PSI - Issue 52

ScienceDirect Structural Integrity Procedia 00 (2022) 000 – 000 Structural Integrity Procedia 00 (2022) 000 – 000 Available online at www.sciencedirect.com Available online at www.sciencedirect.com ScienceDirect Available online at www.sciencedirect.com ScienceDirect Procedia Structural Integrity 52 (2024) 293–308

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Fracture, Damage and Structural Health Monitoring 50 Years of Controversy on Fatigue Crack Closure D. Kujawski a* , A.K. Vasudevan b , R. E. Ricker c and K. Sadananda b a Mechanical and Aerospace Engineering Western Michigan University, Kalamazoo, MI 49008 b TDA Inc, Falls Church, VA 22043 c Univeristy of Maryland, College Park, MD 20742 Fracture, Damage and Structural Health Monitoring 50 Years of Controversy on Fatigue Crack Closure D. Kujawski a* , A.K. Vasudevan b , R. E. Ricker c and K. Sadananda b a Mechanical and Aerospace Engineering Western Michigan University, Kalamazoo, MI 49008 b TDA Inc, Falls Church, VA 22043 c Univeristy of Maryland, College Park, MD 20742

2452-3216 © 2023 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Professor Ferri Aliabadi 10.1016/j.prostr.2023.12.030 2452-3216 © 2023 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Professor Ferri Aliabadi Keywords: plasticity induced crack closure; oxide closure; roughness closure; vacuum experiments; crack tip chemistry. * Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: daniel.kujawski@wmich.edu 2452-3216 © 2023 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Professor Ferri Aliabadi Abstract In this article the 50 years of observations, implications and debates related to fatigue crack closure are discussed. New insights related to plasticity, oxide, and roughness induced crack closures and their role in shielding effects of the fatigue crack tip are re examined. Supporting evidence for these insights comes from the lack of  K th dependence on R in a high vacuum (with partial pressure of 10 -5 Pa or less). The presented new critical chemical-mechanical analyses are based on experimental results reported in the literature that demonstrate the marginal R-ratio effect on  K th of long cracks in vacuum for both planar/wavy slip alloys but show R-dependence in the lab air and in chemical environment. The latter is due to the formation of viscous nature of the oxide, which forms in humid air at the newly expose fresh fracture surfaces. It is demonstrated that the dominant factor related to the experimentally observed R-ratio effects on fatigue crack growth (FCG) behavior (on several alloys) in not related to crack closure but the access of the environment to the crack tip region that affects fatigue damage. In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when crack flanks contact occurs, most of the oxidized metallic species will exist as aquo-complexes, gel, or colloids, that have insufficient shear strength to wedge crack faces during unloading. Dislocation-based models have indicated that the crack tip shielding effect from a single asperity is small. The roughness induced crack closure has been suggested as a mechanical obstruction in the wake of the crack during cyclic unloading for planar slip alloys at the threshold region, the emphasis on the access of the environment to the crack tip for an environmental damage was not considered. Keywords: plasticity induced crack closure; oxide closure; roughness closure; vacuum experiments; crack tip chemistry. * Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: daniel.kujawski@wmich.edu © 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Professor Ferri Aliabadi Abstract In this article the 50 years of observations, implications and debates related to fatigue crack closure are discussed. New insights related to plasticity, oxide, and roughness induced crack closures and their role in shielding effects of the fatigue crack tip are re examined. Supporting evidence for these insights comes from the lack of  K th dependence on R in a high vacuum (with partial pressure of 10 -5 Pa or less). The presented new critical chemical-mechanical analyses are based on experimental results reported in the literature that demonstrate the marginal R-ratio effect on  K th of long cracks in vacuum for both planar/wavy slip alloys but show R-dependence in the lab air and in chemical environment. The latter is due to the formation of viscous nature of the oxide, which forms in humid air at the newly expose fresh fracture surfaces. It is demonstrated that the dominant factor related to the experimentally observed R-ratio effects on fatigue crack growth (FCG) behavior (on several alloys) in not related to crack closure but the access of the environment to the crack tip region that affects fatigue damage. In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when crack flanks contact occurs, most of the oxidized metallic species will exist as aquo-complexes, gel, or colloids, that have insufficient shear strength to wedge crack faces during unloading. Dislocation-based models have indicated that the crack tip shielding effect from a single asperity is small. The roughness induced crack closure has been suggested as a mechanical obstruction in the wake of the crack during cyclic unloading for planar slip alloys at the threshold region, the emphasis on the access of the environment to the crack tip for an environmental damage was not considered.