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 consists 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 direction [1]. While this property appears when the loading axis coincides with the principal stress directions in the isotropic plane, it is of interest to study the failure locus in this isotropic plane of weave for off-axis modes of loading. It is the purpose of this paper to study the general case of failure mode in the isotropic strong-plane of a woven fabric composite when the loading direction of the lamina subtends an arbitrary angle with the principal material directions. Interesting results were derived indicating that for an angle of off-axis loading of the order of theta = +/-15 degrees this quasi-isotropy is maintained. However, for larger angles of off-axis loadings varying between theta = +/-15 degrees and theta = +/-(90-15)degrees the discrepancies from isotropy are mild with the highest at theta = +/-45 degrees. The study was based on the principles holding for the elliptic paraboloid failure surface [2]. A comparison with the results derived from the same tenser polynomial criterion with the assumption of validity of the 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 the respective isotropic value, suggests the validity of the simple relationship 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 woven-fabric composites in the (sigma(1), sigma(2))-transverse isotropic plane present, in general, a weak resistance to failure in the compression-compression quadrant and a strong one in the tension-tension quadrant, where, also, the discrepancies between the results of the (EPFS)- and the Tsai-Hahn criteria are stronger. This suggests a convenient way to test the validity of these criteria by simple biaxial tests, which are easy to perform and may give a decisive answer about the superiority of any of the as yet suggested different versions of failure tenser criteria.