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N.R. Gates et alii, Frattura ed Integrità Strutturale, 34 (2015) 27-41; DOI: 10.3221/IGF-ESIS.34.03 40 Figure 9 : Experimental vs. predicted crack path based on maximum growth rate criterion and proposed crack friction model. These initial results are promising because they demonstrate the ability of the proposed model to predict experimentally observed trends in shear-mode crack growth. However, because this model is new, further investigation is needed to determine its robustness and general applicability. Future application to the torsion with static axial and in-phase axial- torsion data presented in this study will provide a means for some of this verification, but additional analyses for different materials and loading conditions would be necessary as well. In addition, a preliminary analysis of the sensitivity of the model to the various input parameters indicates a fairly strong dependence on the β value, a more moderate dependence on coefficient of friction and asperity angle, and a smaller effect of the average grain size only at shorter crack lengths. A more in depth sensitivity study should be performed as well to help quantify these effects. C ONCLUSIONS espite the significance of shear-mode crack growth mechanisms and crack branching phenomena in practical applications, relatively little research is available regarding these topics. Of the studies that have been performed, few provide a means of quantifying such effects and even fewer consider the natural initiation of fatigue cracks in smooth specimens. The current study was aimed at trying to fill some of the research voids in these areas. Based on the experimental results and analysis presented herein, the following conclusions can be drawn: 1. Microcrack networks and coalescence do not appear to have an effect on the experimentally observed crack paths for the smooth specimen fatigue tests performed in this study, regardless of the applied loading level. 2. The preferred crack growth mode is shown to have a dependence on the applied shear stress magnitude as well as the stress normal to the crack plane. This indicates that fiction and roughness induced crack closure effects play a significant role in the shear-mode crack growth process. 3. An increase in initial mode II crack length before branching is observed with an increase in pure torsion loading level. This indicates that the mode II driving force increases at a higher rate than the attenuation effect due to crack face interaction, which eventually leads to a non-branching condition. 4. A simple model is proposed in an attempt to quantify the complex phenomena involved in crack growth attenuation due to friction and roughness induced closure effects. The model is 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 interface. Resulting crack path predictions are shown to agree relatively well, both qualitatively and quantitatively, with experimentally observed trends for pure torsion loading. Although the model shows promise for application to more complex loading conditions, more analysis is needed for such cases to verify its robustness. R EFERENCES [1] Carpinteri, A., Pook, L. P., Susmel, L., Vantadori, S., Fatigue crack paths 2012, Int. J. Fatigue, 58 (2014) 1. [2] Fatemi, A., Gates, N. R., Socie, D. F., Phan, N., Fatigue crack growth behaviour of tubular aluminum specimens with a circular hole under axial and torsion loadings, Eng. Fract. Mech., 123 (2014) 137–147. D