to the warning message.
The rest of the subroutines are the shutdown subroutine, load cell calibration subroutine,
scan rate subroutine, release subroutine, etc. These subroutine block diagrams are shown
in Appendix A.
Finally, the remainder of the subVIs used in the hierarchy shown in Figure on page 70 are
standard subVIs from the National Instrument's Library that are used for the correct execution of the above mentioned subVIs. Their respective characteristics and operation
information can be found in the NI Library.
4.7.1 Front Panel and Operation Sequence
The user interface is the front panel, which comprises the controls and indicators that are
the interactive input and output terminals of the VI. The screen has been separated in four
major areas.
Set up Phase
The top region of the screen lists the steps the user needs to execute in order to operate the
machine. This procedure is easy since the software walks the users through all the steps he
needs to perform. The first step is to set up the machine, the second step is to set to zero
the load cell offset and the third step is to set up the sample length. When all these steps
are performed, the machine shows the user the ready status. When the machine is turned
on, the software does not know which position is the zero absolute position. The linear
encoder has a reference mark (z-indexing) the machine has to pass in order to obtain the
78
SOFTWARE DESIGN
Figure 4.11 Front panel
real absolute position. The software will not allow the user to operate the machine until the
setup step is performed, with the exception mentioned in Section 4.6. The main VI calls
the SetUp VI, which drives the machine across the mark and feeds the position into the
program. The Set Up subVI uses a physical switch to reverse direction to ensure that the
system passes the reference mark. Once set up, the machine will know its absolute position and the software limits will be engaged and the machine will be protected.
By pressing the set up button another subVI will be called and a window will pop up. The
set up can be done automatically by pressing the Automatic Set Up button. In this case, the
machine will close, pass over the reference mark and obtain the absolute position. Once
this is done, it will update the information to the main VI and allow it to work with no risk
of reading the wrong position information. The window can then be closed and the user
can proceed to the next steps to perform the experiment.
Software Design
79
Safety Control and Data Acquisition Phase
On the right side of the user front panel are the load vs. time and load vs. strain graphs that
display the data in real time. On the left are the indicators that check the status of the
machine. A green LED means that everything is ok, a red LED denotes that something is
wrong. The LEDs indicate if the machine has reached any of the open or close position
limits as well as if the limits switch connection cable was disconnected by accident. If this
happens, the software will stop the motor and prompt a message warning to the user. In
addition, on the left side are the recording status indicators and controls. There is also an
indicators that shows the time elapsed since the beginning of the recording. Once the
recording is stopped the PreRecord subVI is called and another windows will pop up and
show the user a preview of the recorded data. Both axes can be scaled by the user according to the range of loads and strains. The software gives the user the option to save the data
or discard it. When the user presses the button "save the data", the data will be saved in a
file that later can be exported to an Excel spreadsheet file.
Operation Phase
On the central area of the screen are the switches for the modes in which the machine can
run: strain, absolute and increment. Increment mode is generally used to set up the sample
because sometimes when the sample is clamped in the grippers, the sample may buckle a
little bit and therefore the carriage has to be open. The speed can be adjusted from 0.14
mm/s to 83 mm/s. The strain mode depends on the sample length, which is set up in step 3.
The user also has the option to operate the system in the absolute mode. There is an indicator for the absolute position and a button to quit the program.
Certain Consideration to Take into Account when Scanning Materials.
When the material is stretched the surface can flow. This can be observed by scanning in
the direction of the flow and then in the opposite direction. The relative speed between the
sample and the probe will result in a distorted image, showing features larger than in reality if the scan direction is opposite to the relaxation direction. Accordingly, if the scanning
80
SOFTWARE DESIGN
and the relaxation direction are identical, features will appear smaller than they are in reality. In this case, it is prudent to wait until the sample has completed this transitional phase.
Chapter 5
AFM SCAN IMAGE QUALITY AND
EXAMPLES
5.1 Introduction
This chapter covers the verification of the quality of the AFM images with the new tensile
testing machine and describes an example from a real test on a nanocomposite sample
material. Section 5.2 shows scanning images of a calibration grid before and after using
the tensile test machine. Section 5.3 describes an experiment where a sample was scanned
at different strains and finally Section 5.4 illustrates an experiment were also load-strain
and load-time measurements were taken.
5.2 AFM Scan Image Quality
As explained in Section 1.2, any vibration or oscillatory motion that comes from the tensile testing machine or the sample will have an influence on the quality of the images. The
test, which reveals that the final design does not affect the AFM quality imaging, consists
of scanning a manufactured calibration grid. First, to check that the AFM was calibrated,
the calibration grid was scanned on the standard AFM chuck. The calibration grid is a
rectangular piece made of gold on a silicon chip that has squares of 1 x 1 micron. In
Figure 5.1, the typical calibration picture is displayed.
The scanning of this calibration grid is a standard test that shows how well the AFM captures an image. The square edge distance is measured with the AFM software. The user
81
82
AFM SCAN IMAGE QUALITY AND EXAMPLES
01
Data type
2 range
H
Height
100.00 flm
3.00 pm 0
3.00 pm
Data type
2 range
Amplitude
0.2200 U
augridno
Figure 5.1 Calibration grid on standard AFM chuck
(without tensile test machine).
can select two features on the picture and the software provides the distance between
them. In our test, the AFM software measured a value of one micron, which agrees with
the value given in the specification of the calibration grid. Figure 5.1 shows the image
taken with the AFM in the manufacturer standard configuration, that is, without the tensile
testing machine. The picture on the left of Figure 5.1 represents the height of the surface
of the sample while the picture on the right represents the amplitude, which is the first
derivative of the height and provides more details on the surface of the sample. Figure 5.2
shows the scan of the calibration grid mounted on the fixed anti-vibration support.
Figure 5.3 shows the scan of the calibration grid mounted on the flexure mechanism support. This flexure support allows height adjustment for compensating the decrease of the
thickness of the sample as it is stretched. The edge distance from the squares on both pictures were found to be 1 micron, therefore the use of the tensile testing machine on the
AFM provides the same image quality as the use of the AFM in its standard configuration.
AFM Scan Image Quality
Figure 5.2 Calibration grid on fixed anti-vibration support.
3.00 p" 0
Data type
2 range
Height
100.0 n"
Data type
2 range
Amplitude
0.6000 U
3.00 Pm
augridon
Figure 5.3 Calibration grid on the flexure mechanism support.
83
84
AFM SCAN IMAGE QUALITY AND EXAMPLES
5.3 AFM Images at Different Strain Percentage Values
In this example, the sample is a polystyrene-block-isoprene-block-styrene (SIS) triblock
copolymer which forms a cylindrical morphology. The tensile testing machine together
with the AFM allows the examination of the evolution in the microstructure during streching 1 . The SIS sample commercial name is 4211 D and contains 30% styrene.
