Analysis of a rock bolt failed in service

Abstract

This paper describes a fractographic investigation of a rock bolt failed in service in a NSW colliery and relates the failure to service conditions. Optical microscopy revealed that the fracture surface contained a dark thumbnail shaped area 1.9 mm deep. The rest of the fracture surface was quite shiny. There was no necking or other evidence of plasticity. This fracture appearance is consistent with stress corrosion cracking (SCC) followed by fast brittle fracture. There were secondary cracks also indicative of SCC. SEM observation characterized the SCC surface. This failure analysis has indicated that rock bolts can fail in service in a brittle manner with no prior warning at stresses much lower than their ultimate tensile strength. This represents a new failure mode for a critical mine component, that is critical for mine safety. There is no prior experience with this failure mode, and laboratory work is needed to understand the failure mechanisms. With understanding of the failure mechanism, it will be possible to devise counter-measures.