A rationalisation of fatigue thresholds in pearlitic steels using a theoretical model

Ravichandran, KS (1991) A rationalisation of fatigue thresholds in pearlitic steels using a theoretical model. In: Acta Metallurgica et Materialia, 39 (6). 1331-1341 .

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Abstract

From a detailed re-examination of results in the literature, the effects of microstructure sizes, namely interlamellar spacing, pearlitic colony size and the prior austentitic grain size on the thresholds for fatigue crack growth (ΔKth) and crack closure (Kcl, th) have been illustrated. It is shown that while interlamellar spacing explicitly controls yield strength, a similar effect on ΔKth cannot be expected. On the other hand, the pearlitic colony size is shown to strongly influence ΔKth and Kcl, th through the deflection and retardation of cracks at colony boundaries. Consequently, an increase in ΔKth and Kcl, th with colony size has been found. The development of a theoretical model to illustrate the effects of colony size, shear flow stress in the slip band and macroscopic yield strength on Kcl, th and ΔKth is presented. the model assumes colony boundaries as potential sites for slip band pile-up formation and subsequent crack deflection finally leading to zig-zag crack growth. Using the concepts of roughness induced crack closure, the magnitude of Kcl, th is quantified as a function of colony size. In deriving the model, the flow stress in the slip band has been considered to represent the work hardened state in pearlite. Comparison of the theoretically predicted trend with the experimental data demonstrates very good agreement. Further, the intrinsic or closure free component of the fatigue threshold, ΔKeff, th is found to be insensitive to colony size and interlamellar spacing. Using a criterion for intrinsic fatigue threshold which considers the attainment of a critical fracture stress over a characteristic distance corresponding to interlamellar spacing, ΔKth values at high R values can be estimated with reasonable accuracy. The magnitude of ΔKth as a function of colony size is then obtained by summing up the average value of experimentally obtained ΔKeff, th values and the predicted Kcl, th values as a function of colony size. Again, very good agreement of the theoretically predicted ΔKth values with those experimentally obtained has been demonstrated.

Item Type:	Journal Article
Publication:	Acta Metallurgica et Materialia
Publisher:	Elsevier science
Additional Information:	Copyright of this article belongs to Elsevier science.
Keywords:	Induced Crack Closure;Fracture;Deformation;Propagation; Microstructure;Explanation;Roughness;Behavior;Growth.
Department/Centre:	Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited:	09 Nov 2010 08:50
Last Modified:	09 Nov 2010 08:50
URI:	http://eprints.iisc.ac.in/id/eprint/33633

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