A modeling approach is presented that recognizes that the residual properties of composite laminates after any form of loading depend on the damage state. Therefore, in the case of cyclic loading, it is necessary to first derive a damage growth law and then relate the residual properties to the accumulated damage.
The propagation of fatigue damage in notched laminates is investigated. A power law relationship between damage growth and the strain energy release rate is developed. The material constants used in the model have been determined in independent experiments and are invariant for all the layups investigated. The strain energy release rates are calculated using a simple finite element representation of the damaged specimen. The model is used to predict the effect of tension-tension cyclic loading on laminates of the T300/914C carbon-fiber epoxy system. The extent of damage propagation is successfully predicted in a number of cross-ply laminates of the form (90i/0j)ns and the quasi-isotropic laminate (90/+45/-45/0)s. The dependence of damage on load amplitude and specimen size is also well described.
Residual strength is calculated as a function of damage dimensions for (90/0)s specimens using a stress-based failure criterion in conjunction with a Weibull dependence of the 0° ply strength on the volume under stress.