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shear test, +45 ° tension test, and 10° off-axis tension test were highly ranked by .... (0.15") with a notch tip radius 1.3mm (0.05"). .... notch roots and notch flanks for 90 ° and 0° specimens, respectively ... of ICCM VIII, July 15-19, SAMPE, 36-L, 1991. 9. .... to Specimen Along the Load. Introduction. Region. /I r. ;_'. 11.5m. G 2 w.
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NASA-CR-190660

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0-3 AN

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FOR THE IOSIPESCU MODIFIED WYOMING

EXPERIMENTAL PROCEDURE SPECIMEN TESTED IN THE

6_I

COMPOSITE FIXTURE

H. Ho, M.Y. Tsai, J. Morton Department of Engineering Science and Mechanics Virginia Polytechnic Institute and State University Blacksburg, VA 24061-0219

_= E

and G.L. Farley US Army Aerostructures Directorate NASA Langley Research Center Hampton, VA 23665-5225

ABSTRACT

A detailed specimens

description

in the modified

gage instrumentation With

the proper

properties

of the experimental Wyoming

Interpretation

setup

for testing composite

f'Lxmre is presented.

are addressed.

experimental

procedure

Specimen

preparation

of the experimental

and procedure,

consistent

Iosipescu and strain

results

and repeatable

is discussed. shear

are obtained.

INTRODUCTION

Characterization mechanical shear

behavior

test methods

measurements Iosipescu

obtained pure

effect

of a laminated

composite

[1-6] have been

for composite

based

structures. to perform

Among

analysis stress

is needed

Since

these many

none of these

The

specimen, to account

10 ° off-axis

strengths

correction

test also produced

failed under for the coupling lower N92-31566

(NASA-CR-190660) AN EXPERIMENTAL PROCEDURE FOR THE IOSIPESCU COMPOSITE SPECIMEN TESTED IN THE MODIFIED WYOMING FIXTURE (Virginia Polytechnic 24

Inst.

and

State

by

test methods

for the shear-extension

tension

the

no specimen

an after-test

numerous

highly ranked

and the shear

because

of the

and strength

test were

fields in the test section

tension

the late 1960's,

shear test methods,

tension

[7]. However,

[9] in order

by the end constraints.

to the understanding

shear stiffness

were also questionable

For the 10 ° off-axis

shear modulus

is essential

test, and 10 ° off-axis

shear

from these test methods

caused

developed

materials.

on a decision

pure and uniform

shear [8-10].

apparent

shear properties

shear test, +45 ° tension

Lee & Munro produced

of the in-plane

Unclas

Univ.)

p G3/24

0115541

failure

stresses

shear

due to the bi-axial

strain distribution

neighboring

therefore,

+45 ° tension

specimens

in-plane

sequence

(sublaminate

and ply-level

fabric composites,

is attributed

widely

and simple

graphite-epoxy

specimen

investigators Nevertheless, from either

For example,

0 ° or 90 ° specimens

the shear moduli

between

stress-strain

a pure shear

in the 0 ° specimen

longitudinal

different

with the Iosipescu

that occurs

[10,16-19]

stiffness. because

[10,16]

\

\

using the Fig.2,

their specimens.

is caused

shear modulus

studies

strain distributions

showed

that,

were inconsistent.

are: (1) the inability

[10,16],

(2) the different

and (3) the large difference

by the specimen's

shear moduli

[10,16-19],

data for either fiber

shear test method

[10,16-19],

are

different

between

of the 0° specimens

2 \

between

for both 0 ° and 90 ° specimens

The measured the shear

test fixture,

from the 0 ° and 90 ° specimens

the 0° and 90 ° specimens

response

AS4/3501-6

were tested and the results

shear modulus

test

shear modulus

[13-15]

In recent

shear

procedure

was found in the measured

cut from the same panel

associated

of composite

and ease of

for unidirectional

from the same investigator.

