Low-velocity impact fracture behaviour of impact-resistant polymer matrix composite laminates under mixed mode loading

Abstract
The mixed mode (I+II) fracture behaviour of the newly developed 'interlayer-toughened' carbon-fiber/epoxy composite system, T800H/3900-2 (Toray), was studied over a wide range of loading rate including low-velocity impact region (displacement rate : 10^-6–10^l m/s). The MMF (Mixed Mode Flexure) specimen and SHPB (Split Hopkinson Pressure Bar) system were employed for measuring the mixed mode fracture toughness under impact loading. The experimental results showed that the mixed mode fracture toughness was sensitive to loading rate ; the impact fracture toughness was about 30-38 % lower than the static value. The microscopic fracture morphology was rather sensitive to loading rate ; the impact fracture surface was somewhat smoother than the static fracture surface. The effects of mode mixture and their rate dependence were discussed on the basis of the above and previous results by the authors. The validity of the estimation method for mixed mode impact fracture toughness was also discussed on the basis of computational results.