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Test Methods for Fiber Reinforced Polymer (FRP) Composites

John J. “Jack” LeskoDepartment 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.

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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)

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ASTM Standard Test Methods*

DefinitionsD3878--Definitions of Terms Relating to High-Modulus Reinforcing Fibers and Their

Composites

Fiber/Matrix PrepregC613--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

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Key to Successful FRP Testing

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Damage & Strength of Composites

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Composite Damage Modes

A

B

C

Tensile Failure Compression FailureMatrix Cracking & Delamination

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Tensile Strength

8

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

0f

1 2 3 4 5 6 7 8

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Tensile Stress Concentration at a Fiber Break

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Tensile Strength Models

fm

tff

tt V1XVXX

A very crude approximation of tensile strength from the Rule of Mixtures

More sophisticated models includeBatdorf, 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.

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Stresses Around Filler Particles

Monette, et al, J. Appl. Physics, 75 (3), 1994, 1442-1455.

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Pultrusion Fabrication Flaw

90º Tow

0º Tow

“As received” pultruded cross ply laminate (E-glass/Derakane 441-400)

Microcrack -1.2mm long by .25mm wide

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Transverse Strength Models

f2

m2

ffm

ttE

E1VV1YY

11

E

Er

E

YEY

f2

m2

hm2

mt

2t rV

hf 2

3

11

E

Er

E

YEY

f2

m2

sm2

mt

2t rV

sf 4

Gibson, R. F. Principles of Composite Material Mechanics, McGraw Hill, New York (1994)

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0° and Laminate Tension Testing of Composites

Concerns in the Assessment of Modulus and StrengthUniformity 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

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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°

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Straight-Sided Specimen

Advantages: No specimen tapering required; better results with cross-reinforced materialsDisadvantages: Tabbing required; tab s-concentration; tight tolerances in thickness

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Typical Failure Modes in Straight-Sided Coupons

(Acceptable & common in unidirectional specimens)

(Acceptable & common

in 90° or 90° dominated layups)

(May be found in crossply layups; unacceptable)

(Unacceptable)

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Typical Tab Failures in Straight-Sided Coupons

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ASTM D 638 Type I “Dogbone” Specimen

Advantages: No tabbing required; load introduction less of an issueDisadvantages: Careful specimen machining required; not suitable for unidirectional material

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Streamline Specimen

Advantages: No tabbing required; load introduction less of an issue; comparable to straight-sidedDisadvantages: 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

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Linear-Taped “Bowtie” Specimen

Advantages: No tabbing required; load introduction less of an issueDisadvantages: Careful specimen machining required; not suitable for unidirectional material; large specimen

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Effect of Misalignment in Unidirectional Specimens

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Compression Strength

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Compression

L = 41.8 m

LE I

k

ff 14

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Compression Strength

fm

cff

cc V1XVXX

An approximation of crushing strength from the Rule of Mixtures

Co

mp

ress

ion

Str

eng

th

Crushing

Buckling

Slenderness ratio (r/L)

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Compression Strength

L

S S

Matrix

Fiber

Origin of Buckling Fiber's Sine Wave

c

c

2sin

2

GG2

r

kLG

L12

rE

E

V1EVEX

m12m

1232f

2m

122

32ff

1f1

fm

1ff

1c

L

s

4 ff

1

k

IEL 0L1

Ebk

fm

2ff

2

f2

m2

V1EVE

EEE

fm

12ff

12 V1V

Xu, Y. and Reifsnider, K. L. “Micromechanical modeling of composite compressive strength,” Journal of Composite Materials, Vol. 27 (6), (1993), pp. 572-588.

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Compression Strength

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.

fiber

kink band T

T

L

L

1n

y7

31

G

n

1n

yn

1c

1n7

3n1

GX

Ramberg-Osgood shear response

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Compression Testing of Composites

Concerns in the Assessment of Modulus and StrengthUniformity of stress stateEnd loadedShear loadedGage section dimensionsSandwich beamGrippingStressconcentrationTab geometryTabbing materialAlignmentBucklingFailure modesSpecimen machining toleranceFixture characteristics

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Compression Testing of Composites

Classes of Test MethodsShear Loaded - PreferredCelanese & Wyoming modified Celanese IITRI (Illinois Institute of Technology Research Institute) & Wyoming modified IITRI

End LoadedBoeing Compression ASTM D695 & Wyoming modified D695Wyoming End Loaded Side Supported (ELSS)RAE (Royal Aircraft Establishment)Short Block Compression