obtained

must be used.

experimental

fractions

shear

short fiber

and inconsistent

shear moduli

of data scatter

the in-plane

systems

Wyoming

and +45 °

of the Iosipescu

of investigators

fiber volume

a pure shear state in the test section

shear moduli

large

of the measured

by different

a very large degree

The main problems

shear

by a number

to the large scatter in the measured

orientation,

to obtain

the shear modulus

the

that is, the

measurement

careful

than

and the stacking

composites,

of material

inaccurate

rather

measurement,

to measure

property

without

Fig. 1, and modified

The variations

may be caused

results,

measured

geometry,

both 0 ° and 90 ° specimens in addition

However,

with the

of the 10 ° off-axis

The popularity

a wide range

instrumentation.

composites

in Fig.3.

of testing

associated

is a laminate

test methods

shear test for shear

of the experimental

data can be obtained.

other shear

a_dused.

test, the

thickness

For multi-directional

investigated

to its capability

and interpretation

shown

the Iosipescu

+45 ° tension

effect [ 11,12];

The usefulness

the tests can only be applied

and woven

has been

Iosipescu

because

scaling

on the specimen

scaling).

composites.

In the past decade,

handling

is dependent

specimen

[8]. For strength

to a specimen

of unidirectional

composites,

materials

to free edge effects

shear strength

In the

local fluctuations

the +45 ° tension

are susceptible

tests is limited

properties

layers contained

[8]. In addition, it is subject

measured

tension

in the surface

sub-layer

a lamina;

stress state in the specimen.

[14-19].

The lack of

low transverse

for the 0 ° and 90 ° specimens in the test section

in

and are

of the 0 ° and

90° specimens arenonuniformandareof differentforms. Thelargeloaddistributionarea in thespecimen/fixture contactregionof the0° specimen[17]andpossiblelocalhardness alongthespecimen/fixture contactregionof the0° specimen's long edgesresultin inconsistentloadpointsbetweenspecimens.The variationin themeasuredshearmoduli for the 0° specimenis attributedto theeffectof theuncertaintyof loadpoints[10,17]. Due to thehigh transverse stiffnessof the90" specimen,twistingcanoccurwhenloadis applied [10]. Specimentwistingcausesdifferentshearstress-strain responses in thefront and backfacesof the specimen.Thevariationin themeasured shearmodulifor the90° specimenis causedby specimentwisting,whichis materialpropertyandspecimen dependent. In this paper,generalguidelines,ascurrentlypracticedby theauthors,for testingthe Iosipescucomposite specimen in the modified Wyoming fixture are presented. The above mentioned

problems

experimental

are addressed

setup, experimental

in the guidelines procedure,

TEST

in terms of specimen

and proper

interpretation

preparation,

of experimental

data.

PROCEDURE

Specimen Both unidirectional the in-plane

elastic

ply composite

(0 ° or 90 °) and cross-ply shear modulus

panels

The in-plane notches

at the opposite

3.8mm

(0.15")

2.5mm

(0.1")

to 6.3mm parallel

and 90 ° specimens,

Unidirectional comparison Due

tip radius

(0.25")

1.3mm

and perpendicular

Fig. 1. To avoid

in the specimens,

of the uncertainty shear

(0.05").

material.

are machined

Specimen

possible

and cross-

specifications.

