Role of plasticity-induced crack closure in fatigue crack growth

Abstract

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.

On 50 years of fatigue crack closure dispute

“…In addition, they disputed the notion "that all of the load is transmitted through the plastic wedge is not physically reasonable." On the other hand, Toribio and Kharin 30 have analyzed large-deformation elastoplastic simulations of a plane-strain crack under constant amplitude load cycling at different R-ratios and with an overload. They stated that "Simulation reproduces the experimental trends of FCG concerning the roles of stress intensity factor range and overload, but PICC was not detected."…”

“…They concluded that the ΔK eff given by Equation ( 1) is not a suitable driving force for FCG analysis. Therefore, the conclusions drawn from these three articles [30][31][32] imply that using ΔK eff to collapse different R-ratio data may not be a valid procedure since FCG behavior was not significantly altered when different K OP were measured pointing to some other mechanisms that maybe involved. We interpret that these "other mechanisms" are related to the environmental access to the crack tip and its contributions to the environmentally induced damage.…”

“…A similar conclusion was also drawn by Bowman et al 27 that most measurements of closure tend to grossly overestimate its effect on FCG. In addition, they disputed the notion “that all of the load is transmitted through the plastic wedge is not physically reasonable.” On the other hand, Toribio and Kharin 30 have analyzed large‐deformation elastoplastic simulations of a plane‐strain crack under constant amplitude load cycling at different R‐ratios and with an overload. They stated that “Simulation reproduces the experimental trends of FCG concerning the roles of stress intensity factor range and overload, but PICC was not detected.” They also found that despite the absence of closure, load–displacement curves always appeared bent, which raised doubts about the trustworthiness of closure assessment from the compliance slope changes.…”