Evaluation of mode II interlaminar fracture toughness of composite laminates under impact loading

A simple experimental method was established in order to evaluate the mode II interlaminar fracture toughness of composite laminates under low velocity impact loading (impact velocity, V-0 < 20 m/sec). This method was essentially an impact version of the ENF (End Notched Flexure) test using SHPB (Split Hopkinson Pressure Bar) technique, where a slow-gradient ramped incident stress wave was utilized to reduce the kinematic effects on the behavior of the specimen and consequently the dynamic energy release rate was calculated by a simple estimation formula based on static beam theory and compliance method. The validity and limitation of this method were investigated on the basis of finite element results. The stress state and deformation mode of the specimen under impact loading were shown to be approximately equal to those under quasi-static loading when the incident stress wave of adequate amplitude and rise time was applied to the specimen. In addition, the estimation error of this method was shown to be reduced by calculating the energy release rate from the surface strain of the specimen. The experimental results by this method showed that the mode II interlaminar fracture toughness of carbon-fiber/epoxy composite laminates under impact loading was somewhat lower than that under quasi-static loading.