(3" x .75") with two V-

axis, Fig. 1. The notch thickness

handling.

depth

variations

Specimens

modulus

is inevitable

[10].

from with

as the 0°

in the resin content

are trimmed.

from the same panel to render

on shear

is

ranging

axis are designated

of the panels

of how load is wansferred

modulus

Unidirectional

x 19.1ram

to facilitate

the edges

can be used to determine

to the manufacturer's

to the longitudinal

of the effect of fiber orientation

of the measured

center

is recommended

0 ° and 90 ° specimens

to the effect

variation

are 76.2mm

ends of the transverse

respectively,

and poor fiber alignment

according

of the specimen

with a notch

fiber directions

(G12) of a composite

are laid up and cured

dimensions

(00/90 °) specimens

measurement

direct

[10,16-19].

into the 0 ° specimens, The uncertainty

of load transfer

for the0° specimenis aninherentpropertyandtheeffectvariesbetweenspecimens. Therefore,the90° specimenis recommended for accurateshearmodulusmeasurement. Specimens werecutusinga diamondimpregnatedwheelsaw. The longedgesof the specimenweregroundflat andparallelto a toleranceof 0.0254mm(0.001"). Several specimens with backingmaterialwereheldin a viseon a grindingmachinetofacilitate cuttingthenotch. A 150mm(6") diameterabrasivegrindingwheelwith a tip radiusof 1.3ramwaspositionedatthecenterof the specimens heldin thevise. The rotationalspeed andthefeedrateof thegrindingwheelwere2,950RPMand0.4m/rain,respectively.The feedrateof thegrindingwheelshouldbekeptlow to preventdelaminationandsplitting nearthenotches.It hasalsobeendeterminedthata variationin measuredshearmodulus canoccurif thenotchesarenotlocatedatthecenterof thespecimen. Beforeattachmentof thestraingagerosettesto thespecimen,thetestsectionof the specimen(theregionbetweenthetop andbottomnotches)is sandedflat with finegrade sandpaper(600grit). Thespecimenthickness,t, in thetestsectionis thenmeasuredalong with thenotchwidth, w. Usually,threethicknessandwidthmeasurements aretakenand theaveragevalueis usedto calculatethetestsectioncross-section area(A=tw). Foreachfiber orientation,atleastfive specimens areprepared.Becausethethicknessof thepanelmaynot beuniformacrossthepanelandtheedgesof thepanelmayhave irregularities,suchaswavyfibers,thespecimenlocationon thepanelandspecimen thicknessarerecordedfor furtherreference.Any variationin specimenthicknessresultsin local changesin fibervolumefractionwhichcouldresultin changesin mechanical response.The authorsrecommendthepanelsshouldhaveno morethana 3 percent variationin thickness. Specimentwistinghasbeenshowntobea significantproblemwhentestingmaterial with a 90° ply (i.e.90° or 00/90° laminates).Toreducethetwistinginducedstrainsa compliantlayer(maskingtape)is appliedalongtheedgesof thespecimen,seeFig.1. The front andbackshearresponses of a 90° AS4/BMIPES(bismaleimidethermosetmatrixwith polysulfonethermoplasticadditive)Iosipescuspecimenbeforeandaftertheapplicationof themaskingtapesareshownin Fig.4. Theamountof specimentwistingdecreases after the applicationof thetape.In general,thedegreeof twistingdependsonthetransverse 4

stiffnessof the specimen.Formaterialwith low transverse stiffness,the distancebetween thedistributedforces[10]alongtheloadintroductionregionof the specimenandthe specimenmid-planedecreases astheappIiedloadincreases.Thusfor AS4/3501-6 graphite-epoxy compositematerials,the90° specimen(Ey=138GPa)twistswhilethe0° specimen(Ey=8.96GPa)doesnot [20]. Forisotropicmaterials,analuminumalloy specimen(Ey=E=70GPa)twistswhileanaluminumparticulatefilled epoxycomposite specimen(Ey=E=7.75GPa)doesnot [20]. Typicalshearstress-strain dataon thefront andbackfacesof the90° graphite-epoxy andaluminumspecimens areshownin Figs.5and 6. In Fig.5a,the shearstress-strain curvesareobtainedfromthefront andbackfacesof the samespecimenwhentestedfour differenttimesin thefixture. Thatis, thespecimen wasloadedwith onerosettefacingthefrontof thefixture andtheotherfacingthebackof thefixture. Thespecimenwasthenunloadedandrotated180° abouta horizontalor vertical axis, andloadedagain. Thewide separation of theshearstress-strain curvesindicatesthat the specimentwistingis sensitivetohowthespecimencontactsthetestfixture. By taking the averageof thefront andbackshearstrains,thevariabilityof the shearstress-swain data correspondingto thefour specimen-to-fixture contactpositionsof the samespecimenis greatlyreduced,Fig. 5b. Theresponseof analuminumalloy specimenis shownin Fig.6a. The significanttwisting,which is observedin Fig.6a,is eliminatedby the averagingtechnique,seeFig.6b. Thusthetwistingeffectis nota resultof material anisotropy. Instrumentation Measuring oriented

