EXPERIMENTAL INVESTIGATION AND ANALYSIS FOR BEARING STRENGTH
BEHAVIOR OF COMPOSITE LAMINATES
A Thesis by
Amit Yeole
B.E., R.I.T Shivaji University, 2000
Submitted to the Department of Mechanical Engineering
and the faculty of Graduate School of
Wichita State University in partial fulfillment of
the requirements for the Degree of
Master of Science
December 2006
EXPERIMENTAL INVESTIGATION AND ANALYSIS FOR BEARING STRENGTH
BEHAVIOR OF COMPOSITE LAMINATES
I have examined the final copy of this thesis for form and content, and recommend that it be
accepted in partial fulfillment of the requirements for the degree of Master of Science, with a
major in Mechanical Engineering.
__________________________________
Behnam Bahr, Committee Chair
We have read this thesis and recommend its acceptance
___________________________________
Hamid M. Lankarani, Committee Member
___________________________________
Mehmet B. Yildrim Committee Member
ii
DEDICATION
To my parents and my sister
iii
ACKNOWLEDGEMENTS
I would like to express my deepest gratitude to my advisor, Dr. Behnam Bahr for the guidance
and support he has been giving me throughout the entire period of my graduate work and
research which helped me to convert my visions into reality. My special thanks to Dr. Mehmet
B. Yildirim for his assistance, patience, and encouragement. I thank Dr. Hamid M. Lankarani for
reviewing my work and helping me to improve this thesis. My sincere regards and special thanks
to Mr.Waruna P.Seneviratne without whom this work was nearly impossible. His helping hand in
providing all testing facilities in Structures Lab, National Institute for Aviation Research and his
guidance got me past through this work. Lastly, I would like to thank all my friends, colleagues
for their support and help they provided me until this moment.
iv
ABSTRACT
An investigation of joint strength in bolted connections for Carbon unitape material
[0/45/-45/90]3s is presented. The simple case of bearing double shear joint is considered. In the
first stage the objective of work was to test three different material configurations with three lay
up sequences and further reduce to one material and one lay up sequence which yields maximum
Bearing strength. Further this material with maximum bearing strength was used to study the
effects of various geometrical parameters such as edge to width ratio, width to diameter ratio,
hole clearance, external factors such as temperature and clamp up force on the bearing strength.
The second objective of thesis was to build a 3D finite element model for the quasiisotropic material, validate the results with the experimental results and provide a future scope to
use this model for various combinations of material and parametric studies.
v
TABLE OF CONTENTS
Chapter
1.
INTRODUCTION ...............................................................................................................1
1.1
1.2
1.3
1.4
1.5
2.
Experimental and FEM Behavior on Bearing Strength ...........................................6
Scope of ASTM 5961 ............................................................................................11
EXPERIMENTAL PROCEDURE ....................................................................................13
3.1
3.2
3.3
3.4
3.5
4.
Introduction..............................................................................................................1
Background ..............................................................................................................1
1.2.1 Types of Bolted Shear Joints ..........................................................................2
1.2.2 Modes of Failure in Composite Joints ............................................................2
Issues........................................................................................................................3
Problem Statement ...................................................................................................4
Objectives ................................................................................................................5
LITERATURE REVIEW ....................................................................................................6
2.1
2.2
3.
Page
Specimen Manufacturing .......................................................................................13
3.1.1 Type of Carbon Fiber Materials Selected .....................................................13
3.1.2 Specimen Geometry......................................................................................15
3.1.3 Lay up Sequence and Number of Plies .........................................................16
3.1.4 Material Types and Test Matrix....................................................................17
Laminate Cutting ...................................................................................................18
Test Fixture and Specimen Mounting....................................................................19
Test Procedure .......................................................................................................21
Data Processing......................................................................................................22
EXPERIMENTAL RESULTS, DISCUSSION, AND COMPUTATIONAL STUDY.....25
4.1
4.2
4.3
4.4
4.5
4.6
Effects of Lay up Sequences and Comparison of Materials. .................................25
4.1.1 Effects of Lay up Sequences on Carbon Unitape Material...........................25
4.1.2 Effects of Lay Sp Sequences on Plain Weave Material................................29
4.1.3 Effects of Lay Up Sequences on 5-Harness Material ...................................33
4.1.4 Comparison of Carbon Unitape, Plain Weave and 5-Harness Material .......38
Effects of Temperature on the Bearing Strength ...................................................39
4.2.1 Results for Hot dry Condition.......................................................................39
4.2.2 Results for Cold Dry Condition ....................................................................41
Effects of Hole Clearance on the Bearing Strength ...............................................43
Effects of Torque on the Bearing Strength ............................................................44
Comparison of Strengths at Different Failure Modes............................................46
Effects of e/d ratio on the Bearing Strength...........................................................47
vi
TABLE OF CONTENTS (continued)
4.7
4.8
5.
Effects of w/d ratio on the Bearing Strength ..........................................................48
Computational study ...............................................................................................49
4.8.1 Introduction...................................................................................................49
4.8.2 Approach.......................................................................................................49
4.8.3 FE Model using MSC Patran ........................................................................49
4.8.4 Meshing.........................................................................................................50
4.8.5 Boundary conditions .....................................................................................51
4.8.6 Material Properties .......................................................................................51
4.8.7 Material Models ............................................................................................52
4.8.8 Results...........................................................................................................53
CONCLUSIONS AND RECOMMENDATIONS ............................................................58
5.1
5.2
Conclusions............................................................................................................58
Recommendations..................................................................................................59
REFERENCES ..............................................................................................................................58
APPENDICES ...............................................................................................................................61
A. Material Testing For Laminate Properties .................................................................. 62
B. LS-Dyna Key File Input Format ..................................................................................65
vii
LIST OF FIGURES
Figure
Page
1.
Classification of joints according to modes of bolt shearing [1]. ........................................... 2
2.
Modes of failure [2]. ............................................................................................................... 3
3.
Set up for Double shear test. ................................................................................................... 4
4.
Material damaged with and without lateral support (Washer support) [7]........................... 10
5.
Sand used as an analogy to describe the effect of lateral constraint [7]. .............................. 10
6.
Double Shear Test fixture set up........................................................................................... 12
7.
Carbon Unidirectional Tape Material. .................................................................................. 14
8.
Fiber orientation in Plain Weave Fabric material. ................................................................ 14
9.
