Fatigue damage and fatigue life diagrams of a carbon/epoxy cross ply laminate aged by hygrothermal exposure

This study concerns the effects of hygrothermal aging on failure of carbon/epoxy composites subjected to fatigue loading. AS4/8552 laminates with stacking sequence [0(2)/90(2)](s) are exposed to cycles of 4 hat 160 degrees C followed by 8 h of 80% average relative humidity at 70 degrees C, for a total exposure time of 2880 h. Fourier transform infrared (FTIR) spectroscopy, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM) and change in weight, before and after hygrothermal exposure, are used to characterize the composites. Samples of plain epoxy 8552 are also exposed to the same aging conditions and characterized by optical microscopy (OM), FTIR spectroscopy and weight change. Stress-controlled tension-tension fatigue tests are then conducted using a stress ratio R = 0.1 and frequency of 5 Hz. The effects of hygrothermal aging on fatigue are interpreted by fatigue life diagrams as well as by studying evolution of ply crack density and delamination.