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Laboratory Based Investigation of Stress Corrosion Cracking of Cable Bolts

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Laboratory Based Investigation of Stress Corrosion Cracking of Cable Bolts Laboratory Based Investigation of Stress Corrosion Cracking of Cable Bolts Saisai Wu A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Mining Engineering Faculty of Engineering July 2018 THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Wu Other name/s: First name: Saisai Abbreviation for degree as given in the University calendar: PhD School: Mining Engineering Faculty: Engineering Title: Laboratory-Based Investigation of Stress Corrosion Cracking Of Cable Bolts Abstract 350 words maximum Premature failure of cable bolts due to stress corrosion cracking (SCC) in underground excavations is a worldwide problem with limited cost-effective solutions at present. To determine the cause and mechanism of SCC, identify potential technologies and eventually avoid catastrophic failure of cable bolts, a two-step methodology was implemented: (i) a long-term test using groundwater collected from underground mines, and (ii) an accelerated test using an acidified solution. Laboratory experimentation on both representative coupon and full-size cable bolt specimens was conducted. In the long-term tests, simulated underground environments were recreated in ‘corrosion cells’ which contained a newly designed cable bolt coupon together with a mixture of groundwater, coal and clay, to measure the potential for developing SCC. The incidence of SCC failures was not related to groundwater alone. Geomaterials in the corrosion cells accelerated the corrosion of cable bolts by increasing the concentrations of total dissolved solids and electrical conductivity of the water. Following this, an acidic solution containing sulphide, synthesised based on the chemical properties of groundwater from twelve Australian underground mines, was used as the testing solution for the accelerated tests. A coherent failure mechanism for SCC crack initiation, propagation and catastrophic failure of cable bolts was developed. Hydrogen embrittlement was determined to be the dominant mechanism for SCC. SCC only occurred when the environment could promote atomic hydrogen diffusion into the cable bolt, with the crack propagation rate being determined by the hydrogen diffusion rates. SCC resistance of full-size cable bolts was examined using designed tension loading apparatus and periodically increasing strain rate loading mechanism. The cable bolts with more wires took longer to fail. The cable bolts with plain wires provided greater resistance to SCC than those with indented wires. Galvanisation improved the resistance of cable bolts against SCC. The methodologies developed here can be applied to study SCC in other reinforcement materials and the results of this thesis can be used to develop guidelines to assess the environments causing susceptibility to SCC of high-carbon steel and set optimal support regimes in different geological and environmental conditions. Declaration Regarding to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). Signature Witness Signature Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research FOR OFFICE USE ONLY Date of completion of requirements for Award: Originality statement ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed: Date: Copyright statement ‘I hereby grant the University of New South Wales or its agents the right to archive and to make available my dissertation or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this dissertation or dissertation. I also authorise University Microfilms to use the 350 word abstract of my dissertation in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my dissertation or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my dissertation or dissertation.' Signed: Date: Authenticity statement ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my dissertation. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ Signed: Date: Abstract Premature failure of cable bolts due to stress corrosion cracking (SCC) in underground excavations is a worldwide problem with limited cost-effective solutions at present. To determine the cause and mechanism of SCC, identify potential technologies and eventually avoid catastrophic failure of cable bolts, a two-step methodology was implemented: (i) a long-term test using groundwater collected from underground mines, and (ii) an accelerated test using an acidified solution. Laboratory experimentation on both representative coupon and full-size cable bolt specimens was conducted. In the long-term tests, simulated underground environments were recreated in ‘corrosion cells’ which contained a newly designed cable bolt coupon together with a mixture of groundwater, coal and clay, to measure the potential for developing SCC. It was found that the incidence of SCC failures was not related to groundwater alone. Geomaterials in the corrosion cells accelerated the corrosion of cable bolts by increasing the concentrations of total dissolved solids and electrical conductivity of the water. Following this, an acidic solution containing sulphide, synthesised based on the chemical properties of groundwater from twelve Australian underground mines and the characterisation of the localised cable bolting environment, was used as the testing solution for the accelerated tests. The fracture surfaces and fracture patterns of the laboratory-failed specimens and service-failed cable bolts were examined using scanning electronic microscopy and similar characteristics were observed, confirming the ability of the developed methodology to create SCC. A coherent failure mechanism for SCC crack initiation, propagation and catastrophic failure of cable bolts was developed. Hydrogen embrittlement was determined to be the dominant mechanism for SCC. Once a crack reached a critical size, which is determined by the applied stress and the fracture toughness of the cable bolt steel, fast brittle failure occurred and grew along the highly oriented microstructure. The i influence of environmental factors found in underground conditions on SCC of cable bolts was investigated. It was identified that SCC has a direct relationship with pH, stress intensity and the presence of hydrogen sulphide. SCC only occurred when the environment could promote atomic hydrogen diffusion into the cable bolt, with the crack propagation rate being determined by the hydrogen diffusion rate. To further understand the SCC phenomena and examine the SCC resistance of full-size cable bolts, a tension loading apparatus and periodically increasing strain rate loading mechanism were developed. It was identified that cable bolts with more wires took longer to fail. The cable bolts with plain wires provided greater resistance to SCC than those with indented wires as the small cross section of the indented wires increased stress concentration at the base of the indentation and, consequently, accelerated SCC initiation. The potential of galvanisation to prevent SCC was also examined. Galvanised specimens had a higher service life than non-galvanised specimens and improved the resistance of cable bolts against SCC. The methodologies developed here can be applied to study SCC in other reinforcement materials such as investigation of SCC potential of bridge and hoisting cables. The results of this thesis can be used to develop guidelines to assess
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