shear

strain in the test section

at +45 ° relative

expressed

to the longitudinal

of the specimen

requires

axis of the specimen.

at least two gages

The shear strain is

as

(1)

7xy = E+45 - E-45

where

e+45 and e...45 are the extensional

recommend

that for each panel,

gage rosettes

relative

expensive

unstacked

can be used.

at least one specimen

on the front and back

directions

surfaces.

to the longitudinal two-gage

The recommended

strains in the +45 ° strain gages.

rosettes

be instrumented

The authors with stacked

The three gages are orientated

axis of the specimen. oriented

at +45 ° and 0 °

practice,

at +45 ° to the specimens

gage length of the strain

5

For general

gages is 1.5ram.

three-

less

longitudinal

axis

The advantage

of usingthe stackedthree-gage rosettesrelativeto theunstackedtwo-gagerosettesis that the0° gageprovidesa meansof measuringspecimenout-of-planebending.Furthermore, the stackedthreegagerosettemeasures thestrainsovera commonregionin thetestsection whereastheunstackedtwo gagesmeasurethestrainsat two differentlocations[17]. For 0° Iosipescucompositespecimens, thestrainsrecordedby thetwo gagesin the_+45 ° directionsarenot equalin magnitudeandoppositein signasis thecasefor the90° specimens, Fig. 7. Thelackof symmetryin the+45 ° gage readings is attributed to the presence strain

of transverse

field is found

reference

normal

to be uniform

20 (orthotropic

calculated

using

direction

ratio ranging

equation

shear

To identify

(1) [17].

response

and quantify

to have

variations

in the measured

2. When

between

the bearing

rotate

forward.

which

can cause

the twisting surface

the shear

consistent

gages.

Specimen

studied

in

strain still can be at the 45 ° or-45

slides along

occurs.

would

axis.

stiffness

°

post is large, the movable portion

the in-plane

The magnitude is applied

be obtained.

effect

portion

induced

by averaging

6

to

having

of the fixture

induces

90 ° Fig. post

will

an eccentric

is different

on the back between

large variations

the front and back shear between

load

of the specimen,

strains whereas

of the specimen,

can be obtained

specimens.

is attributed

point to the guide

On the front surface

to one surface

responses

unequal

of specimens

of the effect of twisting

However,

to cause

is worn or if the tolerance

of the fixture

shear

it is

a guide post via roller bearing,

If the bearing

to slip in the fixture.

shear stress-strain

[10] by producing

of the load application

of the movable

strains reduce

has been shown

The twisting

and high transverse

about the horizontal

and the guide

on the shear response

twisting

for 90 ° specimens

the eccentricity

If only one rosette modulus

systems

gage is attached

and its influence

of the fixture

the specimen

the converse

specimens.

strains,

portion

The rotation

induced

normal

the value of the single gage strain, an

of the specimen.

of the test fixture

a moment

if a single

twisting

shear moduli

the load is applied,

will produce

for all the material

by doubling

strain

on the front and back surfaces

The movable

The transverse

from 1.0 to 15.4), thus, the shear

However,

specimen

back-to-back

rigid body rotation

[16].

will be obtained.

necessary

plies.

in the Lest section

and the shear strain calculated

erroneous

strains

strain ey in the test section

successive

of

:\

Strain weave

gages may be inadequate

fabric

weave,

composites.

cross-ply

Based

([0/9012s)

Figs.8a-Sb,

moire

distribution

across

architecture

of the woven

cross-ply

shear

fringe

upon

woven

bands

The she_

fabric

two gage rosettes.

