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

Previously, the work factor approach was applied to determine fracture toug hness, G(c) of unidirectional graphite/epoxy CLS specimen, and it is requir ed to apply the approach to determine G(c) of multi-directional CLS specime n. In this work, the elastic work factor, eta (el) form of multi-directiona l CLS specimen was calculated numerically as a function of delamination len gth in order to investigate stacking sequence effect on eta (el) The calcul ated eta (el) was applied experimentally to determine fracture toughness, G (c) of multi-directional ([0/+/-45/0](s)/[0(2)/+/-45(2)/0(2)], [0/+/-45/0]( s)/[0(2)+/-45(2)/90(2)](s), and [90(2)/0(2)](s)/[0(4)/90(4)](s) for the ap and the strap) graphite/epoxy composites. For each case, the G(c) values de termined from the eta (el) approach were compared with those determined fro m the compliance method. Numerical results show that eta (el) is not affect ed by the fiber angles of plies, and eta (el) is linearly related to delami nation length. It was found from the experimental results that G(c) determi ned from the lie, approach is in a maximum 15% range of difference with tha t determined from the compliance method for each case. It was also found fr om the SEM examination that delamination propagated irregularly through int erfaces of lap and strap (8th and 9th plies) for [90(2)/0(2)](s)/[0(4)/90(4 )](s) case while no delamination transfer was observed for [0/+/-45/0](s)/[ 0(2)/+/-45(2)/0(2)](s) and [0/+/-45/0](s)/[0(2)/+/-45(2)/90(2)](s) cases.