Cyclic Hysteresis Evolution as a Damage Parameter for Notched Composite Laminates

Abstract

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′, 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′ 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′ 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.