Fatigue Model for Fiber-Reinforced Polymeric Composites for Offshore Applications

摘要

A model based on cumulative damage has been developed for predicting the fatiguelife of fiber-reinforced polymeric composites used in offshore environments. The model incorporates applied stress, stress amplitude, loading frequency, residual tensile modulus, and material constants as parameters. The model is verified with experimental data from a glass fiber/vinyl ester composite fatigued under different environmental conditions. Specimens are subjected to tension/tension fatigue at four levels of applied maximum tensile stress at two different frequencies while exposed to air, fresh water, and sea water at 30degrees C. Both the residual mechanical properties at specified loading cycles and the number of cycles at which the specimens fail are measured. For the material used in this study, the loss in mechanical properties (residual tensile strength and modulus) in salt water is approximately the same as that in fresh water, and that the fatigue life of the composite in these aqueous environments is shorter than that in air. The S-N curves for specimens subjected to the three environments have approximately the same slope, suggesting that the failure mechanism does not change with these environments. Furthermore, specimens that are fatigued at a lower frequency failed at a lower number of cycles than those tested at a higher frequency. Numerical analysis is performed using the fatigue experimental data to determine the material constants of the composite. The model agrees well with the experimental data, and it can be used to predict the fatigue life of polymeric composites subjected to an applied load in different environments or the residual tensile modulus after a number of loading cycles.