On crack-initiation conditions for Mode I and Mode II delarnation testing of composite materials

An investigation of the influence of pre-crack conditions on the fracture behavior of a unidirectional vinyl-ester/E-glass composite has been undertaken. For this investigation, recommendations from three different testing standards for preparing double-cantilever beam (DCB) specimens were followed. For this purpose two sets of specimens were tested: a set of specimens with 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 testing, with the crack propagation being stable and not influenced by the different pre-crack conditions. As expected, propagation values of the strain-energy release rate were unaffected by the pre-crack conditions and, owing to the extensive fiber bridging, propagation (GIcprop) values were significantly larger than initiation (GIcini) values. However, the influence of the pre-cracking on the initiation values of the GIc was noticed. A similar investigation was also conducted for mode II, with ENF specimens. No influence of the pre-crack condition on the fracture toughness of the composite under the mode II loading was noted. Scanning electron micrographs were taken to provide additional explanations for the observations made during the testing