Test Methods for Fiber
Reinforced Polymer
(FRP) Composites
John J. “Jack” Lesko
Department of Engineering Science & Mechanics
jlesko@vt.edu (540) 231-5259
Introduction to Polymeric Adhesives and Composites Short Course
Copyright, 2004, J J Lesko, ESM, Virginia Tech,
Blacksburg, Virginia. All rights reserved.
1
Partial List of
Standardization Groups
_
_
_
_
USA
– American Society for Testing and Materials (ASTM)
– MIL-HDBK-17 Committee (http://www.mil17.org/)
– Suppliers of Advanced Composite Materials Association
(SACMA)
Europe
– Deutsches Institut Fur Normung (DIN)
– Association Francaise de Normalization (AFNOR)
– British Standards Institute (BSI)
East
– Japanese Industrial Standards
International
– International Organization for Standardization (ISO)
2
ASTM Standard Test
Methods*
Definitions
D3878--Definitions of Terms Relating to High-Modulus Reinforcing Fibers and
Their Composites
Fiber/Matrix Prepreg
C613--Test Method for Resin Content of Carbon and Graphite Prepregs by
Solvent Extraction
D3379--Test Method for Tensile Strength and Young’s Modulus for High Modulus
Single-Filament Materials
D3529--Test Method for Resin Solids Content of Carbon Fiber-Epoxy Prepreg
D3530--Test Method for Volatiles Content of Carbon Fiber-Epoxy Prepreg
D3531--Test Method for Resin Flow of Carbon Fiber-Epoxy Prepreg
D3532--Test Method for Gel Time of Carbon Fiber-Epoxy Prepreg
D3544--Guide for Reporting Test Methods and Results on High Modulus Fibers
D3800--Test Method for Density of High-Modulus Fibers
D4102--Test Method for Thermal Oxidative Resistance of Carbon Fibers
* Found in Vol. 15.03 of ASTM Annual Book of Standards
3
Key to Successful FRP Testing
4
Damage &
Strength of
Composites
5
Composite Damage Modes
C
A

B
Tensile Failure
Compression Failure
6
Matrix Cracking
& Delamination
Tensile
Strength
7
The tensile strength of a composite is controlled by the
interface/phase, influencing the local stress concentrations
and the size of the “ineffective length - ”....
f

f
f


0
1
8
2
3
4
5
6
7
8
Tensile Stress Concentration
at a Fiber Break
9
Tensile Strength Models
A very crude approximation of tensile strength
from the Rule of Mixtures
f
X t  X t Vf  X t
m
1  V f 
More sophisticated models include
Batdorf, S. B. “Tensile strength of unidirectional reinforced composites--I,” Journal of
Reinforced Plastics and Composites, Volume 1 (1982), pp.153-176.
Gao, Z. and Reifsnider, K. L. “Micromechanics of tensile strength in composite systems,”
Composite Materials: Fatigue and Fracture, Fourth Volume, ASTM STP 1156, W. W.
Stinchcomb and N. E. Ashbaugh, Eds., ASTM, Philadelphia, (1993), pp. 453-470.
Reifsnider, K., Iyengar, N., Case, S. and Xu, Y. “Kinetic Methods for Durability and Damage
Tolerance Design of Composite Components,” Keynote Address, Conference on Composite
Materials, Japan Society for Mechanical Engineers, June 26, 1995, Tokyo.
10
Stresses Around Filler
Particles
Monette, et al, J. Appl. Physics,
75 (3), 1994, 1442-1455.
11
Pultrusion Fabrication Flaw
Microcrack 1.2mm long by
.25mm wide
90º Tow
0º Tow
“As received” pultruded cross ply laminate (E-glass/Derakane 441-400)
12
Transverse Strength Models

