Determination of fracture toughness, Gc of graphite/epoxy composites from a cracked lap shear (CLS) specimen

Previously, the work factor approach was applied to determine fracture toughness, Gc of unidirectional graphite/epoxy CLS specimen, and it is required to apply the approach to determine Gc of multi-directional CLS specimen. In this work, the elastic work factor, ηel form of multi-directional CLS specimen was calculated numerically as a function of delamination length in order to investigate stacking sequence effect on ηel. The calculated ηel was applied experimentally to determine fracture toughness, Gc of multi-directional ([0/±45/0]s/[02/±452/02]s, [0/±45/0]s/[02±452/902]s, and [902/02]s/[04/904]s for the lap and the strap) graphite/epoxy composites. For each case, the Gc values determined from the ηel approach were compared with those determined from the compliance method. Numerical results show that ηel is not affected by the fiber angles of plies, and ηel is linearly related to delamination length. It was found from the experimental results that Gc determined from the ηel approach is in a maximum 15% range of difference with that determined from the compliance method for each case. It was also found from the SEM examination that delamination propagated irregularly through interfaces of lap and strap (8th and 9th plies) for [902/02]s/[04/904]s case while no delamination transfer was observed for [0/±45/0]s/[02/±452/02]s and [0/±45/0]s/[02/±452/902]s cases.