Fiber orientation in 3K 5-Harness Material. ......................................................................... 15
10. Standard specimen for procedure ‘A’. .................................................................................. 15
11. Final Cut Specimen............................................................................................................... 19
12. Bearing double shear test fixture. ......................................................................................... 19
13. Fastener and bushings. .......................................................................................................... 20
14. Mounting of specimen on the fixture.................................................................................... 20
15. Extensometer......................................................................................................................... 21
16. Extensometer mounted on the fixture. .................................................................................. 21
17. Tested specimen.................................................................................................................... 22
18. Stress vs strain curve according to ASTM standard [11]. .................................................... 23
19. Bearing Stress vs bearing strain............................................................................................ 23
20. Load vs hole deformation. .................................................................................................... 24
21. Load vs hole deformation, lay up sequence (%): (50/40/10)................................................ 25
viii
LIST OF FIGURES (Continued)
22. Bearing stress vs bearing strain, lay up sequence (%): (50/40/10). ...................................... 26
23. Load vs. hole deformation, lay up sequence (%): (25/50/25)............................................... 26
24. Bearing stress vs bearing strain, lay up sequence (%): (25/50/25). ...................................... 27
25. Load Vs hole deformation, lay up sequence (%): (10/80/10). .............................................. 27
26. Bearing stress Vs bearing strain, lay up sequence (%): (10/80/10). ..................................... 28
27. 2% Offset strengths for Carbon unitape material. ................................................................ 28
28. Load Vs hole deformation, lay up sequence (%): (40/20/40). .............................................. 29
29. Bearing stress vs bearing strain, lay up sequence (%): (40/20/20). ...................................... 30
30. load vs hole deformation, lay up sequence (%): (25/50/25). ................................................ 30
31. Bearing stress vs bearing strain, lay up sequence (%): (25/50/25). ...................................... 31
32. Load vs hole deformation, lay up sequence (%): (10/80/10)................................................ 31
33. Bearing stress vs bearing strain, lay up sequence (%): (10/80/10). ...................................... 32
34. 2% Offset strengths for Plain Weave Fabric Material. ......................................................... 32
35. Load vs hole deformation, lay up sequence (%): (40/20/40)................................................ 33
36. Bearing stress vs. bearing strain lay up sequence (%): (40/20/40). ...................................... 33
37. Load vs hole deformation, lay up sequence (%): (25/50/25)................................................ 34
38. Bearing stress vs. bearing strain, lay up sequence (%): (25/50/25). ..................................... 34
39. Load vs hole deformation, lay up sequence (%): ( 10/80/10)............................................... 35
40. Bearing Stress Vs Bearing Strain, Lay up sequence (%): (10/80/10).................................... 35
41. 2% Offset strengths for 5- Harness Material. ....................................................................... 36
42. Comparison of 2% Offset strengths for three materials. ...................................................... 37
ix
LIST OF FIGURES (Continued)
43. Load Vs hole deformation (1800F). ...................................................................................... 38
44. Bearing stress vs bearing strain (1800F) ............................................................................... 38
45. Load Vs hole deformation (-650F)........................................................................................ 39
46. Bearing stress vs bearing strain (-650F) ................................................................................ 39
47. 2% Offset strength vs temperature........................................................................................ 40
48. 2% Offset strength vs Bolt-hole clearance............................................................................ 41
49. Clamp up force vs time (35 lbf)............................................................................................ 42
50. Clamp up force vs torque (35 lbf and 70 lbf)........................................................................ 43
51. Load vs hole deformation at different torque levels ............................................................. 43
52. 2 % Offset bearing strength vs torque................................................................................... 44
53. Peak strength vs failure modes .............................................................................................. 45
54. 2% Offset bearing strength vs e/d ratio.................................................................................. 46
55. Specimen with fine mesh around the hole. ............................................................................ 48
56. Specimen along with the pin with a fine mesh around the hole. ........................................... 48
57. Specimen with boundary conditions..................................................................................... 49
58. Pin model. ............................................................................................................................. 51
59. Plate model............................................................................................................................ 51
60. Plate and pin assembly.......................................................................................................... 52
61. Von Mises stresses................................................................................................................ 52
62. Von Mises stress distribution................................................................................................ 53
63. Bearing failure mode............................................................................................................. 53
x
LIST OF FIGURES (Continued)
64. Rigid body displacement....................................................................................................... 54
65. Interface force ....................................................................................................................... 54
66. Load vs displacement (CLC Test) ........................................................................................ 62
67. Stress vs strain (CLC Test) .................................................................................................... 63
68. Load vs displacement (Rail Shear Test) ............................................................................... 63
69. Stress vs strain (CLC Test)………………………………………………………………....64
xi
LIST OF TABLES
Table
Page
1.
Laminate Configuration for Unitape Material ......................................................................16
2.
Laminate Configuration for Plain Weave Fabric Material ....................................................16
3.
Laminate Configuration for 3K 5-Harness Material..............................................................17
4.
Material Type.........................................................................................................................17
5.
Test Matrix.............................................................................................................................18
6.
Bearing Strengths at Different Temperature levels ...............................................................40
7.
Bearing Strengths at different Bolt- ole clearance levels.......................................................41
8.
Clamp up Force Values (lbs) at Three Torque Levels...........................................................42
9.
Bearing Strengths at different Failure Modes........................................................................44
10. Bearing Strengths at Different e/d Ratios ..............................................................................45
11. Bearing Strengths at Different w/d Ratios .............................................................................46
12. Laminate Properties for Carbon Unitape Material.................................................................50
13. Comparison of Experimental with FEM Results ...................................................................55
xii
LIST OF ABBREVATIONS/NOMENCLATURE
ASTM
American Society of Testing and Materials.
CFRP
Carbon Fiber Reinforced Polymer.
GFRP
Glass Fiber Reinforced Polymer.
NIAR
National Institute for Aviation Research.
SL
Slope line
OSL
2% offset slope line
xiii
LIST OF SYMBOLS
d
Fastener or pin Diameter.
D
Hole diameter.
Fbru
Yield Bearing Strength.
h
Thickness range.
e
Edge distance.
K
Hole Factor.
L
Length.
PMax
Maximum Load
w
width.
X
Global x-direction
Y
Global y-direction
Z
Global z-direction
xiv
CHAPTER 1
INTRODUCTION
1.1
Introduction
Composites play an essential part in today’s materials as such they are really good in
structural applications where high strength to weight ratio and stiffness to weight ratio is
required. Other advantages which provide a driving force for the use of composite materials is
because of its good corrosion resistance, wear resistance, electromagnetic transparency,
enhanced fatigue life, thermal acoustical insulation, low thermal expansion and low thermal
conductivity etc. Aircraft and spacecraft industries find the best applications in composites as
they are weight sensitive. Composite materials are also well known for their use in Automobile
and Civil industries.
1.2
Background
The increasing use of composite materials and its popularity has brought out the
importance of research in composite joint strength as the joint decides the real efficiency of the
structure. The major classification of composite joints in aircraft structures are the bonded and
bolted joints. Though bonded joints have some advantages over the bolted joints like low weight
and the provide a very good sealing to the structure but are not effective in transferring high
loads as composite material itself has a less resistant to interlaminar stresses and also bonded
joints are very sensitive to environmental conditions. On the other hand bolted joints are very
effective in transferring high loads and they are suitable for any environmental conditions.
1
1.2.1
Types of Bolted Shear Joints
The bolted shear connections can be differentiated into Single shear and Double shear
joints as shown in figure 1. In double shear joints there is a resistance offered to the acting load
by two cross sectional surfaces. Therefore the bolts in double shear will have a bearing strength
of two times the single shear. The laminate hole gets deformed and there is bearing at the point
where the bolt and hole are in contact.
Figure 1. Classification of joints according to modes of bolt shearing [1]
1.2.2
Modes of Failure in Composite Joints
The principal failure modes of bolted joints are (1) Bearing failure (2) Tension Failure (3)
Shear out failure & (4) Cleavage failure as shown in figure 2.
2
Tension Failure
Shear out Failure
Cleavage Failure
Bearing Failure
Figure 2. Modes of failure [2].
1.3
Issues
Past work done and previous background shows that there is a need for more research in
bolted connections. Early research [3] was mainly conducted on pin bearing tests. There was no
clamp up force considered in many of the cases There are many factors such as unpredictable
failure of joints and also there are some other factors like fiber patterns, lay up sequences, clamp
up effects, bolt hole clearance, size and proportion of joints (e.g. the bolt diameter, laminate
3
width edge distance, thickness) and different environmental conditions (cold, room temperature
and hot) which affect the bearing strength of laminate. There is still a lot of scope to study all the
parameters with different materials and different lay up sequence combination.
1.4
Problem Statement
This thesis is aimed at testing carbon fiber composite laminate specimens at coupon level
and finding their bearing strength in Double shear joints in accordance to the ASTM D5961
procedure ‘A’. The test matrix consisted of three carbon fiber materials with different ways of
fiber reinforcements. They were again classified into three sublevels according to three different
lay up sequence. Ultimately the three materials were further reduced to one material with the
maximum bearing strength for further parametric study.
Figure 3. Set up for Double shear test [6].
4
A simple schematic shows the setup for a double shear test in figure 3. The composite laminate is
clamped between two plates of the fixture and bolted with a certain amount of clamp up force.
‘e’ is edge to center distance, ‘d’ the diameter of hole and ‘w’ the width of specimen.
1.5
Objectives
The objective of this thesis was basically to test the composite laminate following bearing
double shear test and investigate the following:
•
Find the highest bearing strength and select between different carbon composites
using three different lay up sequences.
•
2% and Peak bearing Strength
•
Study the effects of stacking sequence.
•
Study the temperature effects.
•
Study the Clearance effects.
•
Study the effects of e/d and w/d ratio.
•
Study the peak strengths for different failure modes.
•
Building 3D Finite Element Model using LS-Dyna software which can be used in
the future research work for studying various parameters.
5
CHAPTER 2
LITERATURE REVIEW
This chapter discusses the work done in the past to study the bearing strength of
composite material and the various parameters affecting the bearing strength characteristics. The
research
involves
experimental
investigations
as
well
as
finite element
technique
implementations.
