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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ratios can be collapsed into a single curve in terms of  K eff . It can be noted that forty years after Elber (1970) published his paper on crack closure, Wei (2010) wrote a book on ‘Fracture Mechanics’ with no reference to crack closure and the analyses rested entirely on the role of environment in fatigue damage. The access of the environment to the crack tip is related to K max applied. Similarly, Hertzberg et al. (2022) in their book on ‘Deformation and Fracture Mechanics of Engineering Materials’ in all editions including the latest 6 th edition also has no reference to crack closure. It is not disputed that two loading parameters (  K and K max ) are needed to characterize fatigue crack growth driving force. The interaction between these parameters can be complex, and the specific mechanisms responsible may require detailed experimental, computational or simulation studies to be fully understood. Additionally, different mechanisms may be dominant under different environmental conditions or loading scenarios. In the present article we discuss our latest analyses Vasudevan et al. (2022), (2022) and Vasudevan and Kujawski (2023) related to PICC, OICC, and RICC and their relevant implications in shielding effects on FCG. We present some insights on these 3 types of closure and the importance on the role of crack tip chemistry on FCG behavior, in particular, at the threshold region. Analysis points out that the common theme in the reduction in  K th with increasing R-ratio in lab air/aqueous solutions is less related to shielding effects of closure and more to the access of environment to the crack tip. A key role in fatigue damage is due to the chemistry and its effect on deformation at the crack tip. 3.0 Review of Key Points for PICC, OICC and RICC 3.1 On Plasticity-Induced Crack Closure [Vasudevan et al. (2022a)] The PICC hypothesis proposes that fracture growth data from various R-ratios can be collapsed into a single curve based on effective stress intensity range, ΔK eff , using equations (1) and (9). This hypothesis has been widely accepted for the past 50 years and is commonly used in the research and analysis of FCG behavior. However, experimental studies have encountered difficulties in determining the K OP (or K CL ) values at near-threshold conditions, primarily due to low sensitivity in the compliance curves used to measure these parameters. Researchers such as Pippan and Hohenwater (2017), Newman (2000), Riddell et al. (1999), and Pippan (1987) have noted that near-threshold K OP values are often simulated or calculated due to the challenges in measuring them directly. Figures 4a and 4b present FCG data for two different alloys, AA7075-T7351 overaged (OA) with wavy slip from Kirby and Beevers (1979) and single crystal PWA1480 with planar slip from Holtz and Sadananda (1998), respectively. The AA7075-T7351 OA alloy was tested in a vacuum of ~10 -3 Pa while the single crystal PWA1480 alloy was tested at 10 -5 Pa. Despite having different slip characteristics, both alloys exhibited a weak dependence of FCG behavior on R-ratio, over several orders of magnitude in FCG rate. The inserts in Figs. 4a and 4b show crack path profiles taken from publications of Petit (2008) and Busse (2019), respectively. The crack profile for the AA7075 T7351 OA alloy was flat, while that for the PWA1480 alloy was zigzag. The overall scatter range in terms of the applied SIF range, ΔK, for the PWA1480 alloy was less than 2 MPa √ , which may be attributed to the experimental resolution of measurements of its rough fracture surface. Trend lines depicted in Fig. 4 represent the average FCG behavior with scatter less than ± 1 MPa√m for the PWA1480 alloy. On the other hand, Fig. 4a sho ws that the scatter in ΔK was much smaller (~0.3 MPa √ ) for the AA7075-T7351 OA alloy with a linear crack path.