Microstructure analysis of carbon-fiber-reinforced polymer laminates subjected to self-heating and fatigue strengthening under tension-tension fatigue loading

In this study, tensile-tensile fatigue testing of unidirectional (UD) carbon-fiber-reinforced polymer (CFRP) prepreg laminates was performed. Two types of specimens (open-hole and unopened-hole ones) were examined at different stress levels to elucidate the impact of fatigue damage on their static strength at different numbers of cycles. The effect of internal heat generation during fatigue on the life of unopened-hole CFRP laminates was analyzed by observing the changes in the microstructure of specimens. The results revealed that the difference in fatigue life between unopened specimens with and without cooling at 70% ultimate tension stress (UTS) level was about 88 times. In the open-hole CFRP laminates, the residual tension strength (RTS) after increases and then decreases with the number of fatigue cycles, and the interface debonding was a significant cause of fatigue strengthening. Open-hole specimens were shown to withstand continuous cycling under severe loads at 5% above their ultimate stress levels. Therefore, the fatigue damage extension of the open-hole laminates can be slowed down under intensive cycling at high stress levels, ensuring good fatigue performance of the material.