Even

responses

Experimental

Wyoming

fixture,

configuration

which

provides

Wheatstone

bridge

is converted

about

using back-to-back strains,

the

see Fig. I0.

through

wall.

pin is lifted to engage

secured

in the fixture

with the adjustable

portion

of the fixture

which

is constrained

completion

controlled

box.

data acquisition

the specimen

portion

using

in the specimen,

wedge clamps.

system.

parts of the fixtures

is centered

is

should

be

of the fixture

an alignment

pin.

and the specimen

Load is applied

to move vertically

analog

The circuit

of the movable

The specimen

The

and the amplified

openings,

and stationary

the lower notch

with a half bridge

amplifier

any rotation

using a cross

of the strain gages.

into the fixture

The movable

the guide rod after load is applied.

Experimental

a circuit

is inserted

to be in the same plane to prevent

The alignment

bridge

compensation

to a micro-computer

specimen

the fixture

to the testing machine

to a signal conditioning

to digital signal

box is connected

against

downward

is

to the movable along a guide

rod.

procedure

An aluminum surfaces

with the

of six plain-weave,

were obtained

to a Wheatstone

temperature

is connected

the composite

adjusted

in

strain

is associated

the front and back shear

Fig.2, is attached

gages are connected

placed

for a plain-

shear

in strain

responses

specimens

are also inconsistent,

Strain

After

to undulating

stress-strain

after averaging

head adapter.

completion

fringe patterns

This variation

composite

of coarse

setup

The modified

signal

determination

[20] with 12k AS4 fiber, shown

correspond

see Fig.9.

fabric.

moire

composite

occur which

woven

stress-strain

shear modulus

u- and v-field

fabric

the test section,

([0°/90°]2s)

unstacked

for accurate

alloy specimen

of the specimen

test setup

produces

machine.

Agreement

any test setup errors.

with stacked

is tested

consistent between

three-gage

within the linear elastic

results

rosettes response

each time the test apparatus

subsequent

on the front and back

tests of the aluminum

region is placed

to ensure

that the

in the test

alloy specimen

minimize

Thereactionforcefromthespecimen,whichis alsotheequivalentappliedshearforce, P, in thetestsection,is obtainedfrom theloadcell connectedto thefixture. The shear stressis calculatedby dividing theshearforceP by thecross-sectional areaA (A=tw) in the testsection.A crosshead loadingrateof 0.5mm/minute(0.02irdminute)is used. Data

interpretation

The shear

stress-strain

on the minimum equivalent

cross-section

area covered

are nonuniform. specimens,

plotted

correction

factors

The correction

factors

forms

depend

for the material

Problems

shear fields

be applied

obtained

in question.

orthotropy

It has been shown

developed

swains over the

arise when

the shear fields

for either 0 ° or 90 °

of shear

modulus

[ 10,17-

from the 0 ° and 90 ° specimens

of the shear stress/swain

on the material

stresses

shear

in the test section

in calculation

shear moduli

shear

of the average

gage rosettes.

should

in the apparent

are the average

as a function

by the strain

due to the different

analysis

in Figs.4-6,

Due to the nonuniform

19]. The difference largely

responses,

distributions

is

in the test section.

and are defined

by a finite element

to be expressed

approximately

[17]

as

CF = 1.036

where

- 0.125

Ex and Ey are the longitudinal

respectively.

