A hysteresis-based damage parameter for composite laminates subjected to cyclic loading

Polymer matrix composites are increasingly being utilized in structural aerospace applications because of their light weight and high specific stiffness and strength. In cyclically loaded applications, structural monitoring of the composite's damage progression can help predict final fatigue failure. Existing measures of fatigue damage in composites include stiffness degradation, crack propagation/strain energy and dynamic parameters like frequency response. The damage tolerant approach requires prior assumptions of the dominant damage mechanisms. Also, dynamic characterization, in laboratory tests, results in an interruption in cyclic loading which can change the test conditions.; In this study, cyclic hysteresis is used in a new damage parameter D' that is related to damage progression. D' is shown to be more sensitive than stiffness degradation, independent of assumptions on the type and location of damage, and is determined from the loading history without additional apparatus. Simulation, incorporating classical laminate plate theory, is used to establish the viability of hysteresis-based fatigue damage measure in composites by studying the effect of different test parameters and postulated damage mechanisms.; Hysteresis data on composites in the literature is scarce and is often monitored at intervals. In the present work, cyclic hysteresis is monitored continuously and determined quantitatively for each cycle. To achieve this, a conventional servo-hydraulic fatigue testing system is modified, including the incorporation of new custom code written to perform command and cyclic data acquisition. The primary specimens fabricated and tested are laminated [0/90] E-glass/epoxy woven composites that are notched with a central hole. Other specimens vary the lay-up and materials that included unidirectional E-glass and carbon fibers.; Hysteresis is computed from load and displacement data and data smoothing is necessary to reduce the inherent noise. The calculated damage parameter D' shows an approximately linear increase with cycling that transforms to an exponential increase just before final fatigue failure. The triple region and the adjusted life ratio models are proposed to fit D' as a function of load cycles. The fit coefficients depend on the composite layup and material. Good predictions for fatigue life are obtained using D'.