RATE-DEPENDENT MODE II INTERLAMINAR FRACTURE BEHAVIOR OF CARBON-FIBER/EPOXY COMPOSITE LAMINATES

Abstract

Effects of loading rate on mode II interlaminar fracture behavior of unidirectional CF/conventional-epoxy (T300/2500, Toray) and CF/toughened-epoxy (IM600/133, Toho Rayon) composite laminates were investigated over a wide range of loading rate from quasi-static to impact at room temperature (displacement rate, δ=10-7-101m/s). A newly developed experimental method using the SHPB (Split Hopkinson Pressure Bar) technique and the ENF (End Notched Flexure) specimen was employed for measuring the accurate fracture toughness at very high loading rates. The mode II fracture toughness at the onset of crack growth showed positive rate dependence (fracture toughness increased with increasing loading rate) at lower loading rates, while it showed negative rate dependence (fracture toughness decreased with increasing loading rate) at higher loading rates; there existed a local maximum value of fracture toughness at intermediate loading rates. The impact fracture toughness was about 13 and 29% lower than the local maximum value for the conventional epoxy composite and toughened epoxy composite, respectively; the toughened epoxy composite was more sensitive to the loading rate than the conventional epoxy composite. Microscopic observation showed that the debonding of fiber/matrix interface was dominant at lower loading rates and that the cohesive fracture of matrix resin was dominant at higher loading rates. The transition point of microscopic fracture morphology approximately coincided with the local maximum point of macroscopic fracture toughness. In addition, the load-displacement relation was non-linear just before the onset of crack growth at lower loading rates but almost linear up to the maximum point at higher loading rates.