A study of the high temperature fatigue behaviour and life prediction of 2.25Cr-1Mo alloy steel through the development of standard-sized and miniature specimen testing techniques

Publication Type:
Thesis
Issue Date:
2008
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- The objective of this research was to characterise and compare the high temperature fatigue behaviour of normalised and tempered 2.25Cr-1Mo alloy steel used in the nuclear and fossil-fuelled power generation industry, using two fatigue specimens possessing different geometries. This involved the design, development and validation of a high temperature fatigue testing system and methodology which incorporated both specimen sizes, associated extensometry and gripping mechanisms. A conventional standard-sized specimen was modified and a miniature specimen was designed and developed for manufacture from a small volume of material, obtained by the chain-drilling technique. A novel high temperature fatigue extensometry system was designed and developed for each specimen size. The extensometry system utilised noncontact capacitance probes, associated equipment and a unique attachment technique, in order to measure and control strain at the shoulders of specimens during fatigue testing. This design was adopted to reduce inherent disadvantages in existing extensometry for fatigue testing, that is generally attached at the gauge length of specimens. An analytical methodology for strain-calibration, correlating the experimental displacements and strain occurring at the shoulders and gauge length of specimens was determined, which enabled the use of the extensometry when attached to the shoulders of specimens during testing. The analytical methodology was then validated through finite element analysis, by modelling both specimen sizes. Finite element analysis was also used to investigate the effects of geometry changes and misalignments in specimens, with respect to the strain-calibration and stress-distributions. Overall, miniature specimen models attained closer correlations to experimental data and were less sensitive to misalignment effects in comparison with the standard-sized specimens. The high temperature fatigue properties of the test material, determined by both specimen sizes, were characterised and modelled using the strain-based and energy- based approaches. Three analytical methods were also employed to provide fatigue data for these approaches and to further evaluate the fatigue properties. The results showed that the fatigue behaviour of the test material characterised by the miniature specimen, compared well with that of the standard-sized specimen and previous literature. This confirmed the successful development of the fatigue test system and methodology of this research. However, some small differences in fatigue properties were found between specimen sizes and were attributed to the difference in geometry of specimens. Therefore, a modelling and analytical method was introduced, that applied a geometrical correction factor to the miniature specimen data. This facilitated in the accurate prediction of standard-sized fatigue data, by the use of miniature specimens.
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