Abstract
Novel experimental methods were proposed to easily and precisely evaluate the mode I, II and I+II fracture toughness of polymer matrix composite laminates under low-velocity impact loading. A ramped incident stress wave was applied to suppress the flexural vibration of the specimen. A strain-based formula was employed to improve the accuracy of evaluation of the dynamic energy release rate. The validity of the proposed method was confirmed by the results of finite element analyses and dynamic experiments. The effects of loading rate and mode mixture on the interlaminar fracture behavior of carbon-fiber/epoxy composite laminates were investigated over a very wide range of loading rate from static to impact. The macroscopic fracture toughness clearly showed loading rate dependence regardless of mode ratio, and consequently the mixed mode fracture criterion varied with loading rate. The microscopic fracture morphology also showed loading rate dependence; cohesive fracture of matrix resin itself was a dominant fracture mechanism at higher loading rate, whereas debonding of fiber/matrix interface was a dominant fracture mechanism at lower loading rate.