Motor
Stainless Steel Roller
The block copolymer is prepared in a slow evaporating solvent. The viscous solution is then applied to two counterrotating cylinders. One cylinder is made of teflon while the
other is made of stainless steel. As the solvent evaporates, a
Teflon Roller Micrometer
Enlsrewt
small vents
film is transferred onto the stainless steel roller. The film is
then annealed at a temperature of 110 C' for about 24 hrs.
The sample has a gage length of 29.6 mm (L) x 5 mm (W) and 0.3 mm (H). This process
results in a highly oriented microstructure. The orientation of the cylinder are perpendicular to the length of the sample.The sample was deformed in 10% intervals up to 140%
strain. The SIS Triblock scanning procedure followed to obtained AFM images is
described hereafter:
- Turn on PCI Daq card.
- Turn on computer.
" Turn on machine power supply.
* Proceed with the machine set up process which drives the machine across
the reference mark and feeds the position into the program (step1 on the
front panel).
- Set up load cell offset to zero the load reading value (step 2 on the front
panel).
* Set up gage length to 25 mm.
" Fix sample on gripper and make adjustment on the gripper separation distance until the sample is straight and the load cell reads 0 N axial load.
1. This investigation and these pictures were performed by Yung-Hoon Ha and Panitarn Wanakamol at MIT.
AFM Images at Different Strain Percentage Values
-
Set sample gage length (step 3 on the front panel). With this step, the user
inputs the sample gage into the program so that the software can calculate
the percentage strain to send the corresponding number of pulses to the
motor controller. The gage length was 29.6mm.
-
Set driving mode to drive the stepper motor (% strain, absolute or incremental mode).
e
Press "start" recording button.
-
Press start on the driving mode section.
85
- Wait until the tensile tester reaches the desired strain.
- Wait until stress relaxation is finished.
- Engage AFM.
- Start scanning.
- When the AFM scan is completed disengage AFM tip.
- Repeat above steps until all desired images at specified% strains are
obtained.
- Stop recording and save data.
- Relieve axial load and remove sample from the grippers.
- Turn off equipment in the opposite sequential order as turned on.
In each figure, the picture on the left is the height plot of the sample surface while the picture on the right represents the phase plot. The phase plot shows the contrast in stiffness of
the different phases of the material and can be used to detect the different domains. Therefore, the phase image is the better image to use to examine the evolution of the microstructure during stretching. In these pictures, the stiff styrene cylinders appear white and the
compliant isoprene domains appear dark. From the micrographs of scans at different
strains, it can be observed that the cylinders are pulled farther apart as the material is
stretched. Note that above 10% strain, the styrene cylinders start to get more noticeable in
the height image, indicating that they are emerging from the surface.
From the micrographs at 0% strain, the styrene and the isoprene domains have a similar
thickness of about 15 nm. Since the isoprene domains are more compliant than the styrene, they will be strained more than the styrene domains. As a result, one expects that the
thickness of the two domains will be different when an axial load is applied. In fact,
86
AFM SCAN IMAGE QUALITY AND EXAMPLES
already at 10% strain, the isoprene domains increase in thickness and the styrene domains
are therefore more spread apart. At 10% macrostrain, the microstrain in isoprene is around
20%. At 20% macrostrain, the isoprene microstrain is about 40%. This tendency continues
throughout the experiment and is the most important uniform change in the polymer structure observed up to 70% strain. The microstrain vs. macrostrain curves of styrene+isoprene domains and isoprene domains are described in Figure 5.4. From the micrographs of
scans at different strains, it can be observed that the styrene domains thickness did not
change significantly throughout the experiment. It can be observed in Figure 5.4 that up to
60% strain, the microstrain of styrene+isoprene domains corresponds to the macrostrain.
From 70% up to 110% macrostrain, the microstrain does not change considerable. At 80%
Microstrain vs. Macrostrain
1.4 -
-
--
..--------
-
---
-
1.2
.
0.6
0.4
-
0.2
*
00
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Macrostrain
----
Th(S+1) ---*--Th(E
Figure 5.4 Microstrain vs. Macrostrain
strain, small distinguished cavitations become clear. At 100% strain, the thickness of the
isoprene domain is not uniform, because the thickness of the isoprene domains are smaller
near the larger cavitation than farther away from the cavitation. Nevertheless, on average
AFM Images at Different Strain Percentage Values
87
the strain of the isoprene domains that seems not to form cavities drops as the macroscopic
strain is increased because the existing cavities grow larger. Therefore, the difference in
macroscopic and microscopic strain corresponds from the strain in the cavities.
Also at 60% strain, waviness starts to appear in the domains. The waviness is also correlated to the increase in surface roughness, which can be observed in the pictures on the left
(the height plots). In these pictures, the z range is reported, which measures the difference
between the highest and lowest points on the surface of the sample. Up to 50% strain, the
z range measures an approximate value of 20 nm; then it starts to increase quite fast (it is,
for example, 50 nm at 60% strain). While the waviness continues to rise, at 80% strain
some small cavitations or crack propagation start to appear. At 90% strain, the crack propagation is much more evident because it forms cavitations which some can reach a length
in the axial load direction of 0.25 microns size. At 100% strain, the cavitation length in the
axial load direction ranges up to 0.4 microns. The surface roughness continues to heighten
at higher strain percentages. Between 80% and 120% strain, the z range measures an average value of 80 nm, which doubles at 130% strain and reaches 300 nm at 140% strain,
indicating a clear increase in surface roughness with cavitation holes that have reached
values greater than 1 micron length in the axial load direction. At 140% strain, the cracks
have propagated into the thickness of the sample (as can be seen from the z-range
increase).
00
Axial Load>
z
-IO
ll
-
3.00
0
Data type
2 range
Height
20.00 nm
jj
0
Load
Axial
Data tupe
2 range
Figure 5.5 SIS: 0% strain - scan area: 3 x 3 microns.
3. 00
Phase
40.00 *
Axial Load 4
CD
0
Data type
2 range
Height
20.00 nm
3.00
,ro
Axial
Load
Data type
2 range
Figure 5.6 SIS: 10% strain - scan area: 3 x 3 microns.
(0
3.00
Phase
40.00 0
0
A *1
0
Data type
2 range
Height
20.00 nm
Figure 5.7
3.00 pm 9'
AxialDataLoad
type
2 range
SIS: 20% strain - scan area: 3 x 3 microns.
Phase
40.00 0
3.00
.
Axial Load
M~wl
9|
Data type
2 range
Height
20.00 nm
Data tsepe
Figure 5.8 SIS: 30% strain - scan area: 3 x 3 microns.
1
Phasea
C
Axial Load4
z
0~ri
0
~r1
t~1
3.00 ow '0
0
Data type
2 range
Height
20.00 nm
Axial
Load
Data type
2 range
Figure 5.9 SIS: 40% strain - scan area: 3 x 3 microns.
3.00 pm
Phase
40.00 *
Axial Load
T
3. 00 ism0
0
Data type
2 range
-
Height
20.00 nS
Figure
Axial Load
Data type
2 range
5.10 SIS: 50% strain - scan area: 3 x 3 microns.
3. 00
Phase
40.00
F'AiA
L
dt
-Y --
-
Data tjpe
2 range
Height
50.00 nN
-- ;;
Axial
Load
Data tjpe +
2 range
Figure 5.11 SIS: 60% strain - scan area: 3 x 3 microns.
