Stress corrosion cracking of rockbolts: a laboratory based approach utilising a controlled mine envrionment

摘要

Stress Corrosion Cracking (SCC) is a failure mechanism which has been observed to affect rockbolts installed in a number of Australian and overseas underground mines. SCC requires the application of a tensile stress on a material placed in an appropriately corrosive environment. SCC is characterised by the growth of fine fractures, originating from the surface of a material, which will continue to grow until they reach a critical length at which thematerial fails by mechanical overload.An extensive literature survey into rockbolt corrosion and SCC identified the need for a dedicated testing system for investigating SCC in full-scale rockbolt specimens. This thesis has focused on the design and development such a testing system. This system included a novel testing methodology, the Bending and Tension Loading Apparatus (BaTLA), which was housed within a Controlled Mine Environment (CME) laboratory, capable of recreating theatmospheric conditions present in an underground mine, as well as conduncting ‘accelerated’ tests using synthetic testing solutions. The development of this laboratory was based on the findings of a detailed field study conducted to characterise the geological, geotechnical, hydrological and atmospheric environment of two underground coal mines which have presented cases of rockbolt SCC. This newly developed laboratory was used to carry out an extensive testing program consisting of over 40, 000 hours worth of experimentation.This extensive testing program assessed the performance of both bending and tension loading in the BaTLA for investigating SCC in rockbolts. Both static and Slow Strain Rate (SSR) loading conditions were examined and each were found to be useful for investigating specific aspects of SCC. It was found that static testing provided the most appropriate means for examining mechanistic aspects of rockbolt SCC such as critical stress threshold, while SSR testing was found to be better suited to comparing material and environmental factors. A comparison of a number of steel grades and finishing treatments showed that lower strength 300 grade steel was far more resilient to SCC than both 1355 and HSAC 840 steel, and that surface treatments such as galvanising and grit blasting both provided notable improvements to a rockbolt’s SCC resilience.The testing program has also utilised small, representative coupon specimens. The coupon specimens used included the standard ASTM International G39 bent beam specimen and a slotted split pin specimen. A comparison of rockbolt steel grades finishing treatments using slotted specimens revealed that 1355 grade steel is much more susceptible to SCC than HSAC 840 grade steel, and that galvanising provided a significant improvement in SCC resilience. The ASTM G39 coupon specimens were used to examine critical stress thresholds for SCC and it was found that rockbolt steel may be susceptible to SCC at stresses as low as 580MPa. Secondary testing carried out by calculating the plastic strain experienced by rockbolts at failure by examining their rib profile supported this observation of a low critical stress threshold, showing that a large proportion of in-situ rockbolts experienced SCC failure at, or slightly above their yield strength.