m
Y t  Y t 1 

Yt  E 2
Yt  E 2
Yt
E2
Yt
E2
m
m
m
m

m

E2
Vf  Vf 1 
f

E2


 E m


2
 1   1
 rs 
f

  E 2


 E m


2
 1   1
 r h 
f

  E 2





 
rs 
rh 
4
2
Vf

Vf
3

Gibson, R. F. Principles of Composite Material Mechanics, McGraw Hill, New York (1994)
13
0° and Laminate Tension
Testing of Composites
Concerns in the Assessment of Modulus and Strength
Uniformity of stress state
• Failure
in the gage section (common problem between test specimens)
• Failure modes
• Material misalignment (1° misalignment can yield a 30% strength
reduction)
• Specimens with cross reinforcement
Gripping
• Transition region concentration (common problem in all specimens)
• Tab geometry
• Grip region geometry
• Grip pressure
14
0° and Laminate Tension
Testing of Composites
Specimen Types Used in Tensile Testing
 Straight-Sided Coupon--MRG Preferred
 With and without tabs
 ASTM D638 Type I “Dogbone” Specimen
 Linear Tapered “Bowtie” Specimen
 30% lower 0° strength compared to straight-sided specimen
 10% lower 0° strength compared to dogbone specimen
 Woven cross-ply strengths  dogbone or tabbed specimen
 Streamline Specimen
 Comparable to straight-sided for 0°
15
Straight-Sided Specimen
Advantages: No specimen tapering required; better results with
cross-reinforced materials
Disadvantages: Tabbing required; tab s-concentration; tight
tolerances in thickness
16
Typical Failure Modes in
Straight-Sided Coupons
(Acceptable & common in
unidirectional specimens)
(Acceptable
& common
in 90° or 90°
dominated layups)
17
(May be found in (Unacceptable)
crossply layups;
unacceptable)
Typical Tab Failures in
Straight-Sided Coupons
18
ASTM D 638 Type I
“Dogbone” Specimen
Advantages: No tabbing required; load introduction less of an issue
Disadvantages: Careful specimen machining required; not suitable
for unidirectional material
19
Streamline Specimen
Advantages: No tabbing required; load introduction less of an
issue; comparable to straight-sided
Disadvantages: Careful specimen machining required; not suitable
for unidirectional material; large specimen (12” [0°/90°]s; 24” [0°]) in
order to keep the shear stresses low at the transition region
20
Linear-Taped “Bowtie”
Specimen
Advantages: No tabbing required; load introduction less of an issue
Disadvantages: Careful specimen machining required; not suitable
for unidirectional material; large specimen
21
Effect of Misalignment in
Unidirectional Specimens
22
Compression
Strength
23
Compression
f
L 4
L = 41.8 mm
24
E1 I f
k
Compression Strength
An approximation of crushing strength from the
Rule of Mixtures
f
Compression Strength
X c  X c Vf  X c
m
1  V f 
Crushing
Buckling
Slenderness ratio (r/L)
25
Compression Strength
f
M a trix
L
4
E1 If
k 
k

S
F ib e r

S
c
L
1   L 0
m
c
E 
f
E2 E2
f
E 2 Vf  E 2
f
m
   12 V f   12

O rig in of B u cklin g F ib e r's S in e W a ve
 Eb
1  V f 
m
1  V f 
s
L
m
2
 E 1 f V f  E 1 m 1  V f    f rf 2  3

G 12
kL
m
m
Xc  
 G 12  2 3  2 G 12  
sin  2  
 E 1
f
2
2
12 L
E1
rf 



Xu, Y. and Reifsnider, K. L. “Micromechanical modeling of composite compressive strength,” Journal of
Composite Materials, Vol. 27 (6), (1993), pp. 572-588.
26
Compression Strength






Ramberg-Osgood shear response

n 1



3  
 
1  
 
G 
7   y  



  

fib e r

k ink
band
T
Xc 



1
 3  n    y
1  n 
 7   n  1
L
L


G
n 1




n
T
Fleck, N. A. and Budiansky, B. “Compressive failure of fibre composites due to
microbuckling,” IUTAM Symposium, Troy, New York, May 29-June 1, (1990), pp. 235-273.
27
Compression Testing of
Composites
Concerns in the Assessment of Modulus and Strength
Uniformity of stress state
 End loaded
 Shear loaded
 Gage section dimensions
 Sandwich beam
Gripping
 Stress concentration
 Tab geometry
 Tabbing material
Alignment
 Buckling
 Failure modes
 Specimen machining tolerance
 Fixture characteristics
28
Compression Testing of
Composites
Classes of Test Methods
Shear Loaded - Preferred
 Celanese & Wyoming modified Celanese
 IITRI (Illinois Institute of Technology Research Institute) & Wyoming
modified IITRI
End Loaded
 Boeing Compression ASTM D695 & Wyoming modified D695
 Wyoming End Loaded Side Supported (ELSS)
 RAE (Royal Aircraft Establishment)
 Short Block Compression
Sandwich Beam
 ASTM D3410, Method C--Flexure
 Axially Loaded Sandwich Column
29
IITRI - ASTM D3410
Advantages: Alignment; high data averages and low scatter; large specimens possible
Disadvantages: Expense; specimen tabbing & machining critical; tab s-concentration
30
Celanese: ASTM D3410
Advantages: Alignment; high data averages and low scatter; long-standing test fixture
Disadvantages: Specimen tabbing & machining critical; tab s-concentration; sensitive to fixture
accuracy; expense (latter two concerns addressed in Wyoming-modified)
31
Boeing Modified ASTM D695
Advantages: Small, thin specimen; reduced material; highly supported against buckling
Disadvantages: No s-e curve; untabbed for modulus; tabbed for strength; tab sconcentration
32
Wyoming End Loaded Side
Supported (ELSS)
Advantages: No tabbing required; simple fixture; inexpensive; simple alignment; some shear loading
Disadvantages: End crushing for highly orthotropic specimens; support s-concentration; specimen
tolerances critical
33
Sandwich Beam Flexure - ASTM
D3410 (ASTM C 393)
Advantages: Simple fixture; reliable results with proper specimen (core) design
Disadvantages: Large specimens (materials expense); failure must occur in compressive
face sheet
34
Axially Loaded Sandwich Column
Advantages: Simple fixture; simple data analysis; standard compression fixture
Disadvantages: Expense in fabricating sandwich panel; end crushing; end s-concentration
35
Other Compression Tests

