Abstract
Elber's hypothesis that DeltaKeff 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 DeltaKeff 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.
Highlights
Paris 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 Km rule applies [1]
Whereas the strip-yield models (SYM) assume FCG is driven by Keff, so that it depends on the interference of the plastic wakes left behind the crack tip along the crack surfaces, the critical damage model (CDM) suppose fatigue cracks propagate by sequentially breaking volume elements ahead of the crack tip, which fail because they accumulate all the fatigue damage they could sustain
The N properties of such materials were measured by standard procedures, following ASTM E606 recommendations. Both the FCG and the fatigue crack initiation properties were measured in coupons machined from the same material lot, to avoid any inconsistency in the data. Both the original CDMs and SYMs can describe reasonably well the measured data, even though they are apparently contradictory from a conceptual point of view
Summary
Paris 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 Km rule applies [1]. Dugdale's model estimates the plastic zone size in a Griffith plate of an elastic perfectly plastic material under plane stress (pl- ) conditions, assuming the pz formed ahead of both crack tips under a given Kmax work under a fixed tensile stress equal to the material yield strength SY (neglecting strain-hardening and stress gradients inside the pz).
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