2.1
Experimental and FEM Investigation on Bearing Behavior
There were numerous types of experiments carried out in detail to study different
parameters considered in a composite joint and find their influence on the bearing characteristics.
They involve the Bearing single shear tests, Bearing double shear test and Bearing bypass
testing. This thesis aimed at studying the double shear joints and hence the Bearing double shear
test.
Smith and Pascoe [3] conducted experiments for pin bearing strengths of eight different
stacking sequences for carbon fiber reinforced polymer laminates. The bolts used were close fit
pins or cap head bolts. The clamp up torque used was 0-23 Nm. Electron microscope scanning
was used to study the development of damage on the loaded surface of the laminate. Fig shows
the results for the pin bearing strength vs. stacking sequence combinations.
The scanning electron microscope results show that the if the 00 degree plies are on the
outer surface they show a tendency of breaking and splitting of layers while if they are on the
inner surface they rather tend to show delamination between layers. The layers with an angle
difference of 900 show some interlaminar shearing in comparison with the angle difference of
450 where the interlaminar shearing plays a very little role. Considering the Glass fiber
reinforced polymers there is no much significant effect of the stacking sequence.
6
By using a clamp up torque of 20 in-lbs the presence of washers show the damage process is
restricted and the failure occurs at a higher load.[3] The effect of torque is that it constraints the
delamination.
Carbon Fiber composites with standard modulus were tested by A.J Nicola and Fantle
[4].The experiment was conducted to find out the effects of fastener hole clearances. The testing
was done for pin bearing and the 4% Hole Deformation Strength was studied for the various
fastener hole oversize. Double lap shear test was conducted on a laminate with eight plies in
which a quasi-isotropic approach is used (equal number of plies oriented in all directions, +00,
+450,-450, and 900) and the other laminate with all +450 and -450 orientations. There was no
clamp up force used in the test. The test was aimed at studying the varying clearances between
0.000 inches - 0.006 inches
It was observed that the experiment results were higher for quasi-isotropic specimens as
compared to the all 450 orientations laminate. Results show that the 4% Hole Deformation
Strength was reduced by 30 %.
The effects of temperature on the Ultimate Bearing Strength were studied by Counts and
Johnson [5].The experiment was performed on quasi-isotropic laminates made up of 64 plies at
hot ,cold and room temperature conditions The three different temperature levels are 3500 F,-58
0
F and 750 F. Basically the approach used was testing two materials IM7/PETI-5 and IM7/K3B
using the Compression test set up which was particularly used to increase the Bearing stresses
without any increase in the load on the bolts. Previous research has shown that the specimens
have the same Bearing strengths whether the specimens are loaded in tension or compression. It
was observed that the ultimate bearing strength increased with a decrease in temperature from
room temperature and decreased with increase in room temperature. The results show that the
7
bearing failure was much faster for the specimens tested at low temperature even though the
failure load was high. Further these specimens were observed with the help of NDE inspection.
From the inspection it is found that 00 plies cracked and there was buckling of 450 and 900 plies.
Sun and Chang [6] tried to develop a model to study the effect of no clamping force, i.e.,
pure bearing strength and clamping force on the bearing strength of composite material. They
used ABAQUS as Finite Element Modeling tool to develop the three dimensional model. The
assumption they have made in this study is that the material which fails in bearing mode does not
tend to compress under the effect of the constraint offered by the washer on around the hole. The
authors have used a progressive failure approach to build their model. The composite 3d brick
elements are used and the preprocessing and post processing is done with the help of 3d Bolt
software.
The authors [6] in their discussion have shown the criticality of the clamp up force. As
the compressive loading forces are applied on the bearing surface the stiffness of material tends
to decrease but the clamp up force which is applied helps to support the surrounding surface and
the impact part and counter react the compressive loads. Sun and Chang [6] observed that as the
load was increased the clamp up force decreased initially and increased slowly after that and then
rapidly increases as the load was increased.
Camanho and Matthews [7] conducted a joint study which includes experiment on double
lap carbon fiber /epoxy quasi-isotropic laminate and they have built a finite element model using
progressive damage approach. The experimental procedure they followed was similar to pin
bearing test with finger tight conditions for the washers .The finite element model was built
using ABAQUS software. Using the symmetry conditions the authors have used a quarter
models for the analysis. Each ply has been modeled using eight node solid elements. The model
8
is based on displacement formulation. The main objective of their work was to study the
following components such as to predict the modes of failure, the corresponding strengths and
stress analysis.
The results [7] show that this model built was able to accurately predict the failure modes
and the strength values. The experimental and the Finite Element Model show good comparable
results. This model also provides a basis for defining the failure from other design factors such as
the maximum deformation of the hole. Finally the ultimate Bearing strength of the material is
predicted.
Sun and Chang [8] worked on the extension of previous work done by them where they
built a 3D Finite Element model to find the Behavior of Clamp up forces and its effect on the
bearing strength. This study [8] was more detail understanding of the idea behind the effect of
the clamp up force. The study included finding out the effect of clamp up area on the hole
surface, the washer size effect, the size and stiffness of the bolt on the bearing strength of the
material. The results obtained prove the assumption made at the beginning in the previous work
that the material doesn’t tend to compress under the effect of constraint offered by the clamping
force. Figures 4 and 5 show the phenomenon of how the clamping action supports the fibers
from spreading and thus increasing the bearing load.
The authors [8] found that as the clamp up area increases the bearing strength is increased
and the bearing load decreased with initial increase in the clamp up force. There is no much
significant effect of the bolt stiffness on the Bearing strength of the material.
9
Figure 4. Material damaged with and without lateral support (Washer support) [5].
Figure 5. Sand used as an analogy to describe the effect of lateral constraint [7].
10
Jurf and Vinson [9] have performed experimental test as well as built a non linear finite
element model. The authors [9] had a objective to study the relationship between width, edge
distance hole thickness and washer diameter.
The finite element model was built using ADINA (Automatic Dynamic Incremental Nonlinear
Analysis). The results obtained showed consistency with the experimental results. The stress
distribution and local failures are accurately predicted.
J.H. Crews, and V.A Naik [2] studied and compared three failure modes .the material
used for testing was 16 ply quasi-isotropic material T300/5208 graphite epoxy. The authors [2]
have also developed a finite element model to predict the effects of various parameters and find
out the failure modes and the strengths corresponding to them. The finite element model assumes
a nonlinear problem as there is a clearance fit in between the bolt and hole. They observed that
the energy dissipated from the bearing failure was more in comparison with the specimen which
failed in tension failure mode.
Shyprykevich [10] has proposed a test method for the bolted joint in the ML-HDBK-17.
He has proposed a test method for single shear, double shear, bearing bypass and fastener pull
through. For the double shear test the author [10] has proposed w/d and e/d ratio as 6 and 3
respectively and “d” as 0.25 inches. The lay up sequence, Laminate thickness and the bolt
diameter selection has to be selected such that they don’t fail in shear or net tension. A clearance
value of 0.000-0.0025 is acceptable. The bolt used is only in finger tight conditions. The final
failure and hole deformation values are important to characterize the joint.
2.2
Scope of ASTM 5961
The ASTM D5961 [11] is specifically designed to find the bearing response of composite
laminates. The standard covers the procedure ‘A’ Double shear test and Procedure ‘B’ single
11
shear test. The test format is in accordance with MIL-HDBK-17 [10] except for a few
modifications. The schematic shows the fixture design for double shear test. The units are
described in S.I as well as inch -pounds. The schematic shows the fixture for double shear test in
figure 6.
Figure 6. Double Shear test fixture set up [11].
The specimens were torqued at 35 in-lbs and the specimens were tested at test speed of 0.05 in
/min .The testing machine had a facility with one head fixed and the other head holding the
fixture in the grips. The specimen is loaded until maximum load is reached and the test is
stopped when the load drops to 30% of the maximum load.
12
CHAPTER 3
EXPERIMENTAL PROCEDURE
3.1
Specimen Manufacturing
This consists of following main phases.
3.1.1
•
Type of Carbon fiber material selected.
•
Specimen Geometry.
•
Number of plies used and Lay up sequence decision.
•
Laminate cutting (Milling).
•
Coupon Grinding and Drilling.
Type of Carbon Fiber Materials Selected.
