PSI - Issue 52

D. Kujawski et al. / Procedia Structural Integrity 52 (2024) 293–308 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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various viscosities, in the wake of a crack on 2.25Cr-1Mo steels show that crack growth threshold  K th changed less than 1.0 MPa(m) 1/2 and the data lies close to the inert He. Bae and Conrad (1989) conducted similar experiments on AISI 316 stainless steel FCG using silicone oil and found that the overall FCG results were not affected by viscous oil in the wake of a fatigue crack (Fig. 8 b). AISI 316 stainless steel is a planar slip alloy with a non-linear branched crack. Trend line in both Figs. 8a and 8b shows the average trend of the data. The above observations are similar to both Fe and Al-hydroxides on a FCG in Fe and Al-alloys. Such observations suggest that the overall effect of viscous oxides play a small role at the FCG thresholds. The cause that reduces  K th with increasing R is mainly the access of environment to the crack tip and the possible hydrogen assisted damage. 3.3 On roughness-induced crack closure [Vasudevan and Kujawski (2023)] The RICC is considered to be more common in a planar slip alloy where Mode-II displacements are prevalent. Suresh and Ritchie (1982) developed a 2-D model to describe the effect of RICC on crack tip shielding for lab air conditions. In their model fracture surface roughness is idealized to be of triangular cross section and roughly equal in size in terms of height ( ℎ ) and width ( ) (Fig. 9). Based on their geometrical considerations they derived the following relationship for the first contact of fracture surfaces during unloading = 2ℎ +2ℎ (9) where = is the ratio of the Mode II displacement, , over the Mode I displacement, . Then, the magnitude of the closure effect, was expressed as the ratio of the closure (K cl ) to maximum stress intensity (K max ) as =√ =√ 2ℎ +2ℎ (10) or in a nondimensional form as =√ 2 1+2 (11) where = ℎ/ is a nondimensional fracture surface roughness factor where can be related to the average grain size by the relation = .

Fig. 10 Schematic illustration of a near-threshold fatigue crack displacement at (a) K=K max and the corresponding d max and (b) after unloading (by Mode I and Mode II) to the first contact between fracture surfaces at K=K cl and corresponding d cl (After Suresh and Ritchie (1982)). The viscous oxide layer forms at humidity level greater than about 40%.