Evaluation of damage evolution of impacted composite laminates under fatigue loadings by infrared thermography and ultrasonic methods

It is widely known that damage induced by low-velocity impact events will significantly affect the damage tolerance and integrity of composite structures. This study investigates the damage evolution and failure mechanism of impacted laminates under fatigue loadings using the infrared thermography and ultrasonic methods. Low-velocity impact damage was introduced by a drop weight with a hemispherical impactor, and compression-after-impact (CAI) tests were conducted to determine the residual compressive strength. Four stress levels were selected to conduct compression-compression fatigue tests with a stress ratio R = 10 based on the static CAI strength. During fatigue tests, the surface temperature variations were monitored online by an infrared camera, and the delamination damage evolution was observed using ultrasonic C-scan methods. The thermal images, delamination contours and stiffness degradation were analyzed. The maximum temperature increase and the stiffness degradation show the characteristic with three stages “rapid-slow-rapid”. For evolution of damage area, two stages were observed. The damage area extends from the impact indentation to two outer free edges in the perpendicular direction until the final collapse of the specimen