Three different types of carbon fiber materials were used throughout the testing
•
Carbon Unidirectional Tape
•
Plain Weave Fabric
•
3K 5-Harness Fabric
The idea behind this was to study and chose the material with higher bearing strength
different types of carbon fiber materials i.e. directions in which the fibers run.
•
Carbon Unidirectional Tape Material
In this type the fibers are aligned in one direction. This direction is said to be the
longitudinal direction. There are no fibers running in any other direction.Figure.7
shows the schematic for unidirectional tape material.
13
Figure 7. Carbon Unidirectional Tape Material.
•
Plain Weave Fabric
This material basically consists of a fabric in which the fibers are aligned
perpendicular to each other i.e. the fibers run in both the major axes the longitudinal
and the transverse direction. The fibers are aligned over and under one fiber at a
time.Figure.8 shows the schematic for fiber alignment in plain weave material.
Figure 8. Fiber orientation in Plain Weave Fabric material.
•
K 5-Harness carbon Fabric
This material has 3000 filaments per yarn and has fibers aligned four under one
and then one under four alternately placed.Figure.9 shows the schematic for the 5Harness material.
14
Figure 9. Fiber orientation in 3K 5-Harness Material.
3.1.2
Specimen Geometry.
Figure 10. Standard specimen for procedure ‘A’ [10].
•
Fastener or pin Diameter d : 0.25+0.000/-0.001
•
Hole diameter D : 0.250+0.001/-0.000
•
Thickness range, h: 0.125-0.208
•
Length, L : 5.5
•
Width, w : 1.5+0.03/1.5-0.03
•
Edge distance, e : 0.75+0.03/0.75-0.03
In accordance to ASTM D5961 as reviewed earlier the specimen geometry for bearing double
shear procedure was followed. Figure 10 shows the standard specimen geometry for the
procedure ‘A’.
15
Further the geometrical parameters were studied .The important parameters such as the
eccentricity (e), hole diameter (D) and the specimen width (w) were altered to study the varying
effects on the bearing strength.
3.1.3
Lay up sequence and Number of plies
The lay up sequence and number of plies were decided on the basis of percentage of 0, 45
& 90 degree orientation of plies as shown in figure 10. They are differentiated as Hard; Quasi
isotropic and soft lay up sequences. Tables 1, 2 and 3 show details of lay up sequence and
number of plies.
TABLE 1
.LAMINATE CONFIGURATION FOR UNITAPE MATERIAL
Lay up %
0°/45°/90°
Ply Stacking Sequence
40/20/40(Hard)
25/50/25(Quasi)
10/80/10(Soft)
[0/90/0/90/45/-45/90/0/90/0]s
[(45/0/-45/90)3]s
[45/-45/90/45/-45/45/-45/0/45/-45]s
Total
Plies
[U]
20
24
20
Nominal
Thickness
(in)
0.120
0.144
0.120
TABLE 2
LAMINATE CONFIGURATION FOR PLAIN WEAVE FABRIC MATERIAL
Lay up %
0°/45°/90°
Ply Stacking Sequence
[0/90/0/90/45/-45/90/0/90/0]s
Total
Plies
[PW]
20
Nominal
Thickness
(in)
0.172
40/20/40
25/50/25
10/80/10
[(45/0/-45/90)3]s
[45/-45/90/45/-45/45/-45/0/45/-45]s
24
20
0.138
0.172
16
TABLE 3
.LAMINATE CONFIGURATION FOR 3K 5-HARNESS MATERIAL
Ply Stacking Sequence
40/20/40
25/50/25
10/80/10
3.1.4
[0/90/0/90/45/-45/90/0/90/0]S
Total
Plies
[PW]
20
Nominal
Thickness
(in)
0.172
24
20
0.138
0.172
[(45/0/-45/90)3]s
[45/-45/90/45/-45/45/-45/0/45/-45]s
Material Types and Test Matrix
The test matrix consisted of three different materials with three different lay up
sequences. Further the unitape material specimens were again classified according to e/d ratio,
w/d ratio, torque levels, clearance and temperature. Tables 4 show the three types of carbon
composites materials used with three different lay-up sequences. Table 5 shows the test matrix
for the material with highest bearing strength i.e. carbon Unitape material for further parametric
studies.
TABLE 4
MATERAL TYPES
Lay up
Material
Sequence
Carbon Unitape
Plain Weave
5-Harness
40/20/40
2
2
2
25/50/25
2
2
2
10/80/10
2
2
2
0/45/90 %
17
TABLE 5
TEST MATRIX
Parameter
Torque(in-lbs)
Clearance (inches)
w/d ratio(in/in)
e/d ratio(in/in)
Temperature(deg F)
3.2
Levels
Number of Specimens
0
2
35
2
70
2
0.001
2
0.003
2
0.006
2
8
2
6
2
4
2
4
2
3
2
2
2
RTD
2
Cold Dry
2
Hot Dry
2
Laminate Cutting
The Laminates were 15”x15” in size. All the specimens were cut along reference axis i .e
along the 0 degree direction of the plies using the milling cutter. The final cut specimen is shown
in figure 11.
18
Figure.11 Final Cut Specimen.
3.3
Test Fixture and Specimen Mounting
The test fixture is as shown below in the figure 12 has a lower and upper locator. The
upper locator is mounted on a spring and is adjustable. The advantage of this is that it can be
moved down to adjust the specimen hole and the fixture hole.
Adjustable
Top locator
Lower
Mounting
Figure 12. Bearing double shear test fixture (Courtesy: NIAR, Structures Lab).
The fixture is clamped in the lower jaws of the MTS machine. The specimen rests on the
adjustable top locator .and it presses the spring along the direction of arrow shown in figure 12
till the specimen hole get aligned with the fixture hole.
19
The bolt is placed in the specimen through the spacers or bushings which are placed inside the
fixture hole on either side of test coupon and clamp the coupon from both the sides. The bolt,
spacer and nut assembly is as show in figure 13.
Figure 13. Fastener and bushings (Courtesy: NIAR, Structures Lab).
The final specimen mounted on the fixture and the fixture gripped in the lower jaws of MTS
machine is show in figure 14
Figure 14. Mounting of specimen on the fixture (Courtesy: NIAR, Structures Lab).
20
3.4
Testing Procedure
The test was carried out on 22 kip Hydraulic MTS machine in accordance with ASTM
D5961. The test speed for this testing was a constant head displacement rate of 0.05 in/min in
accordance with ASTM standards. The grip pressure used to hold the specimen and the fixture is
2500 psi .The specimen is held on one side in the fixture hole with the bolt. The other side of the
specimen is gripped in the wedges.
Figure 15. Extensometer (Courtesy: NIAR, Structures Lab).
To measure the hole deformation uniaxial extensometer was used with accurate
calibration. The maximum displacement measured by the extensometer is -0.21 to +0.21 inches.
Figure 15 shows the extensometer along with pin .With the pin in the distance between two
edges of the extensometers is 1 inch. The extensometer is mounted on the fixture as shown in
figure 16 with the help of rubber bands.
Figure 16 Extensometer mounted on the fixture (Courtesy: Structures Lab).
21
One end of the extensometer is placed on the upper adjustable locator while the other is
placed on the fixed lower mounting .The extensometer is aligned exactly in a vertical position
and care is taken that it is not in a tilted position.
3.5
Data Processing
The data obtained from the Test works the software used for data collection is in a text
file with load, actuator displacement and the extensometer readings. The buffer size used for the
data collection is 2000 points. The final specimen with hole deformation is shown in figure 17.
Figure 17. Tested specimen.
The bearing test is mainly focused in obtaining the peak or Ultimate Bearing strength (UBS) and
the 2% offset bearing strength (2% OBS). The extensometer is dismounted when the hole
deformation reaches a point beyond the 2% offset point The peak bearing Fbru strength is
calculated using the formula
Fbru = PMax/ (K*D*H)…………………………………… (1)
In bearing double shear test the value of load factor ‘K’ is taken as 1.The 2% offset bearing
strength is calculated by offsetting the line drawn along the linear portion of the curve by 2 %.
The point where it intersects the curve gives the 2% offset bearing strength.
Figure 18 shows the ASTM standard bearing stress vs. strain curve. The slope of the
curve gives the chord modulus value. The data obtained from the test file is processed with the
22
specially built macros for the bearing test. Figures 19 and 20 show the results obtained from the
test data reduced using Microsoft Excel.
