Effects of low-energy impact and thermal cycling loadings on fatigue behavior of the quasi-isotropic carbon/epoxy composites

Effects of low-energy impact loading and thermal cycling on fatigue behavior of carbon fiber reinforced epoxy (carbon/epoxy) laminates are examined. A low-energy of 0.62 Joules was adopted to impact carbon/epoxy laminates prior to thermal cycling exposure and fatigue test. The temperature ranged between 60 and −60 °C for thermal cycling and the stress ratio of 0.1 with a frequency of 3 Hz for fatigue loading were used. Impact performances were tested on the virgin specimens and the thermal-cycling exposure specimens. Residual tensile strength and fatigue tests were performed on the laminate composites after being subjected to thermal cycling. The relationship between tensile strength reduction and fatigue performance after thermal cycling was investigated. Stiffness degradation during fatigue testing was monitored; the differences in stiffness for these three composites (virgin specimens, low-energy impacted specimens, low-energy impacted and thermal-cycling exposure specimens) were compared and the coupling effects of low-energy impact and thermal fatigue were studied. Furthermore, the S-N curves were also plotted and the variation was compared on the aforementioned three composites. SEM was used to examine the difference in fracture morphologies on the composites with and without suffering low-energy impact and thermal fatigue.