A FRP Cumulative Damage Model and Its Application to Fatigue Life Evaluation of Composite Laminates
Li Yazhi, Zhang Kaida, Zhang Boping
(Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China)

Abstract: A cumulative damage model is proposed, which can be used to consider the influence of in plane bi-axial stresses and mean stress on the fatigue behavior of unidirectional laminates. With only minimum typical fatigue tests at certain stress ratio for unidirectional laminates, it is possible to predict the fatigue life of multi-directional laminates subjected to cyclic stresses of different stress ratios. This would significantly reduce test costs for laminate structures. The procedure and the correspondent computer codes have bee n developed to evaluate the residual strength and fatigue life of multi-directional laminates. For T300 /QY8911 Carbon/Bismalimide composites, three kinds of typical tests (fatigue of £Û0£Ý16 tension-tension, £Û90£Ý16 tension-tension and £Û0/90£Ý4s shear-shear) are performed to obtain the S-N curves for unidirectional laminates, which are used as input for the above procedure. Both tests and calculations are performed to get the fatigue lives for four kinds of multi-directional laminate coupons with different tensile cyclic stress applied. The pre diction lives agree well with the experimental results. It is also realized that an analytical model is essential to simulate the effect of stacking sequence and the subsequent inter-lamina stresses on fatigue behavior of laminates. In view of the resultant fatigue life, a relative load/strength ratio technique is introduced in the present analysis to diminish this effect. 
Keywords: fatigue life evaluation, damage model, multi-direct ional laminate.