On 50 years of fatigue crack closure dispute

Abstract

In this article, we examine the 50 years of observations and interpretations into the subject of fatigue crack closure in lab air and aqueous solutions: disputes and implications. Over this period, several closure concepts have been invoked to explain why the crack propagation threshold ΔKth should reduce with load ratio R. Among these, three types are the most popular and these are closure induced by crack (i) plasticity (PICC), (ii) oxides (OICC), and (iii) roughness (RICC). Therefore, these three and their role in shielding the fatigue crack tip are critically examined. This analysis finds that the reduction in ΔKth with increasing R‐ratio in lab air and aqueous solutions is weakly linked to decreasing crack closure behind the crack tip but strongly related to the access of the environment to the crack tip region and its influence on fatigue damage at and ahead of the crack tip. Supporting evidence for this interpretation comes from the lack of ΔKth dependence on R in a good vacuum (with partial pressure of 10−5 Pa or less). In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when closure occurs at the crack tip, most of the oxidized metallic species will exist as aquo‐complexes, gel, or colloids that have insufficient shear strength to wedge crack faces during unloading.

Unusual Fatigue Crack Growth Behavior of Long Cracks at Low Stress Intensity Factor Ranges

“…A long crack will not propagate if the following is applied: ∆K eff < ∆K eff,th . Despite more than 50 years of research, there are still unresolved experimental challenges related to the determination of the ∆K eff at threshold [7].…”

“…Later, due to limited plasticity at the near-threshold and inadequacy of PICC to correlate ∆K th with R, other mechanisms such as oxide-induced and roughnessinduced crack closure (OICC and RICC) were suggested to account for R-ratio effects on threshold and near-threshold FCG behavior [5,6]. Recently, we have reviewed and re-examined the effects on these closure mechanisms and their inability to shield the crack tip from the applied load [7] for the observed R-ratio effects on FCG in the air or in an active environment rather than crack closure, since in vacuum R-ratio effect on FCG disappears or is insignificant. Knowing the value of ∆K th at a given R (or ∆K th,eff ), designers assume that below these thresholds a long crack does not propagate in practical applications.…”

“…Then, as small/short cracks grow longer, the associated wake and PICC develop, and they approach long cracks behavior (Figure 2b). This explanation for PICC has been debated for the last 50 years [7], since at near-threshold, the crack tip plasticity is rather limited. Hence, instead of PICC, other mechanisms such as OICC and RICC are often explored.…”

Assessing Fatigue Life Cycles of Material X10CrMoVNb9-1 through a Combination of Experimental and Finite Element Analysis

“…The growth rate of prolonged fatigue cracks (stated as da/dN) at particular stress conditions for a wide range of technical alloys is generally defined by the Paris law [28], which is a widely recognized theory based on linear elastic fracture mechanics (LEFM):…”

On the Link between Plastic Wake Induced Crack Closure and the Fatigue Threshold

“…As outlined in [48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] variants of this approach are now widely used to model the growth of long cracks in conventionally manufactured metals. However, it should also be noted that it has recently been suggested [65] that R ratio effects on the growth of long cracks can be interpreted as a reflection of the effect of the environment on the crack tip region rather than crack closure per se.…”