Issue 48

C. Santus, Frattura ed Integrità Strutturale, 48 (2019) 442-450; DOI: 10.3221/IGF-ESIS.48.42 446 Figure 3 : SEM observations of the initial cracks and evaluation of the orientation angles of two runout specimens for each series. P REDICTION OF THE CRACK INITIAL ORIENTATION he stress distribution at the fretting region is multiaxial, high gradient and singular when the contact is not rounded. For this reason, the Theory of Critical Distances (TCD) is usually applied in the literature along with a multiaxial critical plane method, such as the FS or the MWCM criteria which are shear based, thus they are expected to accurately predict type I shear cracks. On the other hand, the tensile type II is supposed to be better correlated with the SWT, which is a uniaxial based criterion. These two common criteria are formalized as: n,max a Y n,a n,max FS 1 SWT k              (1) where the stress components are graphically defined in Fig. 5: a  and n,a  are the shear and the normal stress amplitudes, respectively, experienced by any possible critical plane and n,max  is the maximum normal stress on the same plane. Y  is the yield strength, and k is a coefficient which controls the mean stress effect. This material parameter k = 0.44 was derived from the fretting analysis by Vázquez et al. [15], considering the same alloy and similar treatment 7075-T651. The critical distance length for the aluminium alloy 7075-T6 was recently obtained in our previous work: Santus et al. [36,37], both according to the primary SIF threshold definition and then proposing an alternative inverse search procedure with an optimized specimen. The threshold-based value obtained for the load ratio R = -1 is: 2 th fl 1 0.056mm K L            (2) T