Cycle-based analysis of damage and failure in advanced composites under fatigue 1. Experimental observation of damage development within loading cycles

Damage development within loading cycles and the possibility of cycle-based modeling of fatigue damage evolution and failure in advanced composites are addressed in this work. In Part I of the work, the fatigue damage development in an unnotched composite laminate is studied by an advanced acoustic emission (AE) technique. The overall time history of the acoustic emission is recorded and separated into the emission from loading and unloading phases of the loading cycles. The source location histories and distributions of the AE events over the fatigue stress range are extracted and analyzed. Qualitatively different time histories and stress distributions for the emission generated during loading and unloading are observed and analyzed, for the first time. It is concluded that most of the emission from the unloading phase is due to internal friction between the crack faces and most of the damage is developed during the loading phase of the cycles. The location distribution of the fatigue damage is found to be fairly random to failure, indicating that the dominating fatigue fracture process in an unnotched laminate is scattered damage nucleation and accumulation. The time history of the overall damage evolution is found to exhibit the classical initial, steady, and final failure development stages. Within the loading cycles, the damage is developed over the entire stress range, with the substantial amount developed at stresses below the maximum fatigue stress. The latter observation sheds light on the reported effects of the loading cycle shape on the fatigue behavior and life of composites. The results of this study provide an insight into fatigue damage development in composites and constitute a fundamental basis for the development of a cycle-based model in Part 2 of this work.