--
Phase
40.00 *
Axial Load
0
Data tupe
2 range
~3.
Height
40.00 nS
00 ON 0
Axial Load
Data tjpe
2 range
Figure 5.12 SIS: 70% strain - scan area: 3 x 3 microns.
3. 00 ON
Phase
40.00
Axial
Loa
0
3.00.o
0
Data type
2 range
Height
50.00 n"
0
AxialDataLoad
43.
tape
2 range
Figure 5.13 SIS: 80% strain - scan area: 3 x 3 microns.
Phase
40.00 0
00
Axial Load 4
0D
I
CD
Data type
2 range
Height
50.00 n"
3.00 Pn 0
Axial
Load
Data type
2 range
Phase
40.00 0
3.00 om
CD
Figure 5.14 SIS: 90% strain - scan area: 3 x 3 microns.
-21
Ax.....d
....
'7,
0.
300
Data type
2 range
Height
80.00 n"
n0
Load
Axial
Data type
2 range
Figure 5.15 SIS: 100% strain - scan area: 3 x 3 microns.
A
3.00
Phase
40.00 0
Axial Load 4
0
0
3.00
Data type
2 range
Height
80.00 nm
0wO
Axial
Load
Data tjpe
2 range
Figure 5.16 SIS: 110% strain - scan area: 3 x 3 microns.
3.00 ow
Phase
40.00 0
Axial Load
>
3.00
0
Data type
2 range
Height
80.00 In
jom
0
Load
Axial
Data type
2 range
Figure 5.17 SIS: 120% strain - scan area: 3 x 3 microns.
3.a00
Phase
40.00 0
Axial Load 4
CD
0
Data type
2 range
Height
150.0 nm
3.00 ji'0
Axial
Load
Data type
2 range
Figure 5.18 SIS: 130% strain - scan area: 3 x 3 microns.
Phase
40.00 0
3.00 pm
eQ
CD
Axial Load#
z
0
Data type
2 range
Height
300.0 nm
3.oO
o0
Axial
Load
Data tjpe
2 range
Figure 5.19 SIS: 140% strain - scan area: 3 x 3 microns.
Phase
40.00 0
3. 00 v.
AFM Images with Load and Strain Measurements
103
5.4 AFM Images with Load and Strain Measurements
The sample used is an SIS triblock copolymer with the load applied parallel to the orientation of the cylinders. The sample has a gage length of 26.9 mm (L) x 3.23 mm (W) and
0.34 mm (H). The sample was stretched at 0.16 mm/s. The scanning procedure followed
to obtain AFM images for this experiment is the same as described in Section 5.3.
Figure 5.20 and Figure 5.21 show the overall load-strain and load-time curves describing
the behavior of the sample during the entire experiment.
First, the sample is scanned. Figure 5.30 shows the microstructure of the sample at 0%
strain. The whiter domain represents the styrene cylinders and the dark domain represents
the isoprene phase. In the micrographs in Figure 5.30, defects on the structure can be
observed.
Load vs. Strain
4.5
4
3.5 3
2.5
0
2
eeoo
1.5
LI'
0.5
I
0
0.05
0.1
0.15
0.2
}
0.25
Strain
0.3
|
0.35
Figure 5.20 All load-strain curves.
0.4
0.45
0.5
104
AFM SCAN IMAGE QUALITY AND EXAMPLES
Load vs. Tin*)
4
Up to 10% strain
Up to 20 % strain
Up to 30% strain
C
Up to 50% strain
0,61
0
200
1720
330
300
3700
Time (sac)
3900
8000
8200
The time is compressed in order to show all stress
relaxation curves
Figure 5.21 All stress relaxation curves.
The sample is then stretched up to 10% strain and reaches a load of about 2.3 N. Most of
the relaxation occurs within the first 80 seconds, as shown in Figure 5.23, and the load
drops to around 1.6 N. However, the material relaxes further and after 22 minutes the axial
load on the sample drops to about 1.3 N, as shown in Figure 5.21. At this point, a scan is
performed and it can be observed that at 10% strain the cylinders start to break as shown
in Figure 5.31. Although the scan location is not the same as at 0% strain, the cracks seem
to propagate from preexisting defects that cause higher stress concentration at nearby cylinders. One way to prove this would be to scan at the exact location at different strain percentages.
105
AFM Images with Load and Strain Measurements
Load
va.
Strain
Load
v.
Time
4-
4
sok
2.05
-
---
f.
,I
I
0
0.01
2.-
002
0.0
0.03
00
0.06
007
0.08
0.00
40
01
00
80
100
140
120
Tine (eec)
160
180
200
240
220
Figure 5.23 SIS: Stress Relaxation.
Figure 5.22 SIS: Load vs. Strain (0-10%).
The sample is then stretched up to 20% strain and the axial load reaches about 2.4 N.
Nearly 23 minutes later, the sample relaxes and the axial load drops to around 1.45 N as
shown in Figure 5.21. At this point the sample is scanned. From the micrograph at 20%
strain, it can be observed that more cracks nucleate from defects and the existing cracks
seem to propagate more and more through the sample.
Load vs. Strain
Load va. Timne
5
4.Z
4
41
3.6 -
-
3.5
3
3-
2.5
25 -
WD, . 0
")C 0
IP
1-
0.:
0.11
012
0.13
0.14
0.15
Strain
016
0.17
018
0.19
Figure 5.24 SIS: Load vs. Strain (10%-20%).
0.2
-0
120
150
200
Tie
250
(eec)
Figure 5.25 SIS: Stress Relaxation.
00
AFM SCAN IMAGE QUALITY AND EXAMPLES
106
The sample is then stretched up to 30% strain and the axial load reaches a value of about
2.2 N. Nearly 65 minutes later, the sample relaxes and the load drops to around 1.35 N as
shown in Figure 5.21. In the micrograph, the same processes as observed at 20% continues at 30%, that is, crack nucleation and crack propagation. At 30% the propagation of the
failures seems to be occurring at some angle to the load direction. This effect could be do
to nearby defects; to investigate this possibility, a larger scanning area could be examined.
Load s. Tirre
Load vs. Suals
5-'-
4.5r
4.5 --
2.5
2
C04
-
3.. .Sv
C
1.5
0.5
.
0.21
-
0.22
0.20
0.24
0.26
0.25
0500i
0 27
0.28
0.20)
0.3
Figure 5.26 SIS: Load vs. Strain (20%-30%).
so
100
soO
200
200
200
Tim (Boo)
300
400
L_
450
Figure 5.27 SIS: Stress Relaxation.
The sample is then stretched up to 50% strain. After about 90 minutes, the sample is
scanned. In the micrograph at 50% strain (Figure 5.34), it can be observed that now most
of the cylinders are broken and the cracks have propagated throughout the sample. There
was no obvious macroscopic necking in the specimen during loading.
