Authors: Elias, E; Vandermaat, D; Craig, P; Chen, H; Crosky, A; Saydam, S; Hagan, P; Hebblewhite, B

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Elias, E, Vandermaat, D, Craig, P, Chen, H, Crosky, A, Saydam, S, Hagan, P & Hebblewhite, B 2013, 'Metallurgical examination of rockbolts failed in service due to stress corrosion cracking', in Y Potvin & B Brady (eds), Proceedings of the Seventh International Symposium on Ground Support in Mining and Underground Construction, Australian Centre for Geomechanics, Perth, pp. 473-483.

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Abstract:
In recent years, increasingly higher strength grades of steel have been used to meet the increasingly demanding requirements placed on rockbolts in underground coal mines. An unfortunate consequence of this has been the occurrence of premature failure of the rockbolts due to stress corrosion cracking. Stress corrosion cracking is a complex phenomenon which involves slow, progressive crack growth under the application of a sustained load (either residual or applied) in what is usually only a mildly corrosive environment. The failure occurs in a brittle manner below the ultimate tensile strength of the material. An understanding of the mechanisms associated with rockbolt failure is imperative in order to develop effective strategies that will reduce the incidence of failure. This paper reports on a forensic investigation directed towards this goal. The work involved fractographic analysis of failed rockbolts retrieved from underground coal mines, identification of the corrosion product in the stress corrosion cracks, and analysis of mine water from affected areas in the mines. It was found that two distinctly different types of mine water were associated with the stress corrosion cracking failures, these being high pH and near neutral pH. The fractographic analysis showed that the micro-mechanism of stress corrosion cracking was quite different for the two types of minewater. Different types of corrosion product were observed within the stress corrosion cracks produced from each of the two different minewaters, providing further confirmation that the stress corrosion cracking mechanism was different in the two cases.

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