ENHANCED MODE-II FRACTURE-TOUGHNESS OF AN EPOXY-RESIN DUE TO SHEAR BANDING

The Brazilian disk specimen is used to measure the fracture toughness of an epoxy resin material under mode-I and mode-II loading conditions . It is found that even though the apparent fracture behavior seems to be brittle for both loading modes, the mode-II fracture toughness is more than three times higher than the mode-I fracture toughness. Such a substantial increase in fracture toughness under mode-II loading con tradicts the prediction of the maximum hoop stress criterion, which ha s been validated for most brittle fractures. Results of unidirectional tension and compression tests of the epoxy resin indicate that under the mode-II loading, shear banding can develop ahead of the crack tip before the brittle fracture occurs. The substantial increase of the mo de-II fracture toughness can be accounted for by including the contrib utions of the shear banding. A mechanical model was developed to study the effects of the shear band on the mode-II fracture toughness. This model suggests that the extension direction of a mode-II crack is sti ll controlled by the orientation of the maximum hoop stress; however, the critical energy-release rate should include contributions from bot h the maximum hoop stress and the shear band. The effect of different stress distribution profiles inside the shear band is studied with the model as well. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. Ali rights reserved.