On crack-initiation conditions for mode I and mode II delamination testingof composite materials

An investigation of the influence of pre-crack conditions on the fracture b ehavior of a unidirectional vinyl-ester/E-glass composite has been undertak en. For this investigation, recommendations from three different testing st andards for preparing double-cantilever beam (DCB) specimens were followed. For this purpose two sets of specimens were tested: a set of specimens wit h a blunt starter crack and the one with a fatigue pre-crack (i.e. sharp). The two sets of specimens exhibited similar fracture behavior during testin g, with the crack propagation being stable and not influenced by the differ ent pre-crack conditions. As expected, propagation values of the strain-ene rgy release rate were unaffected by the pre-crack conditions and, owing to the extensive fiber bridging, propagation (G(Ic)(prop)) values were signifi cantly larger than initiation (G(Ic)(ini)) values. However, the influence o f the pre-cracking on the initiation Values of the G(Ic) was noticed. A sim ilar investigation was also conducted for mode II, with ENF specimens. No i nfluence of the pre-crack condition on the fracture toughness of the compos ite under the mode II loading was noted. Scanning electron micrographs were taken to provide additional explanations for the observations made during the testing. (C) 2000 Elsevier Science Ltd. All rights reserved.