100
Ultimate Strength
2%
90
80
Offset Strength
Bearing Stress (ksi)
70
60
50
25 to 40
ksi chord
modulus
40
30
Offset Line
20
10
0
0
5
10
15
20
Bearing Strain, %
Figure 18. Stress vs. strain curve according to ASTM standard [11]
Bearing Stress (ksi)
Be aring Stress vs. Bearing Strain
200
180
160
140
120
100
80
60
40
20
0
0.000
SL
OSL
0.050
0.100
Bearing Strain (in/in)
Figure 19. Bearing stress vs. bearing strain.
23
0.150
Load vs. Hole De formation
4000
3500
SL
OSL
Load (lbs)
3000
2500
2000
1500
1000
500
0
0.000
0.010
0.020
0.030
Hol e De formation (i n)
Figure 20. Load vs. hole deformation.
24
0.040
CHAPTER 4
EXPERIMENTAL RESULTS, DISCUSSION, AND COMPUTATIONAL STUDY
The experimental investigation gives a fair idea of how the bearing strength varies by
changing various factors externally and geometrically. According to the aerospace standards a
joint is considered to be failed when it reaches a strength value of 2% hole deformation.
Therefore these experiments were more focused on finding out the 2% offset hole deformation
strength. The peak bearing strength has also been a part of study.
The results for various parameters such as material types, their lay up sequences,
temperature effects, torque effects, bolt-hole clearance, e/d ratio, w/d ratio and failure modes
have been discussed in this chapter.
4.1
Effects of Lay up Sequences and Comparison of Materials
4.1.1
Results for Unidirectional Tape Material
Following are the load vs. hole deformation and bearing stress vs. bearing strain curves results
for the lay up sequence (%): (50/40/10) as shown in figures 21 and 22.
Load vs. Hole De formation
7000
OSL
SL
6000
Load (lbs)
5000
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Hol e De formation (i n)
Figure 21. Load vs. hole deformation, lay up sequence (%): (50/40/10).
25
Be aring Stre ss vs. Be ari ng Strain
Bearing Stress (ksi)
140
OSL
SL
120
100
80
60
40
20
0
0.000
0.020
0.040
0.060
0.080
0.100
0.120
Be aring Strain (in/in)
Figure 22. Bearing stress vs. bearing strain for lay up sequence (%): (50/40/10).
Following are the load vs. hole deformation and bearing stress vs. bearing strain curves results
for the lay up sequence (%):(25/50/25) as shown in figures 23 and 24.
Load vs. Hole De formation
6000
Load (lbs)
5000
OSL
SL
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Hole De formation (in)
Figure 23. Load vs. hole deformation, lay up sequence (%): (25/50/25).
26
Bearing Stress (ksi)
Be aring Stre ss vs. Be aring Strain
200
180
160
140
120
100
80
60
40
20
0
0.000
OSL
SL
0.020
0.040
0.060
0.080
0.100
0.120
Be aring Strain (in/in)
Figure 24. Bearing stress vs. bearing strain for lay up sequence (%): (25/50/25).
Following are the load vs. hole deformation and bearing stress vs. bearing strain curves results
for the lay up sequence (%):(10/80/10) as shown in figures 25 and 26.
Load vs. Hole De formation
6000
Load (lbs)
5000
4000
OSL
SL
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Hole De formation (in)
Figure 25. Load Vs hole deformation for lay up sequence (%): (10/80/10).
27
Bearing Stress (ksi)
Be aring Stress vs. Bearing Strain
200
180
160
140
120
100
80
60
40
20
0
0.000
OSL
SL
0.020
0.040
0.060
0.080
0.100
0.120
Be aring Strain (in/in)
Figure 26. Bearing stress vs. bearing strain for lay up sequence (%): (10/80/10).
The three cases of unitape material are compared here for the 2 % offset Bearing strengths as
seen in figure 27. The Quasi isotropic laminates with maximum thickness show the maximum
value of 2% offset bearing strength. The soft (10/80/10)% has a higher bearing strength in
2% offset Bearing Strength(ksi)
comparison with hard (40/20/40)% lay up sequence.
150
145
140
Lay up (40/20/40)
135
Lay up (25/50/25)
130
Lay up (10/80/10)
125
120
115
Lay up sequence
Figure 27. 2% Offset strengths for Carbon unitape material.
28
4.1.2
Results for Plain Weave Material
The Plain weave material was again tested for three different lay up sequences. The lay
up sequences were decided in the same fashion as they were in the unidirectional tape material.
The laminate consisted of 24 plies and the sequence is symmetric and balanced. Following are
load vs. hole deformation and bearing stress vs. strain graph results for the Lay up sequence (%):
40/20/40 shown in figures 28 and 29.
Load vs. Hole De formation
6000
OSL
SL
Load (lbs)
5000
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
0.025
Hole De formation (in)
Figure 28. Load Vs hole deformation for lay up sequence (%): (40/20/40).
29
Be aring Stre ss vs. Be aring Strain
Bearing Stress (ksi)
140
OSL
SL
120
100
80
60
40
20
0
0.000
0.020
0.040
0.060
0.080
0.100
Be aring Strain (in/in)
Figure 29. Bearing stress vs. bearing strain for lay up sequence (%): (40/20/20).
Following are the load vs. hole deformation and bearing stress vs. bearing strain curves results
for the lay up sequence (%): (25/50/25) as shown in figures 30 and 31.
Load (lbs)
Load vs. Hole De formation
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
0.000
OSL
SL
0.005
0.010
0.015
0.020
0.025
0.030
0.035
Hole De formation (in)
Figure 30. Load vs. hole deformation for lay up sequence (%): (25/50/25).
30
Bearing Stress (ksi)
Be aring Stre ss vs. Be aring Strain
200
180
160
140
120
100
80
60
40
20
0
0.000
OSL
SL
0.020
0.040
0.060
0.080
0.100
0.120
0.140
Be aring Strain (in/in)
Figure 31. Bearing stress vs. bearing strain for lay up sequence (%): (25/50/25).
Following are the load vs. hole deformation and bearing stress vs. bearing strain curves results
for the lay up sequence (%) :(10/80/10) as shown in figures 32 and 33.
Load vs. Hole De formation
6000
OSL
SL
Load (lbs)
5000
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
Hole De formation (in)
Figure 32. Load vs. hole deformation for lay up sequence (%): (10/80/10).
31
Be aring Stre ss vs. Be aring Strain
180
Bearing Stress (ksi)
160
140
OSL
SL
120
100
80
60
40
20
0
0.000
0.020
0.040
0.060
0.080
Be aring Strain (in/in)
Figure 33. Bearing stress vs. bearing strain for lay up sequence (%): (10/80/10).
The results show the same pattern as depicted by the unitape material. The three cases of
Plain Weave Fabric material are compared here for the 2 % offset Bearing strengths as seen in
figure 34. The Quasi isotropic laminates show the maximum value of 2% offset bearing strength.
2 % Offset Bearing Strength(Ksi)
followed by soft (10/80/10)% and hard (40/20/40)% lay up sequence.
120
118
116
114
112
Lay up (40/20/40)
110
Lay up (25/50/25)
108
Lay up (10/80/10)
106
104
102
100
Lay up sequences
Figure 34. 2% Offset strengths for Plain Weave Fabric Material.
32
4.1.3
Results for 5-Harness Material
The 5-harness material is also tested for three combinations of lay up sequences. The
laminates are balanced and symmetric. Following are load vs. hole deformation and bearing
stress vs. strain graph results for the Lay up sequence (%): 40/20/40 shown in figures 35 and 36.
Load vs. Hole De formation
6000
OSL
SL
Load (lbs)
5000
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Hole De formation (in)
Figure 35. Load vs. hole deformation for lay up sequence (%): (40/20/40).
Bearing Stress (ksi)
Be aring Stre ss vs. Be aring Strain
200
180
160
140
120
100
80
60
40
20
0
0.000
OSL
SL
0.020
0.040
0.060
0.080
0.100
0.120
Be aring Strain (in/in)
Figure 36. Bearing stress vs. bearing strain for lay up sequence (%): (40/20/40).
33
Following are the load vs. hole deformation and bearing stress vs. bearing strain curves results
for the lay up sequence (%) :(25/50/25) as shown in figures 37 and 38.
