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 contradicts the prediction of the maximum hoop stress criterion, which has 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 mode-II fracture toughness can be accounted for by including the contributions 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 still controlled by the orientation of the maximum hoop stress; however, the critical energy-release rate should include contributions from both 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys