Cyclic Hysteresis Evolution as a Damage Parameter for Notched Composite Laminates

In cyclically loaded polymer matrix composites, structural health monitoring is useful for detecting and tracking progressive damage. Existing approaches use stiffness degradation, crack propagation/strain energy change, or dynamic parameters such as frequency response. These approaches, however, depend on prior assumptions of the dominant damage mechanisms. In this study, a new hysteresis-based damage parameter, D-t, that is a measure of damage progression and failure is shown to be more sensitive than stiffness degradation and can be determined during cycling without the use of additional instrumentation. D-t was measured for cyclically loaded notched glass fiber laminates and was found to be useful as a measure of damage progression. Cyclic hysteresis strain energy dissipated at each cycle was monitored continuously without interruption. A conventional servo-hydraulic fatigue testing system was modified with the incorporation of new custom code for performing command and data acquisition on a cycle-by-cycle basis. In this fashion, progressive damage at each cycle was determined quantitatively in real time during each test. Hysteresis data were obtained from fatigue tests conducted on notched [0/90] E-glass/epoxy laminates in real time on a cycle-by-cycle basis and used to estimate failure. The damage parameter D-t exhibited an approximately linear increase with cycling followed by an exponential increase just before failure. Modeling this behavior allowed for the prediction of damage progression and residual life as a function of load cycles. Hysteresis-based damage parameters for other material configurations were also calculated and found to give good estimates for the cyclic life.