Durability Design US 1998

8/3/2019 Durability Design US 1998

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ABSTRACT

Static Properties and Multiaxial Strength Criterion for Design 0 RNL/CP-99 1of Composite Automotive Structures

f%dF-7fi"s$l3--. B. Ruggles, G. T. Yak, and R. L. BattisteOak Ridge National LaboratoryOak Ridge, Tennessee 3783 1-8051

The Durability of Lightweight Composite StructuresProject was established at Oak Ridge National Laboratory(ORNL) y the US. Department of Energy to provide theexperimentally-based, durability-driven design guidelinesnecessary to assure long-term structural integrity of automotivecomposite components. The initial focus of the ORNLDurability Project was on one representative referencematerial-an isocyanurate (polyurethane) reinforced withcontinuous strand, swirl-mat E-glass. The present paperdescribes tensile, compressive, flexure, and shear testing andresults for the reference composite. Behavioral trends andproportional limit are established for both tension andcompression. Damage development due to tensile loading,strain rate effects, and effects of temperature are discussed.Furthermore, effects on static properties of various fluids,including water at room and elevated temperatures, salt water,antifieex, windshield washer fluid, used motor oil, batteryacid, gasoline, and brake fluid, were investigated. Effects dprior loading were evaluated as well. Finally, the effect 6multiaxial loading on strength was determined, and themaximum shear strength criterion was identified for design.INTRODUCTION

Development of lighter weight, more fbel efficientautomobiles represents a technology area where advancedmaterials, such as polymer matrix composites, can besuccessfblly applied. However, while significant effort is beingdevoted to material development and processing, specificdesign guidance and an understanding of the materialperformance under actual service conditions is lacking. Thereis a recognized need for improved structural design methodsand criteria that address deformation and failure behavior dcomposite materials. The Durability of LightweightComposite Structures Project was established at Oak RidgeNational Laboratory (ORNL) by the U.S. Department dEnergy to provide the experimentally-based, durability-drivendesign guidelines [ I , 21 necessary to assure long-termstructural integrity of automotive composite components. The

project is closely coordinated with the AutomoComposites Consortium (ACC).The ORNL Durability Project initially focusedcharacterizing and understanding the mechanical behavior oSRIM (structural reaction injection molding) isocyanu(polyurethane) reinforced with continuous strand, swirl-maglass. The isocyanurate resin was DOW MM364, andreinforcement was Vetrotex Certainteed Unifilo U750. Tmaterial was chosen by the ACC and supplied in the form25 x 25 x 0.125 in. plaques. Five layers of mat were usedeach plaque, resulting in a fiber content of about 25%volume (40-50% by weight). The 0" direction is parallethe roll direction of the original glass mat.The present paper summarizes tensile, compressflexural, and shear testing and results. Behavioral trendstension and compression are discussed and correspondproportional limits are established. Further, damdevelopment due to tensile loading, strain rate effmts, effects of temperature are discussed. In addition, effectsstatic properties of various fluids, such as water at room elevated temperatures, salt water, antifieex, windshwasher fluid, used motor oil, battery acid, gasoline, and brfluid, are assessed. Effects of prior loading are evaluatedwell. Finally, the effect of multiaxial loading on strengthdetermined, and the maximum shear strength criterionidentified for design.TENSILE AND COMPRESSIVE , ELASTCONSTANTS AND STRENGTH PROPERTIES

Flat specimens with tabs were used in the tensand compression tests. Tensile and compressive specimwere 1.0 x 0.125 x 8.0 in. and 1.0 x 0.125 x 5.25respectively. Tabs were 1.0 x 0.125 x 2.0 in. for tensile a1.0 x 0.0625 x 2.25 in. for compressive specimens.addition to straight-sided tabbed specimens, untabbdogbone-shaped specimens (0.8 in. wide in the gage sectiwere used in a small number of tensile tests. A servocontroMTS axial-torsion mechanical testing machine together wan MTS digital TestStar Materials Testing Workstation wused for computerized testing and data acquisition. For ten

Research sponsored by the office of Advanced Automotive Technologies, U.S.Department of Enerw, under contract DE-ACO960R22464 with Lockheed Martin Energy Research Corporation.


