THE ROLE OF BIAXIAL STRESSES IN DISCRIMINATING BETWEEN MEANINGFUL ANDILLUSORY COMPOSITE FAILURE THEORIES

The irrelevance of most composite failure criteria to conventional fib er-polymer composites is claimed to have remained undetected primarily because the experiments that can either validate or disprove them are difficult to perform. Uniaxial tests are considered inherently incapa ble of validating or refuting any composite failure theory because so much of the total load is carried by the fibers aligned in the directi on of the load. The Ten-Percent Rule, a simple rule-of-mixtures analys is method. is said to work well only because of this phenomenon. It is stated' that failure criteria can be verified for fibrous composites only by biaxial tests. with orthogonal in-plane stresses of the same a s well as different signs. because these particular states of combined stress reveal substantial differences between the predictions of lami nate strength made by various theories. Three scientifically plausible failure models for fibrous composites are compared, and it is shown t hat only the in-plane shear test (orthogonal tension and compression) is capable of distinguishing between them. This is because most theori es are 'calibrated' against the measured uniaxial tension and compress ion tests and any cross-plied laminate tests dominated by those same s tates of stress must inevitably 'confirm' the theory.