numero25

J. Toribio et alii, Frattura ed Integrità Strutturale, 25 (2013) 130-137; DOI: 10.3221/IGF-ESIS.25.19 130 Special Issue: Characterization of Crack Tip Stress Field Role of plasticity-induced crack closure in fatigue crack growth Jesús Toribio, Viktor Kharin University of Salamanca, Spain A BSTRACT . The premature contact of crack surfaces attributable to the near-tip plastic deformations under cyclic loading, which is commonly referred to as plasticity induced crack closure (PICC), has long been focused as supposedly controlling factor of fatigue crack growth (FCG). Nevertheless, when the plane-strain near-tip constraint is approached, PICC lacks of straightforward evidence, so that its significance in FCG, and even the very existence, remain debatable. To add insights into this matter, large-deformation elastoplastic simulations of plane-strain crack under constant amplitude load cycling at different load ranges and ratios, as well as with an overload, have been performed. Modeling visualizes the Laird-Smith conceptual mechanism of FCG by plastic blunting and re-sharpening. Simulation reproduces the experimental trends of FCG concerning the roles of stress intensity factor range and overload, but PICC has never been detected. Near-tip deformation patterns discard the filling-in a crack with material stretched out of the crack plane in the wake behind the tip as supposed PICC origin. Despite the absence of closure, load-deformation curves appear bent, which raises doubts about the trustworthiness of closure assessment from the compliance variation. This demonstrates ambiguities of PICC as a supposedly intrinsic factor of FCG and, by implication, favors the stresses and strains in front of the crack tip as genuine fatigue drivers. K EYWORDS . Crack closure; Crack-tip strains; Fatigue cracking. I NTRODUCTION he capability of opposite crack surfaces to contact prematurely at unloading due to particular patterns of the near- tip plastic straining under cyclic loading, the so called plasticity induced crack closure (PICC), is believed to be the intrinsic feature of fatigue crack growth (FCG), cf., e.g., the Overview [1], although not everybody involved in fatigue studies shares this conviction [2-4]. Despite the phenomenon of crack closure has been focused for decades since its raising by Elber [5], neither agreement between evaluations by different ways nor consensus about the relevance, and even the very existence, of PICC in the course of FCG do exist, cf. [1-3,6,7]. Conclusions about the PICC mechanisms based on both dislocation and continuum plasticity analyses are contradictory, cf., e.g., Deshpande et al. [8] or Pippan and Riemelmoser [9] vs. Louat et al. [3] or Bjerkén and Melin [10], Budianski and Hutchinson [11] vs. Noroozi et al. [12], and McClung et al. [13] vs. Toribio and Kharin [14]. PICC is used to be accepted as the factor directly responsible for the dependence of FCG, apart from the load range, on the load maximum or ratio, as well as on over- or under-loads along the loading routes, forming thus the framework to interpret FCG trends. Crack closure, which can arise from various origins (incidental ones, such as in-crack debris, oxides or other in-crack depositions, as well as ubiquitous ones, such as the crack surface roughness), seems to be out of doubts as a phenomenon that can accompany FCG and affect it under certain circumstances. Nevertheless, specifically PICC remains questionable as the universal intrinsic mechanism that controls FCG. A deal of uncertainty owes here to the difficulties of straightforward evidence of the crack closure phenomenon. Reviews [1,6,7] catalogued a variety of ways of the crack closure verification and assessment ranging from deductions based on the crack growth behavior (i.e., on the presumption of the role of closure in the process) to interpreting the compliance measurements on the assumption that T