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DISCLAIMERThis report was prepared as an acco unt of work spon sored by an agency of the UnitedStates Government. Neither the United States Government nor an y agency thereof, norany of the ir employees, make any warranty, express or implied, or assumes ny legal liabili-ty or respom-bilityfor the accuracy, completeness, or usefulness of any information, app a-ratus, product, or process disclosed, or represen ts that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product, process, or service bytrade name, trademark, manufacturer, or otherwise does not neeessanly constitute orimply itsendorsement,recommendation, or fav oring by the United States Governm ent orany agency thereof.T h e iews and opinions of auth ors expre ssed herein do not necessar-ily s tate o r reflect those of the United States Gov ernment or any agency thereof.


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tests, specimens were mounted in Instron mechanical wedgegrips and strain measurement was accomplished with an MTSextensometer of 1.0 in. gage length. In the case cfcompression tests, specimens were mounted in a WyomingIITRI fixture, and strain gages were used for strainmeasurement.The average in-air room temperature values dstifiess (modulus of elasticity), E, ultimate tensile strength,UTS, and failure strain, Ef, are based on 277 tensile tests,185 in the 0" direction, and 92 tests in the 90" direction. Theaverage values are given in Table 1. The average value cfPoisson's ratio, v , for loading in the 0" direction is 0.31.Results of the tensile tests were further assessed to establish anaverage proportional limit for the material, definedas the point(oPL,PL) in stress-strain space where the tensile stress-straincurve departs from linearity.

Table 1. Room temperature tensile elastic constants andstrength propertiesValues 0" direction 90" directionE (Msi) 1.37 1.68UTS (h i ) 21.3 28.5Ef (%) 2.12 2.17

OPL &si) 5.08 6.19EPL (%) 0.38 0.37

The UTS was found to increase with increasing stiffhess. Therelation between UTS and stifiess can be reasonably wellapproximated with a linear law:oms A66 6 +0.02E.

The proportional limit stress is 24% and 22% of the UTS fathe 0" and 90"directions, respectively. The proportional limitstrain constitutes 18% and 17% of the failure strain for the 0"and 90" directions, respectively.Tensile properties as a function of temperature wereestablished based on tensile tests conducted at -40, 70, 135,190, and 250F. Average values of stiffhess and UTS obtainedat elevated temperatures given in Table 2are noticeably lowerthan those obtained at .and below room temperature.Likewise, tensilestrength and st ifi es s appear to increase withdecreasing temperature, resulting in "stronger" material at-40OF.

Table 2. Tensile properties as a function of temperatureTemp. E Change in UTS (hi) Change in(OF) (Msi) E (%) UTS (YO)250 0.94 -36.1 19.4 -25.5190 1.00 -34.3 19.2 -28.9135 1.14 -24.9 21.1 -22.270 1.48 0 26.0 0-40 1.83 23.6 34.8 33.7

The following simple straight-line approximation may be useto represent reductions in both st ifiess and strength wichanging ambient temperature, T(OF):(dE,Aa,,) x 100%= 11.35- .19 T.

The average in-air room temperature valuescompressive stifhess, Ec, ultimate compressive streng(UCS), and compressive failure strain, Erc, are given in Tab3. These values are based on twelve compressive tests, sevein the 0" direction, and five in the 90" direction.

Table3. Room temperature compressive elastic constantand strength propertiesValues 0" direction 90" directionEc P s i ) 1.37 1.67UCS @si) -24.1 -3 1.5EK (%) -2.41 -2.60

It can be seen that the stress-strain behavior symmetric; that is, compressive and tensile properties are versimilar, with compressive failure strain being slightly highethan the tensile value. The tensile and compressive sets of dacan be approximated with straight lines of the same slope; thais, linear laws with the same coefficients can be used tdescribe the relation between strength and sti&ess for bottension and compression.FLEXURAL ELASTIC CONSTANTS AND STRENGTH

Five 0.125-in.-wide by 1.0-in.-high beams, two ithe 0"and three in the 90" direction, were tested under threepoint loading to determine the in-plane modulus of ruptur(MOR), equal to the elastically calculated maximum bendinstress at failure. The average MOR values were 30.6 ksi an33.5 ksi for the 0" and 90" directions, respectively. ThMOR value for the 0"-direction specimens was 22% higherand the MOR value for the 90" direction was 5.4% higherthan the tensile strength for the corresponding orientation ithe same plaque.Six 1 O-in.-wide by 0.125-h-high beam specimenwere tested to determine the average out-of-plane flexuraproperties given in Table 4.

