TABLE OF CONTENTS Failure theories, such as the Tsai Wu theory, can predict FPF of multidirectional laminates under any state of in-plane stress. Ultimate failure, however, is difficult to predict analytically on the basis of lamina properties because of nonlinear behavior and interaction of failure modes. To check or verify analytical predictions and to generate useful failure envelopes for design purposes, it is necessary to conduct extensive testing of composite laminates under biaxial states of stress. The application of a general in-plane biaxial state of stress, including normal tension and compression and shear components, poses a difficult problem in composite testing. Some of the basic requirements for a biaxial test specimen are:
A significant volume of the material must be under a homogeneous state of stress.
Primary failure must occur in the test section.
The state of stress in the test section must be known or easily determined without the need for secondary measurements or analysis.
It must be possible to vary the three in-plane stress components ( ? x, ? y, ? s) independently.
A variety of specimen types and techniques have been proposed and used for biaxial testing of laminates. They include the off-axis coupon or ring, the crossbeam sandwich specimen, bulge plate, rectangular plate under biaxial tension, and the thin-wall tubular specimen.
Uniaxial loading of a unidirectional lamina along a direction other than one of the principal axes produces a biaxial state of stress (Fig. 8.45). The state of...
