Some Considerations of the Influence of Sub-Critical Cleavage Growth during Fatigue-Crack Propagation in Steels

Research output: Journal Publications and Reviews (RGC: 21, 22, 62)21_Publication in refereed journal

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Original languageEnglish
Pages (from-to)119-126
Journal / PublicationMetal Science
Volume9
Issue number1
Publication statusPublished - 1975
Externally publishedYes

Abstract

A study has been made of fatigue crack propagation in two0·55 % C steels of differing manganese content over a range of mean stresses(characterized by the stress ratio R=Kmin/Kmax, where Kmin and Kmax representthe extreme values of stress intensity during a given loading cycle). Theresults for these materials are compared with those obtained for fine- andcoarse-grained mild steel. In the high-manganese mediumcarbon steel and thecoarse-grained mild steel, it was found that the mechanism of crack growthinvolved substantial proportions of transgranular cleavage cracking, whichresulted in a significant effect of mean stress on the propagation rate and amarked' acceleration in rate near final instability. By contrast in thelow-manganese medium-carbon steel and the finer-grained mild steel no suchsub-critical cleavage fracture was observed, there was no acceleration nearfinal failure, and the propagation rate (above ~ 10-6 mm/cycle) wasindependent of mean stress. 


The results are interpreted in terms of a 3-stage relationship between log growth-rate and log ΔK, and show that, for intermediate and high growth-rates, effects of mean stress are caused directly by the occurrence of ‘static’ modes. The final acceleration in rate near instability is considered to be associated with the critical linkage of such modes. During this stage it is inappropriate to relate the crack growth-rate to the alternating stress intensity, ΔK, because the maximum value of stress intensity, Kmax, is critical in determining how much of the total crack front is composed of transgranular cleavage facets and hence how fast the crack is growing. Discussion is also included on the difference between the terminal stress intensity during fatigue and the KIc value (fracture toughness) under monotonic loading. A possible explanation for such differences is proposed in terms of plasticity-induced heating at the crack tip.