PSI - Issue 52

D. Kujawski et al. / Procedia Structural Integrity 52 (2024) 293–308 Author name / Structural Integrity Procedia 00 (2019) 000 – 000 13 Equation (11) indicates that the magnitude of the RICC depends on the size of the asperity ( ) and to the extent of the shear displacements ( ). The value of =0 if one or both variables equal to zero. In other words, under pure Mode I loading, RICC is not present since =0 and/or the crack fracture surface is smooth with =0 . It should be noted that in this analysis and corespond to the first contact of fracture surfaces during unloading. This first contact of crack surfaces doesn’t constitute complete “locking” nor “fixing” the crack -tip displacement since the inclined fracture surfaces in contact can slide against each other for a slip line fracture. Such sliding is more prevalent in humid air (or aqueous solution) due to a viscous layer of oxides that is formed with metal ions in various states of the transition from aqueous ions to stable end-product phases Vasudevan et al. (2022) (shown in Fig. 9 in blue). Such viscous oxide layer can work as a lubricant and promotes sliding of the inclined fracture surfaces resulting in crack tip displacement during additional unloading below . The observed effect of reduction of  K th (or an increase in (da/dN) rate) with increasing R-ratio is not due to RICC but due to more easily accessible access of the environment to the crack tip. Roughness develops in a planar slip alloy due to the crystallographic crack path. Thus, roughness is intrinsic to such alloys and exists in vacuum as well as in the environment. Therefore, in vacuum surface roughness should not affect  K th or (da/dN) in any significant way near the threshold region. However, at very high vacuum (>10 -7 Pa) the contact between fracture surfaces may cause rewelding 67 due to lack of oxygen to form a monolayer of oxide to prevent welding, which in turn may affect  K th and (da/dN) by bridging the fracture surfaces at low R-ratios. The above geometric model is presented in terms of Eqs. (10-12) that do not include environmental components. It is a purely a 2D geometrical model related to alloy systems with planar slip deformation which results in crack deflection. This deflected crack gives rise to asperity roughness that was related to RICC. As a result, the model should validate the results in vacuum that has very little environmental component. However, the model was used to validate the fatigue threshold data (  K th -R) in lab air (30-50%RH) and in aqueous solutions like water or dilute NaCl. Limited examples on planar slip alloys in vacuum is shown in Fig. 10 which indicates  K th independent of R relationship in vacuum for several planar slip alloys. In the case of 7075-UA, at high R=0.85, DK th drops due to K max approaching K Ic (~25 MPa.m 0.5 ). In addition, Ni-base alloys show the same trend. In the case of PW1480 single crystal alloy (shown in Fig. 2) indicates that RICC is nearly absent in vacuum even though the asperity roughness is present due to the crystallographic nature of the alloy. 305

Fig. 11 Effect of R-ratio on  K th in vacuum for planar slip alloys.

In addition, Petit’s (2008) shows interesting crack path profiles in planar slip AL -4.5Zn-1.25Mg alloys (single and polycrystal) with contrasting images, Fig. 11 a, b. In vacuum (at 4x10 -4 Pa) polycrystal Al-4.5Zn-1.25Mg