Browsing by Author "Humphries, SR"

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    Effect of repeated loadings on the stress relaxation properties of 2.25Cr-1Mo steel at 550°C and the influence on the Feltham 'a' and 'b' parameters.
    (Elsevier, 2010-05-25) Humphries, SR; Snowden, KU; Yeung, WY
    This paper describes studies undertaken to evaluate the stress relaxation behaviour of 2.25Cr–1Mo steel, a material used extensively in high temperature steam components. Repeated stress relaxation loadings were conducted on normalised and tempered 2.25Cr–1Mo steel at 550°C, under constant strain, in a closed loop testing system. Detailed analysis of stress relaxation curves is presented. Application of the data to the Feltham equation for subsequent use in life assessment procedures is discussed and analysed. © 2010, Elsevier Ltd.
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    Effects of prior stress relaxation on the prediction of creep life using time and strain based methods
    (ASME International, 2010-07-18) Payten, WM; Snowden, KU; Dean, DW; Humphries, SR; Edwards, L
    A critical requirement for both next generation conventional and nuclear plants is the development of simplified inelastic design and fitness for purposes procedures that give a reasonably accurate prediction of the complex multi-axial time dependent stress strain behavior. The accumulation of this inelastic strain in the form of coupled creep-fatigue damage over time is one of the principal damage mechanisms which will eventually lead to crack initiation in critical high temperature equipment. Two main procedures that address creep-fatigue loading are generally used, either a time fraction or a ductility exhaustion approach. It is generally accepted that these methods enable conservative predictions within a factor of 2 to 3 and hence are reliable methods for code based design and fitness for purpose type assessments. However, for complex cycles, this may not be the case, for example prior relaxation cycles are found to accelerate the creep rupture of the material with the result that a significant reduction in creep life can be observed. An investigation was undertaken into the influence of prior relaxation on resultant failure using a typical low alloy ferritic power station steel. Both time based and strain based methods were used to predict the damage caused by the stress relaxation cycles followed by operation at steady state. The predictions found that while ductility exhaustion methodologies based on mean properties where adequate in predicting the failure life, time fraction methods were found to be extremely non-conservative for mean properties and only lower bound solutions provided an estimate of remaining creep life. The Monkman-Grant approach resulted in predictions that erred on the conservative side. The results have implications for both current and future conventional and nuclear power stations as it may be difficult for time based approaches to accurately account for complex cycling, shakedown conditions or stress relaxation at welds. © 2013 by ASME
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    Energy-based approach for the evaluation of low cycle fatigue behaviour of 2.25Cr-1Mo steel at elevated temperature
    (Elsevier, 2010-08-20) Callaghan, MD; Humphries, SR; Law, M; Ho, M; Bendeich, PJ; Li, HJ; Yeung, WY
    The energy-based approach for the evaluation of low cycle fatigue behaviour of 2.25Cr–1Mo steel at elevated temperature has been investigated and detailed analyses discussed. Plastic strain energy was determined per cycle and found to characterise both crack initiation and propagation to failure regimes. At cyclic stabilisation, average plastic strain energy may be used as a suitable damage parameter and correlations between experimental and predicted data determined. The fatigue toughness to failure of the material was established and the development of a fatigue toughness to crack propagation analysis is presented. © 2010, Elsevier Ltd.
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    Evaluation of high temperature fatigue behaviour of P22 by miniature specimen testing
    (Trans Tech Publications, 2010-01-01) Callaghan, MD; Humphries, SR; Law, M; Li, HJ; Yeung, WY
    Miniature specimen testing to evaluate mechanical properties, presents a novel opportunity to undertake structural integrity assessments of in-service power generation components, by removing only a very small volume of material. In this study, high temperature fatigue testing of P22 steel was undertaken and a number of fatigue properties determined using a miniature specimen testing methodology. Good comparisons were observed between fatigue properties determined by miniature specimens and the more established standard-sized specimen testing reported in literature.
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    Radiation-induced growth in zircaloy-4
    (International Group On Research Reactors, 2005-09-12) Harrison, RP; Carr, DG; Kim, YS; Boccanera, L; Ripley, MI; Stathers, PA; Humphries, SR
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    Small punch test of LC4/SiCP metal matrix composites
    (Trans Tech Publications, 2010-09-17) Mak, J; Wuhrer, R; Humphries, SR; Booth, N; Heness, G; Yeung, WY; Wei, T; Qin, JN; Ouyang, QB; Zhang, D
    There have been growing demands of high performance metal matrix composites in advanced engineering applications in virtue of their high specific strengths. This paper is to report an assessment of the mechanical properties of LC4/SiCp metal matrix composites using an innovative testing technique, small punch test. The composite materials of this study were produced by stir casting method with particulate reinforcements of 7wt.% and 14wt.% of SiC respectively. Small punch testing was performed on the LC4 base alloy and the two composites materials. The small punch test is a relatively new mechanical testing technique capable of utilizing small disk-shaped samples to determine the mechanical properties of the test materials. In this study, the equivalent fracture strain, εqf of the LC4/SiCp MMCs was characterised and compared with the base alloy. The fracture mechanism of the test samples was examined using scanning electron microscopy. © Trans Tech Publications Ltd.
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    Special testing equipment and validation of measurement methodologies for high temperature low cycle fatigue testing of miniature metallic specimens
    (Springer Nature, 2016-02-25) Callaghan, MD; Humphries, SR; Law, M; Bendeich, PJ; Yeung, WY
    A technique for high temperature low cycle fatigue testing of metallic materials has been developed, to determine fatigue behaviour through the testing of miniature specimens. The miniature specimen geometry was specifically designed, such that it could be manufactured from a small volume of material removed by chain-drilling extraction. An extensometry method to measure and control strain at the specimen shoulders during testing was adopted. This was undertaken to minimise the deleterious contact effects that can occur via extensometry attached at the gauge length of specimens, hence leading to premature failure and inaccurate fatigue data. By the application of this technique, the high temperature low cycle fatigue behaviour of 2.25Cr-1Mo steel was successfully characterised at 540 °C, under a fully reversed strain-controlled regime. The fatigue properties of the steel obtained from testing miniature specimens were shown to correlate well with existing literature for the material under comparable conditions, as determined by the testing of conventional standard-sized specimens. © 2016 Society for Experimental Mechanics
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    Specimen-size dependency and modelling of energy evolution during high-temperature low-cycle fatigue of pressure vessel steel
    (Elsevier Ltd., 2011-08-01) Callaghan, MD; Humphries, SR; Law, M; Ho, M; Yan, K; Yeung, WY
    High-temperature low-cycle fatigue testing was conducted on pressure vessel steel using standard and miniature specimen sizes and the fatigue toughness required for macrocrack propagation was investigated. A definite specimen-size dependency was observed for both the threshold cumulative plastic strain energy and cycles required for macrocrack propagation, which was explained to be influenced by geometric conditions. An analytical modelling prediction was developed that accounted for specimen-size dependency and was successfully applied to predict fatigue toughness to macrocrack propagation. (C) 2011 Acta Materialia Inc.