107
AFM Images with Load and Strain Measurements
La
5.
vs.-
Iim.-
4.6
4.5.
4
3.5
g
2.0
3
1.0
1
--
0.5
032
034
036
Figure 5.28
038
0.4
Sb ainS
0.42
044
0.46
0.4
Load vs. Strain (30-50%).
0.5
0.6
50
100
150
200
250
300
Tirm (
e)
31W
400
450
Figure 5.29 Stress Relaxation.
500
550
M
to
CTI
5.00 Jim 0
0
Data type
2 range
Height
300.0 nf
5.00 uw
Data type
2 range
Figure 5.30 SIS: 0% strain - scan area: 5 x 5 microns.
Phase
40.00 *
Axial Load 4
CD
C-
0
Data type
2 range
Height
300.0 nm
5.00 0e4O
Axial Load
Data type *
2 range
5.00 ON
Phase
40.00 0
Figure 5.31 SIS: 10% strain - scan area: 5 x 5 microns.
0
Axial Load 4
to1
z
Data type
2 range
Height
300.0 n"
5.00 J"60
Axial
Load
Data type
2 range
Figure 5.32 SIS: 20% strain - scan area: 5 x 5 microns.
Phase
40.00 0
Axial Load+
0S.
Data type
2 range
00
Height
300.0 n"
0a Axial Load
Data type
2 range
Figure 5.33 SIS: 30% strain - scan area: 5 x 5 microns.
Phase
40.00 0
S.o00
Axial Load 4
z
0
0
D.00tN
Data type
2 range
0
Height
300.0 nf
Axial Load
Data type
2 range
Figure 5.34 SIS:50% strain - scan area: 5 x 5 microns.
s.00
Phase
40.00 0
5
N
Chapter 6
CONCLUSION AND FUTURE WORK
6.1 Summary
The tensile test machine, together with the AFM, facilitates the study of the correlation
among the morphology, deformation mechanisms, and mechanical properties of nanocomposite materials. The thesis has covered the whole design process: methods and concept
generation, feasibility study, error reduction analysis, detailed engineering analysis, and
software design. The thesis has also presented examples of images taken with the AFM on
the tensile test machine and demonstrated the testing capability to observe the evolution in
structure with deformation of a sample nanocomposite polymer material system.
6.2 Capabilities
The tensile test machine has the capability of testing different types of polymeric-based
micro- and nano-composites by applying a wide range of stresses and strains. On the one
hand, the tensile tester can open up to 125 mm and withstand loads up to 4.4 kN and therefore be able to test very stiff nanocomposite materials such as thermoplastic polymer. On
the other hand, it is able to test very compliant materials or small samples with axial loads
below 3 N. These capabilities are integrated in one design. Moreover, the tensile test
machine not only provides the macroscopic stress-strain behavior of materials under a
wide range of loading and displacement conditions, but also simultaneously allows the
113
114
CONCLUSION AND FUTURE WORK
microscopic structure changes to be observed with nanometer resolution, as shown in
Section 5.3 and Section 5.4.
6.3 Final Design Features
The final design has the following features:
- The tensile test machine can provide integrated in one device a wide range of
measurements in terms of load and displacement, ranging from 0.2 N up to
4.4 kN and displacement from 5 microns to 125 mm.
" The tester is compatible with the Dimension 3100 AFM.
* A special feature is that the center of the sample remains stationary with
respect to the AFM probe while the material is stretched. This is achieved
with a left- and right-handed precision ball screw assembly, combined with a
stepper motor connected through a timing belt. The ball screw is a non-preloaded 0.631 inches diameter Thompson Saginaw ball screw.
- The machine has an adjustable support bridge that can be used to compensate for the decreasing thickness of the sample under tension. This avoids
sample vibration of thin films during the scanning operation, resulting in
scanning images of great quality.
- The control software provides real-time feedback of the measured load to
characterize the state of the stress relaxation of the sample.
The detailed engineering drawings are provided in Appendix B. The details of the final
design are summarized in Table 6.1.
Final Design Features
Tension
Mechanism
Precision Control
Force
Measurement
Left- and right-handed precision ball screw assembly driven by a
stepper motor combined with precision linear guides.
The center of the sample remains stationary with respect to the
AFM probe while the material is stretched.
Open-loop control system with Lab View software and a linear
encoder to ensure 5 micron precision accuracy (at 0 N) that
increases up to 30.9 microns (at full range load).
Three hardware-coded loadcells:
0-25 N with 0.2 N resolution.
0-250 N with 2.5 N resolution.
0-4400 kN with 44 N resolution.
Safety Features
Mechanical end switches to protect the drive system.
Software safety limits.
Software overload protection.
Hardware-coded loadcell.
Safety covers for the belt.
Emergency stop button.
Type of
Measurements
Real-time measurements of force and displacement in order to
characterize the material samples.
Real-time measurement of stress and time to identify the stress
relaxation.
Type of Samples
115
Thin films up to 3 mm thick samples, with width up to 8mm and
sample length between 25 and 125mm.
Compatibility
Compatibility with a Digital Instrument Dimension 3100 Atomic
Force Microscope.
Sample Setup
User interface walks user through all necessary steps.
Data Collection
Data can be exported as text file (for import to Excel, MatLab,
etc.).
TABLE 6.1 Main design features.
116
CONCLUSION AND FUTURE WORK
6.4 Future Design Improvements
Possible improvements that would enhance even further the capability of the tensile test
machine are:
* The use of a gear box with a high gear ratio: this would allow the use of a
smaller motor and also add the capability to test samples at speeds that currently are at the resonance band of the stepper motor.
* Off-set grippers for applying shear loads.
" Integrating the control system of the AFM with the control system of the
tester for automatize testing.
The tensile test machine allows the macro- and micro-structure change observations to
establish the correlation between the macroscopic stress-strain behavior and the observed
microstructure deformation, thereby helping to make possible, perhaps in ways now
unforeseeable, the development of novel nanocomposite materials with a new paradigm in
composite technology.
Future Design Improvements
Figure 6.1 AFM (left) & tensile test machine (right).
117
118
CONCLUSION AND FUTURE WORK
REFERENCES
[1] Avallone, E. A., Baumeister, T., Marks' Standard Handbook For Mechanical Engi-
neers, McGraw-Hill, New York, NY, 1996, p. 3-23.
[2] Bobji M. S. and Bhushan B., Atomic Force Microscopic Study of the Microcrackingof
Magnetic Thin Films Under Tension,Scripta Materiala, volume 44 pp. 3 7 -4 2 ,
2001.
[3] Gary W. Johnson, Richard Jennings, LabVIEW GraphicalProgramming,Third Edition, McGraw-Hill, 2001.
[4] Oderkerk J., De Schaetzen G., Goderis B., Hellemans L and Groeninckx G., Micromechanical Deformationand Recovery Processes of Nylon-6 Rubber Thermoplastic
Vulcanizates as Studied by Atomic Force Microscopy and Transmission Electron
Microscopy, Macromolecules 35 (17): 6623-6629, August 13, 2002
[5] Shigley, Joseph E, Mischke, Charles R. StandardHandbook of Machine Design, Second Edition, McGraw-Hill,1996.
