JOURNAL OF MATERIALS SCIENCE LETTERS19 (2000 ) 339– 342

Fracture behavior of glass/epoxy woven composites under

biaxial loading

J. S. SHIM, J. H. HWANG, W. HWANGDepartment of Mechanical Engineering, Pohang University of Science and Technology (POSTECH),Pohang, 790-784, KoreaE-mail: whwang@postech.ac.kr

Plain woven fabric composites have increased in useas structural materials because they can provide theproperties desired by controlled variation of the type offiber bundle, the fabric pattern and the waviness of thefiber. Textile composites also provide more balancedproperties, being independent of the loading directionin the fabric plane than unidirectional laminates. Thesefabric-based materials are relatively easy to handle andhave low fabrication cost. Traditional two-dimensionallaminated structures suffer from low interlaminar frac-ture toughness and moduli/strength with regard to thethickness axis. In using textiles as composite reinforce-ments we can, therefore, obtain structures of higherimpact resistance and greater tolerance to damage [1].

In spite of the popularity of fabric composites, theirfracture behavior is not easy to predict because of theirgeometrical complexity. It is difficult to perform de-tailed modeling and stress analysis of textile compos-ites [2–4]. In particular the behavior of plain wovencomposites under transverse normal (z) directional loadis difficult to analyze. Therefore, in the present study,the fracture behavior and failure mechanisms of thickglass-fiber woven composites are studied under biaxialloading. Especially, this study examines the character-istics of the material with respect to the thickness axis(z axis or 3 direction).

Fig. 1 shows the configuration of a test specimen.Cylindrical specimens were machined from glass/epoxy woven laminates of 304 plies thickness, man-ufactured by Hankuk Fiber Glass Co. The geometry oftest specimens was designed with regard to factors suchas the radius ratio for neglect of transverse shear de-formation, the stress concentration, the grip conditionfor more accurate loading transfer, the test machine andmethod. An end tap made of glass/epoxy composites, toreduce the stress concentration in the transverse direc-tion, was attached to the specimen using epoxy adhesiveas shown in Fig. 1. Specially designed grips were usedto eliminate sliding between the specimen and grip un-der torsional loading.

We carried out uniaxial testing, biaxial testing andfractographic analysis. In the static tests, purez direc-tional tension, compression, torsion, andx directionalcompression tests were conducted to find the controlvariables. Table I shows the uniaxial test results for thezdirection. We also measured the compressive strengthin thex direction to be 353 MPa. Based on these results,biaxiality tests are performed under two stress levels,denotedR andC. Here R (C), the compressive (tor-

sional) stress level, is defined as the ratio of the appliedstress to the fracture strength under compression (tor-sion) load in the thickness (z) direction. We takeR tohave values 0.02, 0.04, 0.06, 0.2, 0.4, 0.6, 0.7 and 0.8,andC to be 0.2, 0.4, 0.6 and 0.8. Results of the biaxialtesting are shown in Table II. Fig. 2 shows the relationbetween the experimental data and the prediction ofTsai-Wu theory. The prediction is in good agreementwith the experimental results.

Fig. 3 shows that the fractured specimens followingpure z directional compression, torsion, tension andx (or y) directional compression tests. The specimen(Fig. 3a) subjected to compressive load in the thicknessdirection suffered relatively brittle fracture at approx-imately 45◦ angle in shear mode. Plastic deformationand interlaminar fracture due to twisting might be visi-ble in Fig. 3b. In compression fracture in thex direction,we can observe the surface cracking through visiblecrack propagation caused by transverse compression asin Fig. 3c. This results from microbuckling of the testspecimen due to the same loading and fiber directions.

TABLE I Uniaxial test results of 3 directions

Tension Compression Torsion

Strength (MPa) 48 366 62

Figure 1 Configuration of test specimen.

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TABLE I I Biaxial test results under various load ratios

Biaxial ratio 0.02 0.04 0.06 0.2 0.4 0.6 0.7 0.8(R or C)

Torsional strength 68.3 73.7 77.0 82.8 85.8 86.0 79.5 70.6for R (MPa)

Compressive strength — — — 363.4 360.1 354.6 — 345.9for C (MPa)

R: Applied compression/compression strength.C: Applied torsion/torsional strength.

Figure 2 Comparison between Tsai-Wu theory and experiment underbiaxial loading.