Table 4. Room temperature out-of-plane flexuralpropertiesValues 0" direction 90" directionEF Msi) 1.60 1.79MOR @si) 40.5 54.0E@(%) 2.98 3.07

In an effort to account for wide-beam effects, thematerial was assumed to be isotropic and the equationE = [(l-#)/]a (3


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was used to calculate the stifkess values. In addition,. 0.375-in.-wide by 0.125-in.-high beams were tested in three-point bending, yielding average MOR values of 45.14 ksi and50 ,l l ksi for the 0" and 90' directions, respectively. It is seenthat the 1.0-in.-wide and the 0.375-in.-wide beams producecomparable average MOR values. It is recognized that theout-of-plane MOR values aremore than a factor of two greaterthan the UTS. That is, the load-canying capacity is morethan twice that which would be indicated by limiting theelastically calculated maximum stress to the UTS. It is thus

appropriate to provide a higher allowable for primary out-of-plane bending than for primary membrane loading in thedesign guide [l]. A factor of 1.5 is suggested. It must benoted that this observation only applies to out-of-planebending. In the case of in-plane bending, the MOR values aremuch closer to the UTS values.

I25000 -

IN-PLANE SHE AR MODULUS AND STRENGTHThe in-air room temperature values of the elasticshear modulus, ultimate shear strength, and failure shear strainwere determined using Iosipescu specimens [3-51 oriented inthe 0" and 90" directions. As expected, the data from the twodirections were in good agreement. The average in-plane shear

modulus, Gl2, shear strength, $12, and shear strain at failure,f12, were G12 = 0.65 Msi ,#U 16.0 ksi, and .uf12 = 3.2%.It should be noted that the average tensile moduli r fthe plaque that the shear specimens were cut f?om were higherthan the averages for all plaques for both0"and 90" directions.A corresponding adjustment in the measured in-plane shearmodulus provides a value for G12 of 0.59 Msi. Acorresponding adjustment in the shear strength based on thetensile strength data gives a value of 14 h i .The shear modulusof an isotropic material is relatedto the modulus of elasticity and the Poisson's ratio by thefollowing equation:

22500 -B5) 20000 -

(4)The calculated modulus of elasticity based on the average G1 2of 0.59 Msi and a Poisson's ratio of 0.31 is 1.55 Msi. This isconsistent with the measured values of 1.37 Msi in the 0"direction and 1.68 Msi in the 90" direction.EFFECT OF TRAIN RATE

The effect of strain rate on tensile behavior wasassessed through constant strain rate tests performed at strainrates of IO", lo4, 1WZ,and 10 s". While no significantefi'ixtof rate on sti ffuess and failure strain was observed, the UTSappeared to increase considerably with increasing strain rate asseen in Fig. 1.The effect of rate on compressive behavior wasinvestigated in tests conducted at the following constant loadrates (with equivalent strain rates given in parentheses): 7.5Ib/min (10" s-I), 2330 Ib/min (3 x IO4 s-'), and 13,980Ib/min (2 x l o 3 -'). Observations were similar to those madein tensile tests.

STRAIN RATE (Us)Figure 1. Effect of strain rate on ultimate tensile streng

EFFECTS OF PRIOR LOADINGDamage development in tensile loading,manifested by changes in st iaess with increasing stress astrain, was explored in tensile tests with intermittent stifkechecks. During this rest a specimen was loaded to 20% UTand then unloaded to zero stress, at which point a stifkecheck was performed. This sequence, consisting of loading a specific loadstress level, unloading to zero stress, andstiffhess check, was repeated for the load levels of 40, 60 , a80% UTS. Stiffhess check, loading to 80% UTS, aunloading to zero stress were followed by loading to failuPercent change in stiffhess vs maximum prior tensile load shown in Fig. 2.

0 25 SO 75 10

Figure 2. Loss of stiffness due to prior tensile loadings tindicated levels.%UTS

As seen in Fig. 2 no significant changes in stifikess occ' below 29.6% UTS. Above this threshold, stifiess decreaswith increasing prior load, and this decrease may be describewith the following linear law:

(T1 TCAE x 100%= 5.5 - 0.186- x 100%.


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Note, that Eq. ( 5 ) predicts 13.1% stifhess loss at the end cflife. Loading to two-thirds of the UTS, which is the specifieddesign allowable, will result in a 6.9% stiffness degradation.Effects of prior cyclic loading on tensile propertieswere investigated. A series of specimens were subjected totension-tension fatigue cycles to various fhctions of thepredicted cyclic life. The specimens were then monotonicallytested to failure. The residual tensile strength was found todecrease linearly with the fatigue usage factor,n/Nf,where n, isthe number of cycles the specimen has been subjected to and,Nf, is the allowable numberof cycles. The predicted end-ofWestiffness reduction due to prior fatigue loadings was 16.5%,which was somewhat higher than the 13.1% end-of-lifestiffness loss predicted for static tensile loadings.Effects of prior creep on tensile strength and Stiffnesswere explored. Specimens were subjected to a sustained loadfor 20, 41, and 61% of rupture life, and then monotonicallyloaded to failure. Prior creep.had no effect on subsequenttensile strength and stiffness.EFFECTS OF MOISTURE ON STRENGTH ANDSTIFFNESS

The study of environmental effects on strength andstifhess demonstrated that exposure time and weight gainprovide measures of degradation for a given set of conditions.Moisture effects on tensile strength and stiffness are shown inFig. 3. Conditions shown include (1) exposure in distilledwater, (2) exposure in distilled water with superimposed loadsof 25 and 50% UTS [ h m 1, and (3) exposure in 180Fdemineralized water. Exposure in 180F demineralized waterwas introduced for the purpose of accelerating the sorptionprocess. Exposure under load is expected to serve a similarpurpose. Observations of weight gain with exposure timecon fm these expectations. For a given exposure time, weightgain due to moisture absorption is significantly increased bysuperimposed load as well as by elevated temperature.Fig. 3 shows that strength and M e s s decrease withincreasing exposure time. It should be noted, that because thereductions were referenced to average values fkom severalplaques, the solid curve fits in Fig. 3 do not pass throughzero, as they should Therefore, they were adjusted (see dashedlines) to represent zero reduction in strength and/or stifhess atzero exposure time. Furthermore, correlations representingstrength and stifhess reduction due to room-temperatureexposure without load, can be combined into a singleequation:

AR x 100%= 3.44 log,, tE +3.44 .where AR is reduction in strength and/or stifhess and tE isexposure time in hours (tE > 0.1 h). Note, that Eq. (6)yields a17% reduction in strength and/or stiffness after one year cfmoisture exposure.Observations regarding moisture effects on tensilestrength and m e s s can be extended to compressiveproperties. Exposure to moisture resulted in degradation cfcompressive strength and stiffness. Presoaking in 180Fdemineralized water has a significantly greater effect on bothtensile and compressive properties than presoak in room-temperature distilled water.

Z

o3a 20y = 3.479LOG(x)+ 3.44%- -

1 10 100 1000 100EXPOSURE TIME (h):

y = 3.405LOG(x) +0.145-20 I 1 I I I10 100 1000 100

EXPOSURE TIME (h)EXPOSURE IN DISTILLED WATER

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EXPOSURE IN 180FDEMINERALIZED WATEREXPOSURE IN DISTJUED WAE R UNDERLOAD = 50 %UTS (ORNL)

0A

EXPOSURE IN DISTILLED WATER UNDER LOAD = 25% UTS. HENSHAW el alEXPOSURE N DISTILLED WATER UNDER LOAD= 5~2%UTS.HENSHAWel al

Figure 3. Effect of moisture exposure time on tensilestrength and stiffness.

FACTORS FOR OTHER FLUIDSIn addition to distilled and demineralized water, tfollowing seven automotive fluid environments weinvestigated in this study: saltwater, antifreeze, windshiewasher fluid, used motor oil, battery acid, gasoline, and brafluid. Details of the fluid composition as well as sourcesfluidsare given in Ref. [3]. The average changes in strengand stifkess due to exposure for 1080 h and 7540 h apresented in Fig. 4.Strength results in Fig. 4 demonstrate that batteacid has the largest degrading effect followed by windshiewasher fluid and distilled water, and then saltwater. Motor oand brake fluid appear to increase the strength. Stifheresults in Fig. 4 indicate that battery acid and windshiewasher fluid have the largest degrading effect (about a 20reduction) for 7540-h exposure. The other changes in m e sare all less than 10%. A s in the case of moisture, the longexposure times cause greater degradation.