[6] Shigley, Joseph E., Mischke Charles, Mechanical Engineering Design, Fifth Edition,
1989.
[7] Slocum, Alexander H. Precision Machine Design, Society of Manufacturing Engineers Dearborn, Michigan, 1992.
[8] Ugural A. C., Mechanics of Materials,McGraw-Hill Inc,USA, 1991.
[9] Smith T. Stuart, Flexures: Elements of Elastic Mechanics, Gordon and Breach Science Publishers, The Netherlands, 2000.
[10] Meissner 0., J. Schreiber and A. Schwab, Formation of mesostructures at the surface of ferritic steel and a nickel monocrystal under increasing load - an in situ
AFM experiment, Applied Physics A, Volume 66, pp 1113-1116, October 1998.
[11] National Instruments, LabView Basic I: Hands-On Course, Course Software Version
6.0, 320628G-01, September 2000.
[12] Bamberg, Eberhard, Principlesof Rapid Machine Design, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, 2000.
[13] Opdahl, A, Somorjai G. A., Stretched Polymer Surfaces: Atomic Force Microscopy
Measurement of the Surface Deformation and Surface Elastic Properties of
Stretched Polyethylene, Journal of Polymer Science: Part B: Polymer Physics,
119
120
REFERENCES
Vol 39, 2263-2274, 2001.
[14] Lepizzera S., Scheer M., Fond C., Pith T., Lambla M. and Lang J., Coalesced Corel
Shell Latex Films under Elongation Imaged by Atomic Force Microscopy, published in Advance ACS Abstacts, November 1, 1997
[15] T H K, General Catalog, Catalog. No.300-1E, T H K Co., LTD, Tokyo Japan, 2000.
[16] Oberg, E.,Jones, Franklin D.,Horton Holbrook, and Ryffel, Henry H., Machinery's
Handbook, Industrial Press Inc., New York, 2001
[17] Oderkerk J., Groeninckx Q Investigation of the Deformation and Recovery Behavior
of Nylon-6/Rubber Thermoplastic Vulcanizates on the Molecular Level by Infrared-StrainRecovery Measurements, Macromolecules, Volume 35, Number 10, pp
3946-3954.
[18] National Instrument, LabVIEW Basics II, Course Software Version 6.0, Part Number
320629G-01, September 2000 Edition.
[19] Howell, Compliant Mechanisms, John Wiley & Sons, Inc., USA, 2001.
[20] www (2003). http://www.kammrath-weiss.com/kw/english/home.htm. Kammrath &
Weiss GmbH.
[21] www (2003). http://www.deben.co.uk. Deben UK Limited.
[22] www (2003). http://clifton.mech.nwu.edu/~espinosa/Pl, Professor Horacio Dante
Espinosa, Institute for Nanotechnology at Northwestern University.
[23] Dames Chris, Grippo Christian, Lee, Jin-Wook, Mangalgiri Vickram, Sprunt, Alex,:
Optically Sensed In-Plane AFM Tip On-Board Actuator, Design Project, Spring
2002, MIT.
[24] 0. M. Rifai, K. Youcef-Toumi, In contact dynamics of atomic force microscopes.
Advanced Intelligent Mechatronics, 2001. Proceedings. 2001 IEEE/ASME International Conference on, Volume: 2,2001.
Appendix A
SOFTWARE DESIGN
A.1 Software Limit Stop Subroutine Block Diagram
0OC: SOFTWARE LIMIT STOP FUNCTION
|measured Positon
oftwareLiMlStatUS
Ffosn Softeware LSSt--a
step]
Loop
Figure A.1
rTime
stop Status
Software limit stop subroutine.
121
122
APPENDIX A
A.2 Load Offset Calculation to Zero Load Cell Value Block
Diagram
|True------- |-0
1[0..21
tep 2-
FMG
k~ccumulated
Value
ISum
.... l
--- -------
[~ffset Value(NN
TimeLoopStopStatlis
Figure A.2 Load offset subroutine.
A.3 Release Subroutine Block Diagram
aLit sitch-
I,
Lv
----------
True
Jeasured Position
pn Lt S wch
True
eee
Measured Positlon
False
--------d---
El
'~L~J
I~=~==~J
Figure A.3 Release subroutine.
123
APPENDIX A
A.4 Stop Button and Sample Length Calculation Sub Routine
Block Diagram
Stop PLses Progam
True
ample Lengthl
5i0
False
-
PulsesSt
n Ste
Posit
otop
Position Stepj
Figure A.4 Stop button subroutine.
A.5 Machine Set Up Subroutine Block Diagram
CALLS THE SETUP PROGRAM
True
Tru
ud Path
tri Path
open VI Referenc
n
and run, v,
Sample Set Up
t Paramter
--
[
Load CeI Zero
Offset
FMnttronRn5SeUpg.
t VI's Path
So
Figure A.5 Machine set up subroutine.
tau
APPENDIX A
124
A.6 Stop and Quit Program Subroutine Block Diagram
ITrue P
IL'..3k
3
I~vi
tau
top
El1
IF-P.Ope
Figure A.6 Stop & Quit subroutine.
A.7 Pre-record Data SubVI Block Diagram
-True
mGra
....... .....
c P Ope
o ou want to safe data?
r
posed arra
Tre
o ac to Main
N
El........
Figure A.7
Pre-record data subVI.
APPENDIX A
A.8 Pre-record Data Front Panel Sub Vi
Figure A.8 Front panel: pre-record data subVi.
125
126
APPENDIX A
A.9 Set Up Machine Sub Vi Block Diagram
Tme
lE s
--
=rnaw setv
-0r5o
r
11
L~~I
TrL&
m inje
h-tte kbe Trm
01
Ir~,I
StafL~
fa~O
i
W*be
and
he tev skT"
x
3
.....
.....
ac0
ons
s
u
maskSdf~h 1
.tp
...
.~~~(...pr......
ne cbrwoon mp reack-o
J
meL'
mas Swith
o1'
wi~
2|
pUlse
Tre
Tra,
ff
52.
F7bop, ft
r
he s~LWAOfte WMtSm
Fase
Fab
Thie
E ,
_--_
wq - I
EIR
L
bpmae stnusf.|%~4
-
Figure A.9 Set up machine subVi.
EED-4
APPENDIX A
Element
Device #
Pin # at
Connection
Pin
Number at
SCB Box
Signal Type
Connection
Load cell
1
8
9
68
67
AnalogO
Any analog
Limit switch 1
1
9
5
52
53
DIO 0
Digital ground
Limit switch 2
1
7
6
1
17
18
DIO I
Digital ground
Auto set up
limit switch
1
8
3
15
47
Digital ground
DIO 3
Connection
check
1
4
2
19
50
DIO 4
Digital ground
9
Load cell type
25 Newton
1
3
4
16
35
DIO 6
Digital ground
Load cell type
4.4 kN
1
1
6
48
13
DIO 7
Digital ground
Motor direction
1
2
15
49
12
DIO 2
Digital ground
Ground housing
1
25
54
Analog ground
Motor frequency
2
1
44
29
33
PH 24 counter
GND counter
Linear encoder
2
A
B
Z
2
40
3
Counter
Counter
Digital
TABLE 6.2 Hardware connection.
