Abstract
The understanding of shear-mode crack growth mechanisms and crack branching phenomena is of great interest for a variety of practical engineering situations. Despite this fact, relatively little research is available regarding these topics. Of the studies that have been performed, few provide a means of quantifying such effects and most consider crack growth starting from a precrack. The current study is aimed at trying to fill some of the research voids in these areas by investigating the effects of microcrack coalescence, loading level, and superimposed normal stresses on the mode II crack behavior of naturally initiated fatigue cracks. Based on the experimental results and subsequent analyses, it was determined that microcrack networks and coalescence have little to no effect on the experimentally observed crack paths regardless of the applied loading level. Instead, the preferred crack growth mode is shown to have a dependence on the applied shear stress magnitude and stress normal to the crack plane, indicating a significant role of fiction and roughness induced crack closure effects in the crack growth process. A simple model is then proposed to quantify these effects based on the idea that crack face interaction reduces the effective mode II SIF by allowing a portion of the nominally applied loading to be transferred through a crack. The model agrees qualitatively with the experimentally observed trends for pure torsion loading and predicts crack branching lengths within a factor of 2 for all loadings considered.
Highlights
Fatigue crack growth often represents a significant portion of a component’s total fatigue life
Surface replicas have shown the development of small microcrack networks in a number of the smooth specimen fatigue tests performed in this study, crack coalescence was only observed in a limited number of these tests and usually occurred relatively early in the crack growth life
By comparing crack paths between the smooth and precracked specimens in Fig. 2, it is easy to see that they are very similar. This is true even for complex crack paths where cracks initiate in mode II, branch into mode I cracks, and eventually transition back to mode II after growing for some distance (Fig. 2(b-d)). This suggests that microcrack networks and their coalescence did not play a significant role in determining the crack paths for these tests
Summary
Fatigue crack growth often represents a significant portion of a component’s total fatigue life. Cracks initiated naturally at a mechanical notch tend to nucleate and grow a short distance, usually comparable to the length of a few grains, on planes of maximum shear, but almost always turn so that long crack growth occurs on planes of maximum tensile stress [2,3,4,5] In this case, well established mode I crack extension models can be employed to predict growth rates for a given loading history. Owing to the complexity of the task, little research is available on models which attempt to quantify when or if cracks will grow by type R mechanisms, type S mechanisms, or whether or not these two mechanisms are even responsible for the differences in smooth and notched specimen crack paths To simplify this problem for the following discussion, it will be assumed that naturally occurring fatigue cracks always tend to initiate on planes of maximum shear stress. In an attempt to quantify the experimental observations, a model is proposed to account for reductions in effective mode II SIF due to crack face interaction effects
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