Biaxial Failure of GRP—Mechanisms, Modes and Theories

Numerous failure theories have been proposed for GRP. Experimental observations under biaxial loading reveal scatter, multiple failure mechanisms and failure modes, which depend on material type and stress conditions. Biaxial failure theories can be represented as surfaces whose shape depend on both failure theory and the choice of single valued characteristic strengths. Experimental results suggest surfaces which are quadratic functions of the stresses and strengths and which often lie well inside the maximum normal stress boundaries. Failure theories which use a complex stress test to evaluate an interaction coefficient are generally unacceptable because the resulting surfaces are so sensitive to small changes in strength data. It is necessary to consider different classes of reinforcement (unidirectional, woven fabric, and random mat) separately in proposing failure theories. For unidirectional materials quadratic theories only appear to be well defined under tension-tension-shear conditions. For mats and fabrics adaptations of the early Norris theories fitted separately in each stress octant appear to be satisfactory. Failure theories only predict material failure as distinct from structural failure and should be treated with caution when applied to design.