Load vs. Hole De formation
9000
8000
OSL
SL
Load (lbs)
7000
6000
5000
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
0.025
Hole De formation (in)
Figure 37. Load vs. hole deformation for lay up sequence (%): (25/50/25).
Be aring Stre ss vs. Be aring Strain
300
0SL
OS
Bearing Stress (ksi)
250
200
150
100
50
0
0.000
0.020
0.040
0.060
0.080
0.100
Be aring Strain (in/in)
Figure 38. Bearing stress vs. bearing strain for lay up sequence (%): ( 25/50/25).
34
Following are the load vs. hole deformation and bearing stress vs. bearing strain curves results
for the lay up sequence (%) :(10/80/10) as shown in figures 39 and 40.
Load (lbs)
Load vs. Hole De formation
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
0.000
0SL
SL
0.005
0.010
0.015
0.020
Hole De formation (in)
Figure 39. Load vs. hole deformation for lay up sequence (%): (10/80/10).
Be aring Stre ss vs. Be aring Strain
300
OSL
SL
Bearing Stress (ksi)
250
200
150
100
50
0
0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
Be aring Strain (in/in)
Figure 40. Bearing stress vs. bearing strain for lay up sequence (%): (10/80/10).
35
The results show the same pattern as seen previously in plain weave fabric and unitape material.
The three cases are compared here for the 2% offset bearing strengths as seen in figure 41. The
Quasi isotropic laminates with maximum thickness show the maximum value of 2% offset
bearing strength. The soft (10/80/10) % has a higher bearing strength in comparison with hard
2% Offset Bearing Strength
(40/20/40)% lay up sequence.
124
122
120
118
Lay up (40/20/40)
116
114
112
Lay up (25/50/25)
Lay up (10/80/10)
110
108
106
104
Lay up Sequence
Figure 41. 2% Offset strengths for 5- Harness Material.
4.1.4
Comparison of Carbon Unitape, Plain Weave and 5-Harness Material
The three materials with lay up sequence of maximum bearing strengths were compared
It is observed that the Quasi-isotropic material has the maximum strength and the out of the three
materials the unidirectional Tape material shows the maximum bearing strength as seen from the
chart shown in figure 42. On the basis of this result the unidirectional tape material with Quasiisotropic lay up sequence was selected to study the effect of other parameters on the bearing
strength.
36
2% Offset Bearing Strength(ksi)
160
140
120
100
Unitape Material
80
Plain Weave Fabric
60
5-Harness Material
40
20
0
Figure 42. Comparison of 2% Offset strengths for three materials.
Further study was conducted on unidirectional tape material with Quasi-isotropic lay up
sequence to understand the effects of different parameters on the bearing strength.
4.2
Effects of Temperature on the Bearing Strength
4.2.1
Results for Hot Dry Condition
All the tests for the hot dry were conducted at 1800F. The specimens were soaked for
three minutes before testing. The load vs. hole deformation and bearing stress vs. bearing strain
curves shown in figures 43 and 44.
37
Load vs. Hole De formation
4000
3500
0SL
SL
Load (lbs)
3000
2500
2000
1500
1000
500
0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Hole Deformation (in)
Figure 43. Load Vs hole deformation (1800F).
Bearing Stress (ksi)
Be aring Stre ss vs. Be aring Strain
200
180
160
140
120
100
80
60
40
20
0
0.000
0SL
SL
0.020
0.040
0.060
0.080
0.100
Be aring Strain (in/in)
Figure 44. Bearing stress vs. bearing strain (1800F).
38
0.120
4.2.2
Results for cold dry conditions
The tests were conducted at -650F .The specimens were soaked for five minutes before testing.
Figures 45 and 46show the load vs. hole deformation and bearing stress vs. bearing strain curves.
Load vs. Hole De formation
7000
6000
0SL
SL
Load (lbs)
5000
4000
3000
2000
1000
0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
Hole De formation (in)
Figure 45. Load vs. hole deformation (-650F).
Bearing Stress vs. Bearing Strain
180
Bearing Stress (ksi)
160
0SL
SL
140
120
100
80
60
40
20
0
0.000
0.020
0.040
0.060
0.080
0.100
Bearing Strain (in/in)
Figure 46. Bearing stress vs. bearing strain (-650F).
39
0.120
0.140
From Table 7 test results at different temperature levels show that 2% offset bearing
strength increased as the temperature decreased from room temperature 750F to -650F. At the
same time the 2% offset strength decreased with increase in temperature from 750F to 1800F .
The Peak bearing strength showed the same nature. The 2% offset values show less
variation in comparison with the peak bearing strength. Figure 47 shows the strength variation
for different temperature conditions.
TABLE 6
BEARING STRENGTHS AT DIFFERENT TEMPERATURE LEVELS
Peak Bearing Strength
2%offset Bearing Strength
Cold Dry (-650F)
181 ksi
155 ksi
Room Temperature (750F)
166 ksi
144 ksi
Hot Dry(1800F)
115 ksi
102 ksi
2% Offset Bearing Strength(ksi)
Temperature
180
160
140
120
Cold dry
100
Room Temp Dry
80
Hot Dry
60
40
20
0
Temperature
Figure 47. 2% Offset bearing strength vs. temperature.
40
4.3
Effects of Hole Clearance on the Bearing Strength.
The effect of Bolt-Hole clearance was found out at three different levels shown inTable 7
TABLE 7
BEARING STRENGTHS AT DIFFERENT BOLT-HOLE CLEARANCE LEVELS
Bolt-Hole Clearance
Peak Bearing Strength
2%offset Bearing Strength
0.0012 inch
166 ksi
144 ksi
0.0031 inch
161 ksi
133 ksi
0.0062 inch
158 ksi
106 ksi
The specimens were tested at three bolt hole clearance levels. It was observed from figure 48 that
Strength at 2% offset value decreases slowly with increase in Bolt-Hole clearance from 0.0012 to
0.0031 inches and decreases rapidly as the clearance value is further increased from 0.0031 to
2% Offset Bearing Strength(ksi)
0.0062 inches. The Peak Bearing strength values show comparatively less variation
160
140
120
100
0.0012 inch clearance
80
0.0031 inch clearance
60
0.0062 inch clearance
40
20
0
Bolt-Hole Clearance (inches)
Figure 48. 2% Offset bearing strength vs. bolt-hole clearance
41
4.4
Effects of Torque on the Bearing Strength.
Initially clamp up force generated was calculated using a load cell placed in between the
washer and the nut. Figure 49 shows the force value obtained at 35lbf torque.
35 lbf torque
600
Force( lbs)
500
400
300
35 lbf torque
200
100
0
0
50
100
150
200
250
300
tim e (se c)
Figure 459. Clamp up force vs. time (35 in-lbs).
Table 8 shows the clamp up increases gradually with the increasing torque values. Figure.50
shows the clamp up Forces vs. Torque generated at different torque levels.
TABLE 8
CLAMP UP FORCE VALUES (LBS) AT THREE TORQUE LEVELS
Torque
Clamp up Force Generated
0 in-lbs
0 lbs
35 in-lbs
535 lbs
70 in-lbs
990 lbs
42
1200
Clamp up Force(lbs)
1000
800
35 in-lbs
600
70 in-lbs
400
200
0
Torque
Figure 50. Clamp up force vs. torque (35 in-lbs and 70 in-lbs).
Three different torque levels were used to study the effect of torque on bearing strength. Figure
51 shows the peak load vs. the hole deformation at different torque values.
7000
6000
Load(lb)
5000
0 in-lbs Torque
4000
35 in-lbs Torque
3000
70 in-lbs Torque
2000
1000
0
0
0.01
0.02
0.03
0.04
Hole Deformation (in)
Figure 51. Load vs. hole deformation at different torque levels.
43
The results from Figure 52 show that at 0 in-lbs torque level the strength which is pure
bearing strength has the least value and the strength increases rapidly from 0 in-lbs torque to 35
in-lbs torque and the rate of increase further slows down when the torque is doubled.
2 %Offset Bearing Strength(ksi)
150
145
140
135
0 in-lbs Torque
130
35 in-lbs Torque
125
70 in-lbs Torque
120
115
110
105
Figure 52. 2% Offset bearing strength vs. torque.
