The mechanical and toughness properties of six structural steels have been investigated in detail over a range of temperatures between 20°C and -196°C. The true stress - true strain characteristics have been analysed using several different approaches. Strain hardening exponent and transition strain valves were determined from Hollomon's equation σT = K εTn using the simple log σT - log εT plot and the more complex Crussard analysis. The crussard analysis was found to be unreliable in prediction of both transition strain and strain hardening exponent. As deformation temperature was reduced below ambient, the strain hardening exponent values tended to a maximum at some critical ternperature below which the strain hardening exponent values decreased. Transition strain tended to increase with decreasing deformation temperature, until at some critical temperature two stage hardening was replaced by single stage hardening. The variation of true work hardening rate (d σT/d εT) with strain and temperature was determined. At high strains, the true work hardening rate increased lineally with decreasing deformation telnperature whilst at low strains the same behaviour occurred until a critical deformation temperature was attained, when the true work hardening rate decreased. Critical crack opening displacement (C.O.D.) values were deterlnined for all the alloys over the temperature range. The critical event was defined as the onset of slow crack growth, or fast fracture, whichever occurred first. Onset of slow crack growth was detected by a potential drop technique, which was found to give good reproducibility in critical C. O. D. values. The normal transition behaviour was observed in these tests and the effect of increasing grain size on reducing toughness was investigated. No direct correlation between work hardening behaviour and toughness could be deduced and the relationship previously derived by Hahn and Rosenfield, and by Thomason, relating material mechanical properties to plane strain fracture toughness were found to be unacceptable in the light of the data obtained. It has been assumed in the past that the plastic instability strain, as measured in a tensile test, is the relevant strain when considering slow crack growth as the failure mechanism. It is proposed in the present study that a more realistic approach is to consider the critical strain as being that strain at which void linkage occurs.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:466980 |
| Date | January 1976 |
| Creators | Neumann, P. |
| Publisher | University of Strathclyde |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=21715 |
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