Initiation of a mode-II interlaminar crack from an insert film in the end-notched flexure composite specimen

Two- and three-dimensional finite-element analyses have been carried out in order to investigate the initiation of an interlaminar crack in the end-notched flexure (ENF) composite specimen. It is believed that the current practice of using an insert film as a starting defect generates a blunt defect which creates a stress pattern that attracts the crack growth towards the fibre/matrix interface. Results from finite-element (FE) modelling support this concept. The FE results also indicate that with a sufficiently low bond strength at the fibre/matrix interface, the crack can be initiated from the interface instead of from the starting defect. The critical interfacial bond strength for transition of the location for crack initiation from the starting defect to the fibre/matrix interface is 14% of the matrix strength for the two-dimensional model, and 28% for the three-dimensional model. Although the two crack-initiation mechanisms are very different, the fracture surfaces generated are similar. The FE models indicate that after the crack initiation its further growth is always along the fibre/matrix interface. The above conclusions from the FE work were verified experimentally by using a glass-fibre/vinylester composite with a fibre volume fraction similar to that used in the models. The overall conclusions from the study are that the crack-tip bluntness plays an important role in the initiation of the interlaminar fracture in the ENF specimens, and that the interlaminar fracture toughness measured from the ENF specimen can depend strongly on the interfacial bond strength.