Issue 37

J. Vázquez et alii, Frattura ed Integrità Strutturale, 37 (2016) 38-45; DOI: 10.3221/IGF-ESIS.37.06 43 For the SWT parameter, although less significant than in the FS parameter, the same behaviour is observed but now only in the very close area to the surface. While this procedure may give a “ visual approach ” to the crack initiation path, it is important to note that only the maximum values of the parameter are here considered, and no specific information is given about the critical plane orientation (crack initiation angle). Under this circumstance, it is possible that nearby points have completely different critical plane orientations. Third Crack Initiation Analysis Procedure This is a variation of the second procedure, introducing the information of the critical plane. However, this third procedure has the drawback that only assumes straight crack initiation paths. A scheme of this procedure is shown in Fig. 8. First, a straight line starting from the contact trailing edge and forming an angle θ related to the vertical is considered. Then, the parameter value is calculated along this line, but with the characteristic that for all these points, the material plane orientation considered for the parameter evaluation coincides with that defined by θ . This is repeated for different values of θ obtaining the contour plot in Fig. 9. Now, and because all points along a straight line have the same material plane for the fatigue parameter evaluation, it is feasible to pose that those lines having the higher mean values of the parameter along a certain length are more likely to initiate cracks. Fig. 9 also includes the paths of Fig. 6 and the experimental cracks. Now, clearly both, FS and SWT parameters, predict crack initiation paths that point toward the region beneath the contact. To obtain more precisely the preferred direction, the mean values obtained for each parameter in a distance of 50 µm -which is the average grain size for the Al-7075-T65- are represented in Fig. 10 as a function of angle θ . This graph shows that angles of 54º and 9º offer the maximum mean values for the FS and SWT respectively. Lines having these angles are plotted in Fig. 9. First, note that, although with an angle far from the experimentally observed, now the FS parameter produces a crack initiation path that qualitatively has the right direction. On the other hand, once again the SWT parameter offers the best prediction, and also, note that this path is quite similar to the previous one obtained with the first procedure. Figure 8 : Scheme for the third crack initiation procedure. Figure 9 : Polar contour plots for the FS and SWT parameters.