127
128
APPENDIX A
Appendix B
ENGINEERING DRAWINGS
129
cia
0
-~
--
+ .20
--
u
-Q
2mm
-
T
LL
1u-
+0.20
A
+0.Z-
SECTION A-A
C)
4t
CR
C-2
-
0-. (0 0-)
cfl
+
(
i
I I
-
-
i
43.50
-
+0.0l5
010.50 0
0
I
A
'5
T Trough all
K ------
4-
x 0 4.50 TH RU
__ 08.2571 4.50
~21~
-- 4xyM4
0.7 Threaded~TThru
R4
24 5-0
....IS
E SPICIHID
EC
d-iISSOI..E
1.-.
I
K.-. L..Jt L.A
DMINSONS All
IsN:
NI
sdrAc
E
~I' 1K'
ClASNhrislacn
QA
x
:WE.:
.....
777
DMal AN
IA( S-
bllfII
401
x
4006
x0 0.xx
m"
ADP
VN
ODN
1D Is
Tersile Test Machine Modification
Gripjo
Carriage 2
Ulr
A4
1018 Steel
I:
......
------
........
r-3/" 24 UNF T
15mm
1-
Note: thread is 15 mm +0.1
.
.
.
-
A
. ..
.....
Q20
0.r5K0
., .05
N2
Noe: All Corners R 0.2 unless specified
SdPAC mm.
X ,tg.,
x 50.0s
EO I
Tensile Test Machine
Load Cell Holder 2
rm r
Df
..
....
..
....
A4
1018 Steel
SE-111 01,
......................
..
....
.....
.....
...
. ....
......
..
...
.....
....
......... ......
2
..
...
...
......
........................
..........................................
---------...............
D-D
.............
R 0.3
01
z
..........
t)ru
B
TD
Wil
C
--..................
..................
...............
.................
!Ms.:
1041ISS 01-Irwal SPIC11110:
MMSO-AS Aft 14 MIONIIINS
Id"Act-ING.- X
t ().I
011'ArIS:
X
t 0. 015
IMA
XX t 0.01
DAII
,PIA
CbrW*n QApp*
CD
Aw
..............
.................
..................
....
...........
.........
....
DO-DISCAd DIAWWC
WIA(I-AfP
We Is
Tensile,
Testing
Machine
........
.. .......
.......
-----------..
.........
.............
Top Holder Carriage 2
D
....
...
..
..
QA
A4
SCM12.1
: S.111 - C?. -
2
0
I
I
0
-00
dwS 01-61.67 1aSIMI, If D: INS-:
DMINSD04SAtE
I 4
Sur-ACE 'I-IS.: X +.0
ANCIIAf:
OVAww
CXX
IM ilbt VS
-
toe IsA
-
CA-- I
- A - -W -
-
-
----
Tensile Testing Machine
-0.01
ChdanI Nippo
Encoder Holder
WAIllilkl:
D-- -10.
A4
'5 4A : " "1
..
.. III.Q.,
7
........ ......
2.
Any type of teeth shape
1K
650-5.0
4.60
26
26
±0 0
DW
+002
S04JAI W.4WhNI
A
IIAIE
44XX
PCsS
Through
*OrA
2ao6
Tersile Test Machine
.01
srAIdri
Gripper For U Shape Carriage
1018 Steel
A4
A
1..
+0.07
14 0
-1.
dc
JJ'
DM18
4SAflIWE.1.1MIIDDF
1.iPACI
i E
X
t
TlA40
10C IS
D1
X t 0.05
A -IC d.AI
13FAW4
Chsa dp
VNE04
Tensile Testing Machine
XX ± 0.01
3
D0DSOtDFAW140
$91ACS-AVP
:
0.1.
4AdE
..
..
..
DARE
....... ....
Strip Holder 2
. POVD
V - -10.
A4
Akjrymnum
2
.............
28,1
RS.5
7
Z
6.-g
.........................
........
NR
a divs 2 mm
7
4.2
..
...........
............
............
.. A
lix fvA e lxl,
Tfu
i=2 d
lsw
Threaded ,T 22.4
4x
................
.......
.-.1.11..
........
.....
..
..
..........
SPICPJID:
Ph
MS QI.IrwSI
DImD4s Art woohlballits
AW-dtAg:v
xx
..................
DI9d fA4D
.No.:
Af.S.Av
IDG is
Tensile
;0.01
................
DfAW4;
VIVIC04
.....
............
................
............
...
..........
DAR
...
.. .....
...
. .........
---------
Block.Carrioge 2
..........
.....................
.C-VD .
............
Test Machine
................
..............
..
Ionic
'OA
wAffelk.:
...
.......................
A4
.....................
..........
...
--------------..........
..............
.................................
......................
1018 Siee1
.........................................................
...........
.................
..............
.......
........................................................................
- 1- ..........
............
.......
I
Oil
........
..............
.....
.............
...........
Note: For.R6 cut
use Ball End Mill
4A, 04
3.4
...........
.......
...
......
!DM14SID-JS ANN 14 mHeMIIINS
141AV;02S
Awda'ACY
............
......
....................
-MMI
=14AIDWI
..
........
DrAW-1,
Ind r A 4 0
IfiACS-Ag'
-- t-.- ...................
S..
CA. I DN.A
............
NUT
FLANGE
..............
.................
INII:
..
.............
i........................
C-CD
A4
S-11110-2
- 9It
L --
2x
~9io
0
________
3
_30 Thr
-7e
ri rI
SdIWACE 11
c
A.CII
t
X
-XX
OIC'hsANEGd
0.1 1
Tensile Testing Machine
0.05
0-01
SQAIldWI
DAI
Stop Switch Holder
+AIVD
QA
Akminum
~C.1O.
:sca,E:-
A4
Ss-m
on01
1-1- .............
..
.......
..
....
A
k
/ A-A
-y
26
S
C
~
-26.46
Plecse Align holes wiih holes from Gripper2LArgoU Drowings
III
E4IVACI 614.E
v.EFANCS:
MEt
D4D
rA4D
I,I t AI
-
ISCA IiD VA- WOc
C
M604
Tensile Testing Machine
X ± 0.05
XX t 0.0
MWAIME
DkME
DAISal.Oiistidh
Support for Mini Load Cell
AP"D
5.,
7",--
wuIT|Ai:
A4
n
ScC11:
s-IlmIO
I
L
0
-------- --3
UI
SECTION C-C
16x~ (7b4. ' THRI.
L_ rZ#34.5
--------~
~ -------~
ST9
R2.
7
4
V15.5THPU
L (
T
1,U
/2.
