This paper describes a program that has been developed for stress corrosion cracking testing of rockbolt steels both in-situ in underground mines and in a controlled laboratory environment. Two types of double bent beam specimen have been used, a pin loaded specimen stressed to just above the yield stress, and a bar loaded specimen stressed almost to the ultimate tensile strength. Testing of the specimens in an acidified 3.5% sodium chloride solution containing hydrogen sulfide has verified the efficacy of both designs.
Underground coal mines rely extensively on effective ground control through the use of resin-grouted rock bolts. Consequently, any failure of rock bolts drastically decreases the stability of the roof, which has serious implications for both mine operations and personnel safety. In recent years it has been recognized that a substantial proportion of rock bolt failures, at least in the Australian context (Crosky et al. 2002, 2004, 2012), have resulted from stress corrosion cracking (SCC). It is considered that strata movement disrupts the protection provided by the grouting resin, exposing the rock bolts to groundwater. The combined action of the corrosive environment provided by the ground water, and installation and/or service-induced stresses, causes cracks to initiate and grow, eventually leading to catastrophic failure of the rock bolts.
SCC cracks can initiate at surface flaws that either preexist, or are formed during service by corrosion, wear and other related processes. These cracks then propagate by either a transgranular or intergranular model. In some cases both transgranular and intergranular crack propagation can occur.
As SCC is a delayed failure process, the cracks in the material initiate and propagate at slow rates (for example 10–9 to 10–6 m/s) (Jones & Ricker 1992) until the stress in the remaining ligament of metal exceeds the fracture strength. The main stages leading to SCC failure are as follows (Jones & Ricker 1992):