Characterization of mode II delamination behaviour of poplar plywood and LVL

The aim of this work was to characterize the interlaminar fracture toughness of poplar laminate veneer lumbers (LVL) under mode II loading. To this end, the Four-point End Notched Flexure test (4ENF) was chosen, and crack propagation was continuously monitored during the test using a stereo-correlation set-up. The R-curve was then obtained by a compliance calibration method, expressing the evolution of the critical strain energy release rate as a function of the crack length for the tested specimens. Two different interfaces were investigated, with the interfacial fibre orientations of 0°/90° and 0°/0°. The results show that the interlaminar fracture toughnesses are about 2.2 and 2.5 kJ/m2 for interfaces 0°/0° and 0°/90°, respectively. These values are higher than those of carbon-epoxy laminates. Fracture surface morphologies were analysed to understand the effect of fibre orientation on the fracture toughness. Finally, the fracture toughness was calculated with a compliance-based beam method, and good agreement was found between the two methods.