84.1
-------
'7en
GE3.EAI1 b .401
1.I
0444 10 14
m1 firs141
.
s1m
ct
"
:
NUTAD
DO
19E4 AT
x xs
x 001
1
^',I
C10510
DTA- 1C
Tensile Testing Machine
Top Carriage
I,,-,
-CD
.'o
1018 Steel
i
SCA,:13
_4.
A4
|
S
IfI0I
TT
1 71/2" TThrou
~
-
--
1/4"
R7/"a2mm
- R1/4"
or Oreoler
A
01 /2"T Throu
149 r--
(07/8"T2mrn
I13
Bottom Surface
tr
C
LC-~
II
4(*
I
/
~
/
~2rrv
11 -A
Top Surface
SECTION A-A
SCALE 1: 1.5
SC
,,
Chamfer 05 x 45 deg
+ 0.1
M 9 K24
sxco
s
X
0.1
Tensile Test Machine
Note: Use precision aluminum book
with 0.002" tolerance
-
Din Projection System
Chuck-Base Support
A4
Alu minurm
- --------------
&-111 1C2.1
I
......
..........
.............
.5
11.46
SECTION A-A
101111pcj -.
It : g 1.
-.
I -DU
Pass Thro,,&%
utdcA.-ID
IRM S.Aff
=ts
.1 20 De
XX 10-0 "1
.
..............
Tensile Test Machine
...........
...
.......
.............
...
.............
.
wA-4
...........
15.70
.............
14 .6-iD
........... ....
.................
Support ForMini Load Cell Metric
..........
..............
..........
...................
....
..................
.......................
......
- - ..................
...................
j CA
! 0w; -or.
.............
.............................
.............
10 18 Steel
..........
.. .....
.................
..............
.............
..........
A4
I
30 --.45.
71
iho
t;o
g
I
I
,.-
t
A/)
g
2y(0
T 12
I
I
2.J4ThrEaded
ei
is --0,01
(,)
t .
DA4
Al...minum.
Tensile Test Machine
LimitSwitchl Support
A4
luminm
Nil I 106 1
2
0 Wi~j'y~j1
ili
.
75.1
A_
.
~..
.~'
rs
DilI
4SK
A1
O4EfA4CIS:
*X
f
OD5
-CO
A
A
SQ4AIII'l
III:
Christian
.....
D ...
McDhie
<Tensile Testing Machine>
MX1A 1:
hs
siCA4l
DMCstin
<
!DM41A.4D
RSIA(S-ANI
AMUl
±riD
E4
Adaptor Small Load Cell
...
....
A4
S
CA,
1E2:4
A
I
~Z10-3
15.0
16.0
U U1F
N
A
~1
I
£
A-A
0
..
....
'I:-
/
0
DO 401 SCA i 1 0 RAW 14r,
ur-tAdCEI
.1
0WrAES: X
lt
t Ofli
+
<Tensile Testing Machine>
II
chnsian..
WWSK 4
IL
.......
Adaptor Small Load Cell
....................
..............
,VAC
JQ.
A4A
2
...................
............
...........
....
*
1
*< t..............
01 .7G-.. ........
01-4.7r 0
0
4>
O:3
.05
o
DINFA-11)
DMiWSKDWS Aft 14.h.V4[11WS
WC IS
SUPACI $1411,
OLAWCIS:
X
X
Ax
.
.......
......
4%ml
Tensile Test Machine
:! 0.05
20.01
Sr.4AldWI
DRAWN
.............
1C-CD
DAIR
...........
Chuck-Pin
......
...
........
..........
...
OA
.............
AA
118-Stee
SCAWfI
2S
~
u
r
~~i1?
1
0'0
~~gLx
(N6C
?4~t
'I
-
qCK6
'0
'0
or-,0
i
f) If)
tf
-22.2
.
I0
0 -
265
0
4zI T
41
-5
-1
+
-t
c4
0
+
I-
I
1.
- 18x A/Gxi.I,
5 Tti~.ac1c~d 121.8
180./
C
+
--
/
-<4
/
/
K_______24
FINH:
ENS *THERWNINPECIFIED:
D ENSIOHAREINHMILUMETER$
URFA CE FIN NTOLERAHCE$:
X
±0.1
x
XX
AN GULA14
WGHATURE
---
DOHOT$CALEDkAWHG
IIEAKSKAtP
EDGEn
t0.08
±0 t.01"
NAME
30
DEBUR AND
DATE
~~
Tensile Testin
TfLE:
...
..
-----
Base
-4
MFQ
j
1 1 0 -u11
A
TE8
0 AL:
D WG H O.
1018 Steel
2
WEIG HT:
6 CA LE:
...........
...
......
..
..................
....
............
1- ..............
................
- .1...........
fmdf
tA4D
At$-Afl
OMINS104SANIN. mkilmIlIfS
X
t 0
WIA
Note: Woll thickness 0.125" (3.175)
X
0.05
t 0.01
...................
- ............
..............
- .................
.............................
...............................
CAR
s,4AWI
Or*w4. ChdAan 44pp*
.............
C.CD
...........
....
.....
D0401SCAH DIAWNG
...........
-- .....
......
nV6104
..........
............
Tensile Testing Machine
...........
....
...........
.
...............
Switch Holder
......
......................
..........
......
....
Din Projecton System
A4
Miminum
i SCAIIJ2
: S-111 101 -
APPENDIX B
149
150
APPENDIX B
Appendix C
CALCULATIONS
C.1 Minimum Shaft Diameter
C.1.1 Failure due to Yield
Failure of the shaft due to yield can be estimated using the Von Mises criterion [5]:
(C.1)
Sy'
where Sy is the yield strength and o' is the equivalent Von Mises stress, given by:
'+=
+3
(C.2)
2
introducing the expressions for the shear c and normal stress ax:
T r
J
16.T
-
d3
=
and
X
= 4F
A
-d2
(C.3)
one finally obtains:
2
=
4' F r
3.
2
4 F2
+ 48(T2
(C.4)
r
151
152
APPENDIX C
C.1.2 Failure due to Buckling
The screw shaft is subject to compressive loads as the sample is stretched. Therefore it is
important to estimate the critical load F, at which bucking occurs [5]. Since both ends of
the ball screw are fixed through the mechanical system, a fixed-fixed boundary condition
is assumed. The critical force F, is estimated assuming a commonly available shaft diameter of 16 mm with a pitch of 5.08 mm and a Young's modulus of steel of 207 GPa.
F
E
=
- C
(C.5)
dr
64
L
where E is the Young's modulus:
E
dr = 16mm -
F-L
(C.6)
= 13.46mm, LC = 125 mm and E = 206843 N/mm 2 , C=4
2C
2
4 - 2 206843
N
n
C.7)
(13.46mm)
Fmm
(125mm)2
64
Therefore the maximum axial load is much less than the critical force that will produce
buckling:
Fc = 0.84 106 N
(C.8)
4400N
C.2 Required Power Calculation
Assuming an outer diameter of 16 mm (OD) and with a lead (1) on a square type screw of
5.08 mm using the following standard formula [5] the torque T is:
T = Fx-dm [it. g dm
2
- dM -V0
-1
Where:
P
= 1 for squared threads
(C.9)
)pU = 0.01
APPENDIX C
dm = d - p/2
153
p = l/ns
1 = p - n,
Hence when replacing values:
d. = 16 - 2.54 = 13.46 mm
p = 5.08/2 = 2.54
i - 0.01 - 13.46 + 5.08
T - 440ON - 13.46.
