Issue 41

S. E. Ferreira et alii, Frattura ed Integrità Strutturale, 41 (2017) 129-138; DOI: 10.3221/IGF-ESIS.41.18 129 Focused on Crack Tip Fields A model to quantify fatigue crack growth by cyclic damage accumulation calculated by strip-yield procedures Samuel Elias Ferreira, Jaime Tupiassú Pinho de Castro, Marco Antonio Meggiolaro Pontifical Catholic University of Rio de Janeiro, PUC-Rio, R. Marquês de São Vicente 225, Rio de Janeiro, 22451-900, Brazil ferreirase@hotmail.com , jtcastro@puc-rio.br , meggi@puc-rio.br A BSTRACT . Elber's hypothesis that  K eff can be assumed as the driving force for fatigue crack growth (FCG) is the basis for strip-yield models widely used to predict fatigue lives under variable amplitude loads, although it does not explain all load sequence effects observed in practice. To verify if these models are indeed intrinsically better, the mechanics of a typical strip-yield model is used to predict FCG rates based both on Elber's ideas and on the alternative view that FCG is instead due to damage accumulation induced by the cyclic strain history ahead of the crack tip, which does not need or use  K eff ideas. The main purpose here is to predict FCG using the cyclic strains induced by the plastic displacements calculated by strip-yield procedures, assuming there are strain limits associated both the with the FCG threshold and with the material toughness. Despite based on conflicting principles, both models can reproduce quite well FCG data, a somewhat surprising result that deserves to be carefully analyzed. K EYWORDS . Fatigue crack growth models; Strip-yield mechanics; Crack closure; Damage accumulation ahead of the crack tip. Citation: Ferreira, S.E., Castro, J.T.P., Meggiolaro, M.A., A model to quantify fatigue crack growth by cyclic damage accumulation calculated by strip-yield procedures, Frattura ed Integrità Strutturale, 41 (2017) 129-138. Received: 28.02.2017 Accepted: 15.04.2017 Published: 01.07.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION aris and Erdogan clearly demonstrated that stable fatigue crack growth (FCG) rates da/dN can be correlated to stress intensity factor (SIF) ranges  K , at least in the central region of typical da/dN  K curves, where theirs da/dN  A  K m rule applies [1]. Following their idea, many others proposed similar rules to consider the effects of other parameters that can affect FCG rates as well, such as the peak load K max or the load ratio R  K min /K max , FCG thresholds  K th (R) , and the toughness K C [2]. In particular, after discovering crack closure under tension loads, Elber postulated that additional fatigue damage could only be induced after the crack tip is fully opened under loads greater than K op , the crack opening load [3-4]. His da/dN  f (  K eff  K max  K op if K op > K min ) hypothesis can plausibly explain many characteristics of the fatigue cracking behavior, such as FCG delays and arrests induced by overloads (OL), reductions on OL-induced delays after underloads (UL), or the existence of R -dependent FCG thresholds, which can very much affect P