4.5
Comparison of Strengths at Different Failure Modes
The aim of this investigation was to find out the maximum strength of the joint at
different failure modes as discussed in chapter 1 shown in figure 2. Table 9 shows the maximum
load and peak strength at three different failure modes i.e. Bearing, Tensile and Shear out Mode.
TABLE 9
BEARING STRENGTHS AT DIFFERENT FAILURE MODES
Failure Mode
e/d Ratio
w/d Ratio
Max Load
Peak Strength
Bearing
6
3
5790 lbs
166 ksi
Tensile
3
3
5500 lbs
158 ksi
Shear out
6
1.5
4438 lbs
127 ksi
44
It is observed from figure 53 that the maximum load sustained was at bearing failure
mode followed by the Tensile and shear out failures.
180
160
Peak Strength
140
120
Bearing
100
Tension
80
Shear Out
60
40
20
0
Figure 53. Peak strength vs. failure modes.
4.6
Effect of e/d Ratio on the Bearing Strength
The three different levels of e/d ratio were chosen as shown in table 10 to find their effect
on the bearing strength of material.
TABLE 10
BEARING STRENGTHS AT DIFFERENT E/D RATIOS
e/d Ratio
Peak Bearing Strength
2% Offset Bearing Strength
4
174 ksi
151 ksi
3
166 ksi
144 ksi
2
160 ksi
142 ksi
The results show that the increasing e/d ratio affects the bearing strength of Material. The
Bearing Strength increases with the increase in e/d ratio as shown in figure 54.
45
2 % Offset Bearing Strength
154
152
150
148
e/d=4
146
e/d=3
144
e/d=2
142
140
138
136
Figure 54. 2% Offset bearing strength vs. e/d ratio.
4.7
Effect of w/d Ratio
The three different levels of w/d ratio were chosen to find their effect on the bearing
strength of material. Table 11 shows the values of bearing strength at three different w/d ratios.
The peak bearing strength as well as the 2% offset bearing strength increases with the increase in
w/d ratio.
TABLE 11
BEARING STRENGTHS AT DIFFERENT W/D RATIOS
w/d Ratio
Peak Bearing Strength
2% Offset Bearing Strength
8
190 ksi
168 ksi
6
166 ksi
144 ksi
4
99 ksi
98 ksi
46
4.8
Computational Study
4.8.1
Introduction
An understanding of structure is important to study the behavior of the bearing
phenomenon. The aim of computational study is to build a model which can be used in the future
course to study a series of different combination of parameters without going for experimental
study for each case. There are some models developed in the past by some authors [6] &[9] using
different software packages like ABAQUS and ADINA.Prerforming this work in LS-Dyna is
again a future contribution to the literature.
A non linear finite element analysis was carried out to develop a three dimensional model
for simulating the bearing double shear test using LS Dyna, a code developed by Livermore
Software Technologies. This is a general purpose code used for analyzing large deformation and
dynamic response of structures. It is also capable of doing implicit analysis.
4.8.2
Approach
The bearing double shear is a static test with a test speed of 0.05 in/min. The test runs for
1-2 minutes with specimen pulled in tension. To simulate this condition an explicit analysis
could consume lot of computational time. To avoid this computational time an implicit approach
was used to simulate this process. The time steps in implicit code are 100 to 1000 times lesser
than explicit ones.
4.8.3
FE Model using MSC PATRAN
The three dimensional model was developed using PATRAN as a preprocessing
software. The approach used was to draw the 2D model for the specimen and generating surfaces
which can be extruded to create the 3D model. The bolt was modeled as a pin with a clearance of
0.001 inches between the bolt and the hole.
47
4.8.4
Meshing
The meshing technique followed was to mesh the area around the hole with a fine mesh.
Eight node brick elements were used to mesh the whole model. A coarse mesh was used for the
bolt while the surface of the hole in contact with the bolt had a fine mesh. This was important to
develop a better contact between the elements of the bolt and hole. Figure 54 and 55 show the
meshed specimen.
Figure 55. Specimen with fine mesh around the hole.
Figure 56. Specimen along with the pin with a fine mesh around the hole.
48
4.8.5
Boundary Conditions
One end of the plate was constrained in all directions i.e. translational x, y and z while the
other end on the hole side is constrained in translational y and z direction. The pin is constrained
in global y and z direction and the rotational x, y and z directions. The bolt is free to move in
translational x-direction. Displacement of 0.04 inches was applied at the edge on the hole side of
the specimen.
Y
X
Figure 57. Specimen with boundary conditions.
4.8.6
Material Properties
The solid model approach required the laminate properties instead of ply properties and
the later are readily available. To obtain the laminate strength properties, experiments were
conducted to find out the in plane compressive, tensile and shear strengths. The stiffness
properties can be found out with the help of dedicated software built on the basis of composite
theory. “Class” is one example of this software. Table 12 shows the properties used in the finite
element model.
49
A detailed discussion of these tests is done further in the Appendix A with appropriates
charts showing the results. During the whole experiment the tests were conducted in accordance
with the ASTM standards.
TABLE 12
LAMINATE PROPERTIES FOR CARBON UNITAPE MATERIAL
4.8.7
Direction
Ultimate Strength
Modulus
Tension
117 ksi
6.7 Msi
Compression
91 ksi
6.5 Msi
Shear
64 ksi
2.45 Msi
Material Model
The input file for the Ls-Dyna is in the form of text file. The detailed input format of key
file is discussed in the appendix B. Some of the important cards used in LS-Dyna code
include:
•
MAT_22:MAT_DAMAGE_COMPOSITE is the material model used in
composite modeling. A solid modeling approach is used in this model.
•
MAT_20: MAT_RIGID is used for modeling of bolt.
•
BOUNDARY_PRESCRIBED_MOTION_RIGID is used to give displacement
motion to the bolt.
•
CONTACT_AUTOMATIC_SURFACE_TO_SURFACE is used to define the
contact between the hole and pin.
50
4.8.8
Results
The post processing was done using LS-Post. Figure 57 shows meshed model for pin.
Figure 58 and 59 show the meshed model and the plate-pin assembly respectively.
Figures 61 and 62 show the Von mises stress distribution for the plate. It is clearly seen.
from figure 63 that the hole deforms with a bearing failure mode. Figure 64 shows
displacement vs. time while figure 65 shows Force vs. Time.
Figure 58. Pin model.
Figure 59. Plate model.
51
Figure 60. Plate and pin assembly.
Figure 61. Von Mises stresses.
52
Figure 62. Von Mises stress distribution.
Figure 63. Bearing failure mode.
The result in Figure 64 how the pin displacement varies with respect to time. The displacement is
given with a constant rate of 0.05 in/min. Figure 65 shows the force variation with respect to
time. The force increases till it reaches a peak and there is a hole deformation process and after
that peak the force keeps on dropping without any further rise and never reaches the peak again.
53
Figure 64. Rigid body displacement.
Figure 65. Interface force
54
Table 13 shows the comparison of experimental vs. finite element analysis results for the
peak loads attained. The finite element results show that the results for the 2 % offset bearing
strength show a very close approximation than the peak load values. The values for peak load
showed a 25 % variation while for 2 % offset bearing strength showed a 16 % variation
TABLE 13
COMPARISON OF EXPERIMENTAL WITH FEM RESULTS
Finite Element
Experimental
Variation
Analysis
Peak Load
4230 lbs
3153 lbs
25%
2 % Offset bearing strength
98 ksi
84 ksi
16%
55
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
The most important contribution of this experiment was that the whole test data was
obtained at a torque value of 35 in-lbs. The strength of bolted joints in composite materials is
sensitive to material properties as well as geometric parameters. A very important factor in
understanding the joints in composite materials is to understand the heterogeneous nature of this
material and the failure process.
The material and lay up sequence comparison showed that among the three materials and
three lay up sequences the carbon unitape material and Quasi-isotropic lay up sequence showed
maximum bearing strength. The clearance effect has shown that with an increase in the bolt hole
clearance from 0.001 to 0.006 inches the strength decreased by 22-25%. The temperature effects
show that the material when compared with the room temperature experienced a 23 % decrease
in ultimate bearing strength when tested at hot condition (180 0F) whereas it showed an 8 %
increased strength when tested at cold temperature (-650F). The effect of torque showed that the
strength rapidly increased from 0 in-lbs torque to 35 in-lbs and further increased slowly when the
torque value is doubled. The bearing strength value increased with the increasing values of e/d
and w/d ratios. The failure modes showed that the bearing failure mode has maximum failure
strength followed by tensile and shear out failure modes.
