Fatigue Life of Composite Structures: Analytical Models

The importance of length scales across which damage is created and it evolves in composite structures under fatigue loading is discussed. Qualitative characteristics of damage creation and growth during fatigue of composites are presented on the basis of experimental results. An approach to create analytical models for predicting cycles to failure is presented. It differentiates between small scale undetectable damage evolution which is captured by wear-out models and longer scale detectable damage which is characterized by special purpose analytical models. Two wear-out models are presented and are shown to be in good agreement with test results. A model to predict cycles to failure when the damage type and failure mechanism do not change is presented for constant amplitude and spectrum loading. Comparisons with tests show that accurate definition of damage and how it affects residual strength is necessary in order to develop a reliable analytical model. Application to uni-directional laminates under tension-compression shows good agreement with test results.