Frequency-Dependent Fatigue Damage Accumulation in Fiber-Reinforced Plastics

Abstract

The damage reversibility phenomenon in Fiber Reinforced Plastics (FRP), its interrelationship with frequency dependence, and its significance in fatigue life prediction are investigated through several series of static, constant-amplitude, and intermittent fatigue tests on specimens made from thick cross-ply and angle-ply woven roving Glass-Fabric-Reinforced Vinyl-ester (GFRV). It is shown that the viscothermoelastic behavior of this material causes it to undergo a severe, but reversible, thermal degradation during fatigue cycling. In order to fully comprehend various features of this type of degradation and its reversibility, the evolution of temperature and the material’s hysteretic and stiffness properties are carefully studied through a series of continuous and intermittent fatigue tests. The pace of thermal degradation is found to increase with increasing frequency for two reasons: (1) dissipated energy per cycle increases, and (2) trapped heat inside the material increases. The trend of S-N data is found to be strongly controlled by the loading frequency. This is especially true for the matrix-dominated lay-up, where not only does the lifetime of the specimen change with frequency by two or three orders of magnitude for certain stress levels, but also an endurance limit is introduced when the loading frequency is lowered. The presence of an endurance limit at lower frequencies is attributed to the change of the damage accumulation mode from mostly thermal to mechanical. Careful monitoring of dynamic compliance, dissipated energy per cycle, and temperature indicates that the evolution of these parameters during the fatigue life is both stress-dependent and frequency-dependent; thus the latter two parameters must be implemented in a realistic damage-accumulation model. The role of damage reversibility in fatigue life is shown to be automatically accounted for when loading frequency is implemented in the predictive model.