2
n - 13.46 - (0.01
5.08)1
T = 3.858 N m
(C.10)
(C.11)
Since LinearSpeed = Lead - rpm and assuming a desired speed of 25 mm/min.
2mm
25rpm
=
mn
5.08mm
= 4.92rpm
(C.12)
1
(>
=
4.92rpm. 2 - 71 = 0.515s-
(C.13)
60#
min
Therefore the required motor power is:
Power = 3.858Nm - 0.515s~
=
1.99W
(C. 14)
.--M
154
APPENDIX C
C.3 Truck Stiffness Calculation
The following analysis is an approximation of the stiffness from a system formed by a carriage and trucks. The trucks are placed on the linear guides. In this section, the carriage is
assumed to be infinitively stiff, in Section 3.3.4 a FEM analysis for the carriages is performed. The purpose of the analysis is to compare two different design strategies: two
adjacent trucks or two trucks separated by a gap on each side of the carriage.
C.3.1 Design Strategy I: Using One Truck on Each Side of Carriage on
the Linear Guides:
Klinear guide truck
Stiffness per unit lenght for one truck.
Figure C.1
For this analysis a coordinate system with the oriY-axis
gin at the center of the truck is choosen. Assuming
an homogenous stiffness distribution along the xaxis, the stiffness of an infinitesimal small portion
0N--
--- - I
i
I
Truck
I
X-axis
is:
K' =
dx
(C.15)
APPENDIX C
155
where K/i is defined as the stiffness per unit length. The vertical displacement 6' is a linear function of x. It depends on the position and angle 0, hence using the small angle
approximation:
6 ' =x0
(C.16)
Per each small increment on x, the vertical force is:
dF' = K' - 6'.
(C.17)
At each infinitesimal segment dx, the moment is equal to the vertical force dF' times the
moment arm. dM = dF' - x .
(C.18)
Therefore replacing equation C.15 and C.17 in equation C.18:
dM =
K'
'-x
=
K
-dx
-x -
(C.19)
Integrating from -1/2 to 1/2:
= 2
-(E.x2.)dx
M =
p(K
2
3
x
2. 3
K
1
2
010
(C.20)
2
M = --12
Kmoment -
M
(C.21)
-.0
2
K-1
12
(C.22)
156
APPENDIX C
C.3.2 Design Strategy II: Using Two Trucks Next to Each Other
/
/
Kinear guide truck
Kinear guide truck
L=1
Figure C.2 Two adjacent trucks.
Using two trucks of same length I each, the analysis follows as before:
M =
K- 0.-x2 dx = 2. -
M =-
Kmoment
2
3
=
E- 0 .x2dx = 2
2
K - 0 -1
M -:2 -K- 12
K-. 0 --X
(C.23)
(C.24)
(C.25)
APPENDIX C
157
C.3.3 Design Strategy III: Using Two Trucks on Each Side of Carriage
Separated by a Gap:
I
1/
.4
L
yL.
x
Kinear guide truck
K inear guide truck
Figure C.3 Two truck separated by a gap.
Y-axis
L
I
/
Truck
X-axis
ruck
- -- -- - - - - -
x
(L+l)/2
77
----------
-~
(L-1)/2
Figure C.4 Integration limits.
Using two trucks separated from each center by a distance L, defined as the distance from
the center of one truck to the center of the other truck, the stiffness can be calculated as
follows.
Using equation C.20 with the appropriate integration limits:
158
158
APPENDIX C
APPENDIX C
L +l
32
L+l
M=
2.
-L
2
2-
1
x
dx = 2 -K 0
3 L2
3 (L
M = 2 -K - 0 [(L+
M
=K
12. 1
(6.1)
*[(L+1)3- (L-l1)3]
M =
-12- 1
=
)3
(C.26)
- l.(3 - L3 +12)
(C.27)
m (3e - L2 + 12)
(C.28)
SM
(C.29)
Kmoment
0
Hence Kmoment with two linear guides separated by a distance L is stiffer than Kmoment
with one truck or two adjacent truck.
Kmoment =
- (3 -L2 +
6
)
2
K moment = 2.K-
if L>l
(C.30)
C.4 Socket Head Cap Screw Grade and Torque Estimation for
the Clamping Force:
Assuming that the coefficient of friction is approximately 0.15 [16], the normal force is:
F = p-N
N = 440N - 29333N
0.15
(C.31)
(C.32)
APPENDIX C
APPENDIX C
-dM + d 0)2
For a Unified thread, the tensile stress area is As =
159
159
(Machinery's Hand-
book, 1483). For a M4xO.7, the average major diameter is 3.91 mm and the average pitch
diameter is 3.495 mm (Machinery's Handbook, 1771).
Hence the tensile stress area is:
As = 10.77-6m 2
N
(C.34)
= Pb
Gallow
(C.35)
-
Gallow = 2.67 - 10
Since two screws are used,
sallow =
(C.33)
Pa
(C.36)
1.335 - 109 Pa. The recommended grade has to be
above SAE Grade 7 (H or I) [16]. For this design a SAE Grade 8 was chosen.
The total torque necessary to develop the axial load Pb [16] is equal to:
L
Pb - 2
-n
d2 * R1
+
(2 -cosa)
+
(d + b) 4 )
(C.37)
Assuming 60 threads, a = 30 then d 2 is about 0.92 d. With no washer under the bolt
head, b is approximately 1.5 d.
T = Pb- (0.159 - L + (0.531 - g + 0.652 - 92) - d)
(C.38)
If all the threat and bearing friction coefficients are equal then:
T = P'b- [0.159 -L + 1.156 -
- d]
(C.39)
Where Pb was replaced by P'b = 2 because two screws are used.
L = 8 mm, t = 0.15, d=4 mm
(C.40)
160
160
APPENDIX C
APPENDIX C
(C.41)
T = 28.82 N m
C.5 Height Estimation Between Bottom Surface of the Head
Screw and the Top Surface of the Sample
The total specimen area under the grippers has to be under the compression force, otherwise a region of the specimen can slip and the test will loose its validity.
Assumption: the stress cone expands with a 30-degree angle
Head diameter 7 mm for a M4xO. 7 screw
Stress Cone
7 mm diam.
Top Gripper
Plate
Sample
Bottomn Gripper
SppPort
Cross Section View
Figure C.5 Stress cone.
Min. distance between screws: 14.6mm. Screw diameter 4mm.
_
(10.6 - (1.5 - 2)) = 3.8mm
(C.42)
6.58mm
(C.43)
Y = tanx
2
3.8
-
tan 30
Hence the minimum distance between the top of the sample and the bottom of the screw
head has to be 6.58 mm.
C
U
-.
-~