Cracking behaviours of layered specimen with an interface crack in Brazilian tests

Crack initiation and propagation from a pre-existing interface crack are of great importance in assessing the integrity and stability of layered rocks, rock/concrete systems and other layered structures. In this study, a numerical model, based on the inserted cohesive zone model (CZM) method, was developed to simulate the complex crack behaviours in layered discs with a preexisting interface crack, which were further validated by Brazilian tests. Different typical crack patterns were identified: interface cracks, wing cracks and secondary cracks. The results indicate that the interface cracks can be observed for disc samples under smaller loading angles while the secondary cracks can be observed for disc samples under larger loading angles. The formation of the typical cracks is also controlled by the interface strength. If 0 degrees < loading angle < 60% interface cracks can form for disc samples with lower interface strength. By increasing the interface strength of disc samples, interface cracks disappear while wing and boundary cracks can be found. If the interface strength further increases to a higher level, the secondary cracks will disappear. If the loading angle > 60 degrees, no interface crack can be found even with a very small interface strength, and the secondary cracks remain, regardless of the interface strength. The material match and confining pressure are also important factors that influence the crack patterns. The stress intensity factors of both tips of the pre-existing interface crack are also discussed. The energy release ratios and the corresponding phase angles for both crack tips are quite different, while the moduli ratio and the crack length-to-diameter ratios are noticeably larger.