Under purez directional tensile loading, the fracturesurface shown in Fig. 3d is similar to that for torsionalloading, but nonlinear behavior such as plastic deforma-tion was not found. In addition, biaxiality fracture underlow compressive stress levels withR values of 0.02,0.04, and 0.06 tends to show rupture like a pure tor-sion fracture. By contrast specimens for whichR is 0.7and 0.8 tend to fail like the purez directional compres-sion case. The fracture behavior of textile compositesunder high compressive stress levels therefore causescompressive force rather than torsion. For intermediatestress levels in whichR is 0.2, 0.4 and 0.6, there is notendency for torsion fracture because of plastic defor-mation. Under torsional stress levels withC= 0.2, 0.4,0.6, and 0.8, specimens tended to fracture like the purecompression fracture case in the thickness direction.

To characterize the fracture morphology of plainwoven composites under biaxial loading, the fracturedand delaminated surfaces were observed using SEMfractography (Fig. 4). Arrows in the figures denote thepropagation direction of the load. Fig. 4a shows a lowermagnification photograph under purez directional tor-sion, i.e., the compressive stress levelR= 0. The de-lamination fracture regions between fabric fiber bun-dles and resin are visible in Fig. 4a. From this figure wecan observe the matrix fracture induced by a pure tor-sional load and the absence of fiber breakage. In Fig. 4bfor R= 0.06, fiber breakage, the compression fracturephenomenon, and hackles resulting from the sliding offibers due to shear are all visible. Several resin richregions and grooves between fibers and matrix—so-

Figure 3 Fractured specimens after uniaxial test.

called channels—are also present in Fig. 4b. This phe-nomenon can be explained as follows. The lower filltow of fiber bundles moves to contact the upper warptow because of the compressive force. This contact offiber warp/fill tows increases the shear force applied tothe material. Therefore the fibers breakage occur in theregion adjacent to the contacted warp and fill tows. Theaction of the load, which is normal to the fibers, alsogenerates the channel in the matrix [5]. It is seen inFig. 4c, which is a higher magnification photograph ofFig. 4b, that a torsional load results in debonding of thefiber/matrix interface and fiber breakage. This impliesthat the stresses on fiber bundles are redistributed dueto the combined compressive and torsional loadings. Atypical fractograph for glass/epoxy woven compositefor C= 0 is shown in Fig. 4d. This photograph showsseveral fiber breakages in the intersection region of thefiber bundles. It is found that most of the compres-sive load on textile composites is applied to the fiberbundles, and the resulting fiber fractures appear in theintersection region of fiber bundles.

From macro- and microscopic fractograph analysis,the failure mechanisms of textile composites can bededuced. Fig. 5a illustrates breaking of the warp and filltows under pure compressive loading. The compressiveforce causes fiber breakage of a warp tow constrained

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Figure 4 SEM fractographs of glass/epoxy woven laminates.

by two fill tows. Also, torsion under biaxial loading hasa close relation to the formation of pockets of pure resin.By increasing the compression load, the shear force isapplied as a compressive force in the inclined warptow as shown in Fig. 5b. Internal forces then result infiber breakage in some region of the warp and fill tows.The larger the compressive force applied, the greaterthe fiber breakage of the warp/fill tows. For values of

more thanR= 0.7, the test specimen tends to fail duewholly to the compressive force, as stated above. Underpure torsion, matrix cracking propagates parallel to thematrix because of the influence of shear force. Fabriccomposites are, therefore, liable to fail as the torsionalload increases.

In summary, the biaxial strength of glass woven com-posites is much more sensitive to the magnitude of the

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Figure 5 Fracture mechanism under compression and biaxial loadings.

applied compressive force than to torsion. The fracturemorphology of fabric based materials under pure tor-sion is characterized by matrix cracking, and underpure compression by the fiber fracture of warp/fill tows.Also, the fractured surfaces under biaxial loading carry

such characteristic features as fiber breakage, channelsand hackles. The failure mechanism under pure com-pressive loading distinguishes the breaking of warp/filltows due to the compressive force. The transverse shearforce in biaxial loading acts like a strong internal forcein combination with the compressive force.

AcknowledgmentThis research is supported by the Korea Science andEngineering Foundation (KOSEF) under Grant No.981-1003-021-2. The authors are grateful for this fi-nancial support.

References1. B. Z . J A N G, “Advanced Polymer Composites” (ASM Interna-

tional, 1994) p. 104.2. T . I S H I K A W A andT.-W. C H O U, J. Mater. Sci.17(1982) 3211.3. K . W O O andJ. D. W H I T C O M B, J. Comp. Mater.30(9) (1996)

984.4. J. W H I T C O M B andK . S R I R E N G A N, Composite Structures34

(1996) 13.5. L . A R C A N, M . A R C A N andI . M . D A N I E L , in “Fractography

of Modern Engineering Materials: Composites and Metals, ASTMSTP 948,” edited by J. E. Masters and J. J. Au (1987) p. 41.

Received 16 Juneand accepted 9 August 1999

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