Rate Dependence of Mode II Interlaminar Fracture Toughness of Interlayer-Toughened Carbon-Fiber/Epoxy Laminates.

Abstract

Rate dependence of mode II interlaminar fracture toughness of interlayer-toughened carbonfiber/epoxy laminates (T800H/3900-2, Toray) was investigated over a wide range of loading rate from quasi-static to impact (displacement rate, δ=0.01 mm/min-15m/sec). The fracture toughness was measured by using the ENF (End Notched Flexure) specimen with a universal testing machine at quasi-static loading rates and with a SHPB (Split Hopkinson Pressure Bar) system at impact loading rates. The load-displacement relation was non-linear below the maximum load point at all loading rates. Hence, the fracture toughness was calculated with considering the microscale crack growth preceding the macroscale unstable crack growth, which was enabled both by using a ramped incident stress wave and by estimating the equivalent load from the surface strain of the specimen at impact loading rates. The fracture toughness, wholly, showed positive rate dependence (i. e. the fracture toughness increased with loading rate). However, it, partly, showed negative rate dependence. The crack path was near the boundary region between the base lamina and the interlayer at quasi-static loading rates. This type of fracture was characteized mainly as the interface debonding between the carbon fiber and the epoxy resin, resulting a rough fracture surface. However, the crack path was inside the interlayer at impact loading rates. This type of fracture was characterized as the failure of the interlayer resin, resulting a smooth fracture surface. It was supposed that the transition of the frature characteristics might cause the rise of the fracture toughness at impact loading rates.