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w4ar80

,1080 h

wga8I0

~-0

-5

-10

- 1 - I I I 1 I I 1 I

FLUID

10 I

0

-10

-20

-30 STFENGTHSTIFFNESSI

FLUIDFigure 4. Effect of prior exposure to various automotivefluids on tensile strength and stiffness.

Observations made for tensile properties can beextended to compression. Compression tests on specimenspresoaked inbattery acid for 1080h produced a 17% reductionin stiffness and a 14% reduction in strength. These values aresomewhat higher than the corresponding ones in tension.Iosipescu specimens were soaked in distilled water for2200 h, battery acid for 2200h, or 180F demineralized waterfor 1080h. Percent changes in shear modulus, shear strength,and shear strain at failure for the different environments aregiven in Table 5 .

Table 5. Effects of prior exposure in automotive fluon shear propertiesProperty Distilled Battery 180FWater Acid Demin.Water

9 2 -8 -3 -137 12 -14 0 -3512 -16 8 -27

Visual examination of the Iosipescu specindicated that the battery acid had attacked the matrix, Iefibers exposed on the surfhce. However, the soak in bacid causedan 8% increase in the shear strain at failure.soak in 180F demineralized water was the most damacausing a 35% decrease in the shear strength. Note thacorresponding reduction in tensiIe and compressive strwas approximately 50%.MULTIAXIAL STRENGTH CRITERION

Because the designer is usually only provideduniaxiaI strength values, while the stresses in componenoften multiaxial, it is necessary to have a multiaxial strecriterion. Because the material in this study is generaIlyplate or shell form that is loaded so that the stresses are iplane of the plate, only a biaxial strength criterion is requiMany merent strength criteria have been propSome of them account for different strengths in tensioncompression and for merent strengths in different directHowever, such criteria become quite complex for the desto use and require extensive materials testing to determinthe required constants. To keep the criterion simple an dfor the designer to use, the strength is assumed to be thein tension and compression and in all directions in the pof the plaque. Thus, the only strength value used istensile strength in the 0 direction.Five candidate strength criteria are shown in FigNote that all five criteria predict the same uniaxial strenThey differmost under equal biaxial tension or compresand when the maximum principal stress is tension andminimum principal stress is equal in absolute valueopposite in sign. This is the conditionof pure shear.According to classical elastic plate theory, whconcentric ring-load is applied to a simply-supported circplate, the stress state at the bottom surface is equal bitension everywhere inside the ring load. Thgrefore, thiswas chosen for evaluating the failure criterion in thequadrant.Tests were run on 3.7-in.-diam. circular dsupported on a plate with a 3.5-in.-diam. hole and loaded wa plunger that made contact at a radius of 0.75 in.. Becausedeflection at failure was more than twice the disk thickneswas necessary to perform a large-deflection elastic fielement analysis to determine the stresses at failure. The dfailed where the plunger contracted the disk. That islocation of maximum stress according to the analysis. analysis showed that the radial stress was somewhat higthan the tangential stress at the location where the disks fai


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t

I

Maximum shearstress criterion

strain criterionFigure 5. Candidate failure criteria.

It was necessary to account for the observed differencein tensile strength and out-of-plane flexure strength (MOR)became the disks were basically subjected to out-of-planebending. The corrected maximum stresses in the disks atfailure were 22.5 ksi in the radial direction and 17.0 hi in thetangential direction.The average maximum stress in the disk is comparedto the failure criteria in Fig. 6. The average failure point wasplotted at two locations in stress space that are symmetricalabout a 4 5 O line. The average maximum stress in the diskagrees well with the maximum shear and maximum stresstheories of failure. The maximum work theory agrees almostas well, but is unconservative. The modified maximum worktheory is extremely conservative, and the maximum straintheory is extremely unconservative. "herefore, either themaximum stress or maximum shear theory would be areasonable failure criterion in the first quadrant, where they areidentical. Note, however, that those two criteria are verydifferent in the second and fourth quadrants.The principal stresses in the case of pure shear aretension in one direction and equal compression perpendicularto the tensile stress. Therefore, shear strength provides thedata needed for evaluating the criteria in the second and four&quadrants. The average data point fiom the shear test isplotted in both the second and fourth quadrants in Fig. 6.The average 90" direction tensile strength andcompressive strengths are also plotted. Clearly, the maximumstress theory is unconservative in the second and fourthquadrant. Only the maximum shear and modified work criteriaare both conservative relative to all the average data points.The maximum shear criterion is more conservative than themodified work criterion in the second and fourth quadrant.However, the modified work criterion is extremelyconservative in evaluating the first quadrant. Unfortunately,data in the third quadrant are not available for the Murecriteria. The maximum shear criterion is recommendedbecause it iswidely used by designers and is very simple.