The computational model was a source of motivation for future research which can be
done to study various material and parameter combination. Here specifically implicit approach
was used and the model was validated for the quasi isotropic material with pin bearing condition
simulated to get the pure bearing strength. The finite element analysis results show a variation of
56
25% for the peak load while the 2% offset strength shows a 16% variation in comparison with
experimental results. Due to the presence of some variation in finite element analysis results it
was concluded that further results can be approximately found out by finite element analysis to
study the effect of other parameters on the bearing strength of composite material .This can be
done by further working on the finite element analysis work to get more closer results to actual
experimental results.
Finally to conclude the results for the various parameters were in well accordance with
the literature and followed almost similar behavior patterns.
5.2
Recommendations
The experiments were carried out on carbon fiber reinforced polymer material (CFRP).
Similar experiments can be carried out on glass fiber reinforced polymer material (GFRP). The
test was confined to double shear type. Similarly, this test can be performed for Single shear
joints with single and double fasteners. The material can also be tested for different fasteners
diameters.
Currently there is some interesting research going on in high speed testing of composite
material. The bearing test can be performed at high speeds and the effect of high speed testing
the on bearing strength of the material can be studied.
57
REFERENCES
58
REFERENCES
[1]
Jones, R.M., “Mechanics of Composite Materials,” Taylor and Francis, Philadelphia,
1999.
[2]
Crews, J.H, and Naik, V.A, “Failure Analysis of a Graphite /Epoxy Laminate subjected
to Bolt Bearing Loads,” NASA Technical Memorandum 86297, August 1984.
[3]
Smith, P., and Pascoe, J., “The Effect of Stacking Sequence on the Bearing Strengths of
Quasi-Isotropic Composite Laminates,” Composite Structures, Vol. 6, pp. 1-20, 1996.
[4]
DiNicola, A.J., and Fantle, S.C., “Bearing Strength Behavior of Clearance Fit Fastener
Holes in Toughened Graphite/Epoxy Laminates,” Composite Materials: Testing and
Design (Eleventh Volume), American Society for Testing and Materials (ASTM),
Philadelphia, pp.220-237, 1993.
[5]
Counts, W.S., “Temperature Effects on Ultimate Bearing Strength of Polymeric
composite Joints,” Journal of Composite Technology & Research, Vol.24, No.1, pp.1723, January 2002.
[6]
Sun, H.T., and Chang, F.K., “The Response of composite joints with bolt Clamping
Loads, Part 1 Model Development,” Journal of Composite Materials, Vol. 36, No.l, 2002.
[7]
Camanho, P.P., and Matthews, F.L., “A Progressive Damage Model for Mechanically
Fastened Joints in Composites Laminates,” Journal of Composite Materials, 2000.
[8]
Sun, H.T., and Chang, F.K, “The response of Composite Joints with Bolt Clamping
Loads, Part 2: Model Verification,” Journal of Composite Materials, Vol.36, No.l, 2002.
[9]
Jurf, R.A., and Vinson, J., “Failure Analysis of Bolted Joints in Composite Laminates,”
Composite Materials Testing and Design, ASTM, Philadelphia, Vol.9, pp. 165-190, 1990.
[10]
Schyprykevich, P., “Characterization of Bolted Joint Behavior: MIL-HDBK-17 Vol.17,
59
No.3, pp.260-270, July 1995. Accomplishment at Standardization,” Journal of Composite
Technology and Research.
[11]
Anonymous, “Standard Test Method for Bearing Response of Polymer Matrix composite
Laminates,” ASTM Standards, Designation: D5961/D5961M-05.
[12]
Hodgkinson, J.M., “Mechanical Testing of Advanced Fiber Composites,” Woodhead
Publishing Limited, 2000.
[13]
Barbero, E.J., “Introduction to Composite Material Design,” Taylor and Francis,
November 1998.
[14]
Bhamare, V.V., “Transverse Impact Characteristics of Adhesively Bonded Composite
Single Lap Joint,” Masters Thesis, Wichita State University, 2006.
[15]
Maker, B.N., “Implicit Analysis with LS-Dyna: Version 970,” Livermore Software
Technology Corporation (LSTC), May 2004.
[16]
Goteti, V., “Parametric Modeling of Bolted Joints between components made of
Particulate Composites,” Masters Thesis, West Virginia University, Morgantown, WV,
2003.
60
APPENDICES
61
APPENDIX A
Material Testing For Laminate Properties
The laminate properties were not directly available to input in the Finite element model.
While finding these properties different test were conducted in accordance with ASTM
standards. The tensile Modulus and tensile strength was found by testing the laminate for tension
test using ASTM D3039 standard. Strain gauges were used a means of measuring the strain.
The compression modulus and strength was found with the CLC test (Combined Loading
Compression Test) according to ASTM D695The shear Modulus and shear strength was found
with rail shear test in accordance with ASTM 7979.Following are some results for the Tests
discussed as shown in figures66-69.
Load vs. Displacement
7000
Loa d [ l b]
6000
5000
4000
3000
2000
1000
0
0.000
0.020
0.040
0.060
0.080
0.100
D i spl ac em e nt [ i n]
Figure 66. Load vs displacement (CLC Test)
62
0.120
Stre ss vs. Strain
25000
y = 6.50x + 257.46
R2 = 1.00
Stress [psi]
20000
15000
10000
5000
0
0
1000
2000
3000
4000
Strain [m icro in/in]
Figure 67. Stress vs strain (CLC Test)
Loa d vs. Displa c e me nt
12000
Load [lb]
10000
8000
6000
4000
2000
0
0.00
0.02
0.04
0.06
0.08
0.10
D i spl a c e m e nt [ i n]
Figure 68. Load vs displacement (Rail Shear Test)
63
0.12
Stre ss vs. Stra in
16000
y = 2.4501x + 276.17
R2 = 0.9999
14000
Stress [psi]
12000
10000
8000
6000
4000
2000
0
0
2000
4000
6000
Strain [m icro in/in]
Figure 69. Stress vs strain (Rail Shear Test)
64
8000
APPENDIX B
LS-Dyna Key file Input Format
*KEYWORD
*TITLE
*CONTROL_BULK_VISCOSITY
1.5
0.06
*CONTROL_CONTACT
0.1
0
2
0
1
0
0
10
0
4
1
1
0
*CONTROL_ENERGY
2
*CONTROL_HOURGLASS
4
0.05
*CONTROL_OUTPUT
0
3
0
0
0
0
*CONTROL_SHELL
20
2
-1
0
2
2
1
*CONTROL_TERMINATION
5
0
0
0
0
*CONTROL_IMPLICIT_GENERAL
$ imflag
1
dt0
iefs nstepsb
0.01
0
0
igso
0
*CONTROL_IMPLICIT_NONLINEAR
$ nlsolvr ilimit maxref
2
$
0
0
0.0
dctol
ectol
0.0
0
rctol
0
dnorm divflag inistif nlprint
0
0
0
0
$
*CONTROL_IMPLICIT_LINEAR
65
lstol
$ lsolvr prntflg negeig
0
0
0
$
*CONTROL_IMPLICIT_AUTO
$
iauto
0
iteopt
0
itewin
0
dtmin
0.0
dtmax
0.0
$
*DATABASE_BINARY_D3PLOT
0.5
*DATABASE_EXTENT_BINARY
0
0
0
1
1
1
0
0
0
1
2
1
1
1
*DATABASE_RBDOUT
0.5
*DATABASE_RCFORC
0.5
$ Material : composite
$
$ Material : titanium
*MAT_RIGID
3 0.0004211 1.59e+07
1
0
0.25
0
0
0
0
*CONTACT_AUTOMATIC_SURFACE_TO_SURFACE
3,1,3,3
0.05,0.05,0.0,0.0,0.0,1
*CONTACT_AUTOMATIC_SURFACE_TO_SURFACE
3,2,3,3
0.05,0.05,0.0,0.0,0.0,1
66
$
$ This is the unit load curve
$
*DEFINE_CURVE
1,,5,0.05
0,0
1,1
$
*BOUNDARY_PRESCRIBED_MOTION_RIGID
1
1
2
1
67