Block Compression Test



Advantages: Simple untabbed specimen; simple
fixture; inexpensive
Disadvantages: Thick specimen required; end
crushing; end -concentration; misalignment
sensitive
RAE Compression Test


Advantages: No tabbing required; simple fixture;
inexpensive; shear and end loading
Disadvantages: Not widely used; tolerance
sensitive for thickness taper; misalignment upon
debonding; specimen buckling
36
Shear
Strength
37
Shear Strength Models

m
SS  SS 1 

SS  G 12
SS  G 12
SS

m
G 12
SS
G 12
m
m
m
m

G 12
Vf  Vf 1 
f

G
12





 
 G m


12
 1   1
 rs 
f


  G 12

 G m


12


 1  1
 rh 
f

  G 12


rs 
rh 
4
2
Vf

Vf
3

Gibson, R. F. Principles of Composite Material Mechanics, McGraw Hill, New York (1994)
38
Shear Testing of
Composites
Concerns in the Assessment of Modulus and Strength
In-plane: 12
Interlaminar: 13
Uniformity of Stress State




Failure in the gage section (common problem between test
specimens)
Failure modes: buckling out of plane; scissoring
Material alignment
Uniform shear
Load Introduction



Transition region concentration (common problem in all
specimens)
Loading arrangement and assessment of results
Grip region geometry
39
Shear Testing of
Composites
In-plane: 12






Iosipescu ASTM D5379 (Preferred for shear strength)
(45)ns Tension ASTM D3518 (Preferred for modulus)
Off-axis Tension
Rail Shear ASTM D4255
Torsion of bar (circular/rectangular)
Torsion of a tube ASTM D5448
Interlaminar: 13


Short Beam Shear ASTM D2344
Iosipescu ASTM D5379 (experimental)

bonded laminates
40
Shear Directions
3
S13
S12
S23
2
S12
S23
1
S13
41
Material Coordinate
System
1, 2, 3
Iosipescu Shear Test ASTM D5379
Advantages: Excellent shear strength measurement; small specimen; 0°, 90°, [0°/90°]ns layups
Disadvantages: Tight tolerances on specimen; alignment; twist failure; quality fixture required;
expense
42
(45)ns Tension ASTM D3518
Advantages: Simple; uniform stress state; no fixture; damage growth representative of laminates
Disadvantages: Tabbing; alignment; strength dependent on layup; scissoring; t12 and t13 failure; edge
delamination; s-concentration due to tabs
43
Short Beam Shear ASTM D2344
Advantages: Simple test and fixture; small specimen
Disadvantages: Load introduction; no strain measurement; no modulus measurement;
improper assumption of parabolic stress distribution; mixed mode failure
44
Stress Distribution in a Short Beam
Shear Specimen
Elasticity Solution
Beam Theory
45
Interlaminar
Fracture
46
Double Cantilever Beam (DCB)
Test Data – ASTM D5528
700
P
Load, P(N)
600
a1
a2
a3
500
400
D
a
300
an
200
100
0
0
0.01
0.02
Displacement, m
47
0.03
0.04
DCB Data Reduction: Modified
Beam Theory
Cube Root of Compliance
C 1/3 (J/m2)1/3
GI 
0.07
3P
2
m (a  x)
3
2
b=width
2b
P
y = 0.429941x + 0.001997
R2 = 0.9997
0.06
D
0.05
0.04
•Find C:
0.03
•Find fit:
0.01
C 1 / 3  m(a  x )
m
1
0
0
x
a
•Plot C1/3 vs a
0.02
-0.05
C  D P
0.05
0.1
Crack Length, a [m]
48
0.15
0.2
DCB Data Reduction: Compliance
Calibration Method
GI 
m2PD
b=width
2 ba
P
D
log(C)
a
•Find C: C  D P
m2
1
•Plot log(C) vs log(a)
•Find the slope m2
log(a)
49
DCB Data Reduction: Compliance
Calibration Method
GI 
3PC
2/3
2 m 3 bh
b=width
P
D
a/h
a
•Find C:
m3
C 
•Plot a/h vs C1/3
1
•Find the slope
m3
C1/3
50
D
P
Edge Notch Flexure (ENF)
P
1/C
PMax
P95%
Load, P
Pnl
a
L
L
Mid-span Displacement, D
b=width
E f lex 
95% of 1/C
L
3
4 bCh
3
Of the uncracked region
a corr
 8 E f lex Cbh
 
3

51
3



1/3
2
G II 
9 C a corr Pi
2 b (2 L  3 a corr )
3
3
QUESTIONS
C
A
B
52