Sandwich BeamASTM D3410, Method C--FlexureAxially Loaded Sandwich Column

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IITRI - ASTM D3410

Advantages: Alignment; high data averages and low scatter; large specimens possibleDisadvantages: Expense; specimen tabbing & machining critical; tab s-concentration

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Celanese: ASTM D3410

Advantages: Alignment; high data averages and low scatter; long-standing test fixtureDisadvantages: Specimen tabbing & machining critical; tab s-concentration; sensitive to fixture accuracy; expense (latter two concerns addressed in Wyoming-modified)

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Boeing Modified ASTM D695

Advantages: Small, thin specimen; reduced material; highly supported against bucklingDisadvantages: No s-e curve; untabbed for modulus; tabbed for strength; tab s-concentration

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Wyoming End Loaded Side Supported (ELSS)

Advantages: No tabbing required; simple fixture; inexpensive; simple alignment; some shear loadingDisadvantages: End crushing for highly orthotropic specimens; support s-concentration; specimen tolerances critical

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Sandwich Beam Flexure - ASTM D3410 (ASTM C 393)

Advantages: Simple fixture; reliable results with proper specimen (core) designDisadvantages: Large specimens (materials expense); failure must occur in compressive face sheet

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Axially Loaded Sandwich Column

Advantages: Simple fixture; simple data analysis; standard compression fixtureDisadvantages: Expense in fabricating sandwich panel; end crushing; end s-concentration

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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

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Shear Strength

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Shear Strength Models

rV

hf 2

3

rV

sf 4

Gibson, R. F. Principles of Composite Material Mechanics, McGraw Hill, New York (1994)

f12

m12

ffm

G

G1VV1SSSS

11

G

Gr

G

SSGSS

f12

m12

hm12

m

12

11

G

Gr

G

SSGSS

f12

m12

sm12

m

12

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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

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Shear Testing of CompositesIn-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

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Shear Directions3

2

1Material Coordinate

System1, 2, 3

S23

S23

S12

S12

S13

S13

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Iosipescu Shear Test ASTM D5379

Advantages: Excellent shear strength measurement; small specimen; 0°, 90°, [0°/90°]ns layupsDisadvantages: Tight tolerances on specimen; alignment; twist failure; quality fixture required; expense

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(45)ns Tension ASTM D3518

Advantages: Simple; uniform stress state; no fixture; damage growth representative of laminatesDisadvantages: Tabbing; alignment; strength dependent on layup; scissoring; t12 and t13 failure; edge delamination; s-concentration due to tabs

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Short Beam Shear ASTM D2344

Advantages: Simple test and fixture; small specimenDisadvantages: Load introduction; no strain measurement; no modulus measurement; improper assumption of parabolic stress distribution; mixed mode failure

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Stress Distribution in a Short Beam Shear Specimen

Elasticity SolutionBeam Theory

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Interlaminar Fracture

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0

100

200

300

400

500

600

700

0 0.01 0.02 0.03 0.04

Displacement, m

Load

, P(N

)Double Cantilever Beam (DCB) Test Data – ASTM D5528

a

P

a1a2

a3

an

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DCB Data Reduction: Modified Beam Theory

y = 0.429941x + 0.001997R2 = 0.9997

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

-0.05 0 0.05 0.1 0.15 0.2

Crack Length, a [m]

Cub

e R

oot o

f Com

plia

nce

C 1/

3 (J

/m2 )

1/3

x

1m

•Find C:

•Plot C1/3 vs a

•Find fit:

a

P

PC

)xa(mC / 31

23 23( )

2I

Pm a x

b G b=width

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DCB Data Reduction: Compliance Calibration Method

2

2I

m P

ba

G

m2

1

•Find C:

•Plot log(C) vs log(a)

•Find the slope m2

PC

a

P

log(C

)

log(a)

b=width

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DCB Data Reduction: Compliance Calibration Method

3

2/33

2I

PC

m bhG

m3

1

•Find C:

•Plot a/h vs C1/3

•Find the slope m3

PC

a

P

a/h

C1/3

b=width

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Edge Notch Flexure (ENF)P

a

L L

b=widthLo

ad,

P

Mid-span Displacement,

95% of 1/C

1/CPMax

P95%Pnl

3

3

4bCh

LEflex

313

3

8/

flexcorr

CbhEa

2

3 3

9

2 (2 3 )corr i

IIcorr

Ca P

b L a

G

Of the uncracked region

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QUESTIONS

A

B

C