FAILURE MODES OF WOVEN FABRIC COMPOSITES LOADED IN THE TRANSVERSE ISOTROPIC PLANE

Woven fabric undirectional laminae in the form of plain weave present an important advantage over typical fiber reinforced laminae, which co nsists that, at the strong weave plane, their failure locus approaches very close to a circle, thus suggesting that the failure mode on this plane is invariant and independent of the orientation of the fiber di rection [1]. While this property appears when the loading axis coincid es with the principal stress directions in the isotropic plane, it is of interest to study the failure locus in this isotropic plane of weav e for off-axis modes of loading. It is the purpose of this paper to st udy the general case of failure mode in the isotropic strong-plane of a woven fabric composite when the loading direction of the lamina subt ends an arbitrary angle with the principal material directions. Intere sting results were derived indicating that for an angle of off-axis lo ading of the order of theta = +/-15 degrees this quasi-isotropy is mai ntained. However, for larger angles of off-axis loadings varying betwe en theta = +/-15 degrees and theta = +/-(90-15)degrees the discrepanci es from isotropy are mild with the highest at theta = +/-45 degrees. T he study was based on the principles holding for the elliptic parabolo id failure surface [2]. A comparison with the results derived from the same tenser polynomial criterion with the assumption of validity of t he Tsai-Hahn relationship indicated significant discrepancies between the two criteria. However, the type of anisotropy valid for such woven -fabric materials, whose eigenangle omega approaches values close to t he respective isotropic value, suggests the validity of the simple rel ationship H-33 = 2H(11) for the tenser coefficients, which results to the particular value for H-12 = 0. Thus, for such materials the (EPFS) -criterion and (FTP)-criterion with the assumption that H-12 = 0 give similar results. Finally, it is derived that the failure loci for wove n-fabric composites in the (sigma(1), sigma(2))-transverse isotropic p lane present, in general, a weak resistance to failure in the compress ion-compression quadrant and a strong one in the tension-tension quadr ant, where, also, the discrepancies between the results of the (EPFS)- and the Tsai-Hahn criteria are stronger. This suggests a convenient w ay to test the validity of these criteria by simple biaxial tests, whi ch are easy to perform and may give a decisive answer about the superi ority of any of the as yet suggested different versions of failure ten ser criteria.