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

Abstract

Rate dependence of mode I interlaminar fracture toughness of interlayer-toughened carbonfiber/epoxy laminates (T800H / 3900-2, Toray) was studied over a very wide range of loading rate from quasi-static to impact. A SHPB (Split Hopkinson Pressure Bar) system using a ramped incident stress wave was employed to estimate the accurate fracture toughness at high loading rates. Finite element results proved the validity of a simple estimation formula, which is based on static beam theory, for impact fracture toughness. Experimental results showed that the fracture toughness is strongly rate-sensitive. The fracture toughness decreased stepwise interposing a transition region where the fracture toughness dropped rapidly with increasing loading rate. The decrease of fracture toughness in the transition region was larger in the initial stage of crack growth than in the propagation stage. The crack path was inside the interlayer region in the initial stage, but it shifted to near the interface between the interlayer and carbon-fiber/epoxy lamina in the propagation stage. Rough fracture surfaces including microcracks of resin were observed at low loading rates, but relatively smooth fracture surfaces were observed at high loading rates. The transition of fracture characteristics approximately corresponded to the transition of fracture toughness.