The corrected

(2)

x log(Ex/Ey).

shear

and transverse modulus

stiffnesses

G×y is expressed

of the specimen, as

Gxy = CF X G*

where

G* is the apparent

front and back

shear

the 90 ° specimens of the specimen. However,

increased

Iosipescu

specimens

using

The measured

shear

shear moduli

of load points between

the shear modulus

of the 0° specimen

and the difference greatly,

see Fig. 11.

Zave]Ygage, with '_ave = P/A and'Ygage = average

The distribution

is obtained

the measured

to uncertainty

shear modulus,

strains.

and 90 ° graphite-epoxy

reduced

(3)

between

of the measured is shown

the averaged moduli

for several

for several specimens. reduced

shear

modulus

in Fig. 11. The

90 ° specimens

shear modulus



for

application

shear moduli

are consistent.

show a range

of values

of correction

while the shear modulus

the measured

for several

swain of the front and back surfaces

0 ° specimens After

shear

of

due

factors,

of the 90 ° specimen

of 0 ° and 90 ° specimens

is

Twistinghasbeenidentifiedasplayinganimportantrolesfor specimenswith high transversestiffness,(i.e.,90° and00/90° specimens).In additionto differentfrontand backshearstress-strain responses, twistingalsocauseslow failure stresses. Iosipescuspecimens (isotropic,0° or 90° unidirectionalcompositematerials)donot fail atthenotchaxiswherethecross-section areais a minimum. Themixedmodefailure is attributedto thepresenceof thetensilenormalstrains,Ex,andey, notch

roots and notch flanks

inherent

problem

The effect

due to the notch

design

of strain gage transverse

for 90 ° and 2.6%, transverse

for 90 ° and 0 ° specimens,

sensitivity

coefficient

[16,17],

of the

and is an

of the specimen.

sensitivity

for 0 ° graphite-epoxy

respectively

at the intersections

has been found

specimens

instrumented

to be negligible,

about

2%

with strain gages with a

kt = 2% [21].

CONCLUSIONS

For

0 ° and 90 ° graphite-epoxy

measurement

could

the experimental

occur

specimens,

if a proper

guidelines

significant

experimental

variations

procedure

for shear testing of the Iosipescu

in the shear modulus

is not followed. composite

In summary,

specimens

are as

follows.

1. The thickness

of the composite

panel should have

2. The edges of the panel from which constant

fiber

volume

fraction

the specimens

and straight

less than 3% variation. are cut should be trimmed

fibers.

3. Use slow feed rate in the cutting

of the notches.

4. Apply

long edges to reduce

masking

5. For accurate back 6. Apply

rosettes

tape at specimen

shear modulus should

a correction

modulus

measurement,

to ensure

specimen

90 ° specimens

twisting.

are recommended.

Back-to-

be applied. factor

of the composite

to the measured

shear modulus

material.

9

to obtain the corrected

shear

Following theproposedguidelinesandby properinterpretationof experimentalresults, extremelyconsistentin-planeshearmodulusdatausingtheIosipescuspecimentestedin the modifiedWyomingfixturecanbeobtained.

ACKNOWLEDGEMENTS This work hasbeensupportedby U.S.Army Aerostructures DirectorateunderNASA LangleyResearchCenterResearchGrantNAG-l-1053.

10

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D.L. and Howell, H.B.,"Analysis Journal of Composite Materials,

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in Fiber

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8. Ho, H., Tsal, M.Y., Morton, J. and Farley, G.L., " A Comparison Of Popular Test Methods For Composite Materials," Composites: Analysis, Manufacture, and Design, Proceedings of ICCM VIII, July 15-19, SAMPE, 36-L, 1991. 9. Pindera, M.J. and Herakovich, Composites with the Off-axis (1986). 10.

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J.,"Strength

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J.,"Scaling

13.

