Assessment of initiation criteria used in interlaminar fracture tests of composites

Finite element (FE) simulations of delamination growth in mode I double cantilever beam (DCB) and mode II end-notched flexure (ENF) specimens were conducted in order to evaluate the non-linearity (NL) and 5% offset or maximum load (5%-Max) criteria. The results showed a good performance of the 5%-Max criterion, while the NL criterion was inadequate for the ENF test. However, it was also found that large process zones in common ENF specimens may lead to significant toughness underestimations. In order to obtain accurate results it was necessary to increase the starter crack length and the support span.