Theory and Computation of Nonlinear Damage Accumulation for Lifetime Prediction

Nonlinear damage accumulation is modelled for the lifetime prediction in order to capture the loading sequence effect, which is the influence of the chronological order of the loading values on the lifetime. The prediction results from the solution of the damage evolution equation, which is defined according to the theory of continuum damage mechanics and applied together with a cohesive zone model for structural adhesive joints. The damage model consists of a creep and fatigue damage part, both taking into account the influence of the mean stress and the load multiaxiality on the predicted time to rupture. The analytical investigation of the model shows the meaning of the model parameters and propose their identification by means of tests with static and constant amplitude loading. In order to capture the loading sequence effect by nonlinear damage accumulation, the fatigue damage part is enhanced with a factor, which influences the predicted lifetime due to variable amplitude loading in the case of pure fatigue damage, while the prediction for constant amplitude loading is unaffected. The influences of the enhancement on the predicted lifetime and the damage evolution are discussed. The comparison of lifetimes with numerical predictions proves the validity of the proposed approach.