Mode III Fracture Toughness Measurement Using the Transverse Shear Cracked Plate Specimen

Background Nowadays there is no currently accepted test specimen to measure the mode III fracture toughness of rigid monolithic and composite materials. Objective To propose and evaluate a new and simple test specimen called the Transverse Shear Cracked Plate to characterize the mode III fracture toughness of an epoxy resin. Methods The proposed specimen consists of a cantilever 38 mm x 38 mm square plate with a centered crack subjected to a transverse deflection. Specimens with two thicknesses (2.5 and 4 mm) and six crack lengths were tested, 72 in total. Finite element analysis was performed to calculate the energy release rates. Results A linear force-displacement behavior was commonly observed prior to fracture. The crack extension was typically longitudinal, uniform and unstable, splitting the specimen in halves. Radial patterns of prominent twist hackles that tend to curve toward outside the fracture origin were observed in the fractured surfaces and river markings appeared near one specimen edge as indication of mixed mode I/III loading. Analysis of the energy release rates showed a dominant parabolic-like mode III component along the crack front, although minor mode I and II components appeared near the edges. Conclusions Except for two cases that exhibited fragmentation, there is no apparent effect of the crack length on the fracture toughness, averaging 2.04 and 2.15 kJ/m(2) for 2.5 mm and 4 mm thick samples. Based on the simulations and measurements, the proposed test specimen is suitable to measure mode III fracture toughness.