I

I *90"Figure 6.Available data indicate maximum shear

criterion applies.

SUMMARYIn-air room temperature tensile elastic constants strength properties were established for both 0" anddirections.Proportional limit was established and found to consti22 to 24 % of the UTS.Ultimate tensile strength was found to increase wincreasing stifhess, and a linear law correlating UTS s t i f i hess was proposed.Tensile properties as functions of temperature westablished for temperatures between -40 and 250F.equation for predicting changes in elastic properties wambient temperaturewasproposed.Compressive properties and proportional limit westablished and found to be very similar to thosetension.The in-plane MOR was no more than 22% higher tthe tensiIe strength, but the out-of-plane MOR was mthan twice the tensile strength.The room temperature in-plane shear 'modulus strength were determined.No significanteffect of loading rate on stifhess and faistrain was observed. However, UTS increaconsiderably with increasing strain rate. This observatwas extended to compression.Effects of prior static, cyclic and creep loadings winvestigated. Static loadings above a 29.6% Uthreshold reduced the subsequent sti&ess (up to 13.1%Residual tensile strength decreased linearly with faticycling (up to 22% at n/Nr = 0.8). With a design facto20 on cycles to failure, the maximum reductionnegligible (about 1.5%).A factor of 20 on cycles to failure will limit the stifnloss during fatigue cycling to 10%or less.

100% UTS).


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Prior creep strains (up to 61% of the creep ductility) haveno effect on subsequent tensile strength and stifkess.Effects of moisture exposure on tensile. and compressivestrength and stiflkess were explored. Exposure time andweight gain were shown to provide measures CEdegradation for a given set of conditions. A singleequation was proposed to represent strength and m e s sreductions due to moisture exposure.Effects of seven automotive fluid environments on tensilestrength and stiffness were investigated. Battery acid hadthe largest degrading effect followed by windshield washerfluid, distilled water, and fmally saltwater.Shear and bending tests on circular disks provided data forevaluating the applicability of various multiaxial strengthcriteria. The maximum shear stress criterion using thezero direction tensile strength as the base value wasrecommended for design.

REFERENCES1. J. M. Conun, et. al., Durability Based Design Criteriafor an Automotive Structural Compo site: Part I .Design Rules, OWL-6930, Lockheed Martin EnergyResearch Corporation, Oak Ridge National Laboratory,

February 1998.2. J. M. Conun, et. al., Durability-Based Design Criteriaforan Automotive Structural Composite, Proceedings ofthe 13 Annual ESD Advanced Composites TechnologyConference, September28-29, 998.3. ASTM Standard D 5379/D 5379M-93,Standard TestMethod fo r Shear Properties of Composite Materials Lythe V-Notched Beam Method, American Society ftrTesting and Materials.4. Walrath, D. E., and Adams, D. F., The Iosipescu ShearTest as Applied to Composite Materials, Experimental

Mechanics, Vol. 23,No. 1,March 1983,pp. 105-110.5 . Adams, D. F., and Walrath, D. E., FurtherDevelopment of the Iosipescu Test Method,Experimental Mechanics, Vol. 27,No.2, June 1987, pp .

1 13-119.6. Henshaw, J. M., Meyer, L. J., Houston, D. Q., andHagerman, E. M., Stressed Environmental Degradationof Automotive Composite Materials, AdvancedComposites Conference Proceedings, ESD - TheEngineering Society, April 1997, p. 367-380.7 . J. M. Corum, et.al, Durability of Lightweight CompositeStructuresfo r Automotive Applications: Progress Reportfor Period Ending September 30, 1995, ORNLITM-13176, Lockheed Martin Energy Research COT., OakRidge National Laboratory, March 1996,

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Durability Design US 1998