The prediction of fatigue strength and life in composite laminates†

Abstract

A theoretical formulation for predicting fatigue strength and life in composite laminates is proposed. First, the Tsai‐Hill failure criterion is modified to become the extended Tsai‐Hill fatigue failure criterion for the prediction of fatigue characteristics of unidirectional laminates. The fatigue data of [0]16, [90]16, and [45]16 coupons are the base‐line data involved in the parametric study to estimate fatigue strengths and lives of other off‐axis laminates. The predicted values are found satisfactorily close to the empirical data. In association with classical lamination plate theory (CLPT), the extended Tsai‐Hill fatigue failure criterion can also be applied to quasi‐isotropic [0/45/‐45/90]2S and cross‐ply [0/90]4S laminates. Nevertheless the difference is larger for [±45]4S laminates because of the coupling effect of plastic deformation and friction among the layers of relative slippage. Next, based on Marco‐Starkey theory we develop the nonlinear cumulative damage as well as the reduced non‐damage theory associated with four damage parameters and the failure criterion of multi‐block fatigue loading to predict the life due to variable cyclic loadings. The difference in cycle fraction between two damage loading conditions is generated, so that we can easily shift to a different damage curve and predict the life due to a multi‐block cyclic loading. Fatigue tests of two‐step and multi‐block loading are also discussed. The measured lives are in good agreement with those predicted by our proposed four models.