Scaling

Effects

" An Evaluation Measurement,"

in Fiber

in Angle-ply

Adams, D. F. and Walrath, D. E.,"Current Method,"Journal of Composite Materials,

of Unidirectional Mechanics, 26 (1) : 103-112

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laminates"

of the Iosipescu Journal of

NASA

Contractor

NASA

Contractor

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11

15.Wilson,D.W.,"Evaluationof theV-n0tehedBeamShearTestThroughan InterlaboratoryStudy",Journal of Composites Technology and Research, 131-138

12 (3) :

(1990).

16. Ho, H., Tsai, M.Y., Morton, J. and Farley, G.L., "An Experimental Investigation Iosipescu Specimen For Composite Materials," Experimental Mechanics, 31 (4) • 328-336, 1991. 17. Ho, H., Tsai, M.Y., Morton, J. and Farley, G.L., "Numerical Analysis Of The Iosipescu Specimen For Composite Materials," Composite Science and Technology, in press. 18. Pindera, M.J., Choksi, G., I-Iidde, J.S. and Herakovich, C.T.,"A Methodology for Accurate Shear Characterization of Unidirectional Composites," Journal of Composite Materials, 21 (12) • 1164-1184, 1987. 19. Pindera, M.J., Ifju. P. and Post, D.,"Iosipescu and Metal Matrix Composites," Experimental 20.

Shear Characterization of Polymeric Mechanics, 30 (1) • 101-108, 1990.

Ho, H., Tsai, M.Y., Morton, J. and Farley, G.L.,"An Evaluation Specimen For Composite Materials Shear Property Measurement," Virginia Polytechnic Institute and State University, Aug. 1991.

Of The Iosipescu CCMS-91-18,

21. "Errors Due to Transverse Sensitivity in Strain Gages," M-M Tech Note TN-509: Transverse Sensitivity Errors, Measurements Group, Inc., Raleigh, NC.

12

Of

FIGURE CAPTIONS Fig.1 Modified Iosipescushearspecimen. Fig.2 Modified Wyomingfixture. Fig.3 Normalizedvaluesof in-planeshearmoduliobtainedfromIosipescusheartest on AS4/3501-6.Valuesarenormalizedwith respecttoG* 12=5.12GPa,the averageobtainedfrom + 45 ° tension and 10 ° off-axis tests [8]. The ranges shown in the figure are moduli deviation.

of repeatability

[15] or denote

one standard

Fig.4

Typical shear stress-strain data (a) before (b) after application of masking tapes to the specimen along load introduction region of a 90 ° AS4/BMIPES Iosipescu specimen.

Fig.5

(a) Front and back surface shear stress-strain data for a single 90 ° graphite-el_.xy specimen. Specimen was loaded in different orientations by rotating the sp.ecn'nen about the x, y and z axes. (b) Average of front and back surface shear strmns as a function of shear stress for 90 ° graphite-epoxy specimen.

Fig.6

(a) Front and back surface shear stress-strain data for a single aluminum specimen. Specimen was loaded in different orientations by rotating the specimen about the x, y and z axes. (b) Average of front and back surface shear stratus as a function of shear stress for aluminum specimen.

Fig.7

Strain vs. stress for typical (a) 0 ° and (b) 90" graphite-epoxy Gages are aligned at +45 ° and 0 ° directions.

Figs.

8a&b

Typical moire fringe pattems of the plain weave [0/9012s specimen at an applied shear stress of 142 MPa. (a) u-field, (b) v-field. Carrier rotation is applied such that the deformation information is equally contained in the uand v-fields.

Fig. 9 Shear strains across the notches, strain, for plain weave [0/9012s Fig. 10

Fig.11

specimen.

normalized specimens.

with respect

to the average

shear

(a) Shear stress-strain data from front and back faces, (b) average of front and back shear stress-strain data of six plain weave [0/9012s specimens. Calculated application

G12 for three graphite-epoxy of correction factors,

0 ° and 90 ° specimens,

13

before

and after

y

Application of Masking Tape to Specimen Along the Load Introduction Region

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fiber angle 91Y.

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Modified

Iosipescu

shear

specimen.

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