Abstract
This work raises some new questions about the validity of blindly assuming that Elber’s effective stress intensity factor is the actual fatigue crack driving force, and that as so it can be used to explain all load sequence effects on fatigue crack growth (FCG). Although plasticity-induced crack closure can be a quite reasonable heuristic explanation for many non-elementary FCG behaviors, it has some limitations that cannot be ignored. In fact, this never settled discussion is particularly important for the simulation of FCG lives under real service loads, a most important practical issue. After arguing that ?Keff can spoil the use of the most important similitude principle in FCG problems, simple but convincing experimental data that cannot be explained by this classical idea is presented here. This data involves the shape of fatigue crack fronts and the FCG behavior under nominally plane stress and plane strain conditions.
Highlights
Fatigue crack growth rates are very much susceptible to brusque changes in crack driving forces, which may cause important load sequence effects by significantly altering subsequent rates as compared to the rates induced by identical driving forces that have not been previously affected by sudden load changes
fatigue crack growth (FCG) lives cannot be properly estimated in many if not most practical applications if such load history effects are neglected. Such effects can be induced by several mechanisms that can be divided into three main classes [1]: (i) fatigue crack closure induced by plasticity, roughness, phase transformation, and/or oxidation, all mechanisms that act on the crack faces, before the crack tip; (ii) blunting, kinking, or bifurcation of the crack tip, mechanisms that act at the crack tip; and (iii) residual stresses and/or strains, mechanisms that act ahead of the crack tip
In many practical cases one of those mechanisms can be so dominant that the others may become negligible, but in other cases they may be not. Such mechanisms can act competitively reducing the effects of the others, or else, they can act symbiotically
Summary
Fatigue crack growth rates are very much susceptible to brusque changes in crack driving forces, which may cause important load sequence effects by significantly altering subsequent rates as compared to the rates induced by identical driving forces that have not been previously affected by sudden load changes. Arrest, or even acceleration of FCG rates after tensile overloads (OL) or abrupt decreases in the applied stress intensity factor (SIF) range K and/or peak Kmax; sudden fracture caused by very large OLs; and reduction of OL-induced delays after compressive underloads (UL).
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