PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 137–144 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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5. Conclusions Cyclic accumulation of plastic strain and creation of the domain of compressive residual stresses improve the HAC behaviour through the increase of the failure load in aggressive environment by delaying the entry of hydrogen into the fracture process zone near the crack tip due to the existence of negative gradients of hydrostatic stress in the vicinity of the crack tip in the most severe fatigue pre-cracking program. From the mechanical point of view, the numerical results of a high-resolution elastic-plastic finite element analysis show that hydrostatic stress distributions depend markedly on the fatigue pre-cracking level K max , and this dependence is stronger at the first stages of the monotonic load phase associated with the SSRT during which the heaviest fatigue program produces compressive residual stresses in the near-tip area. The depth of the maximum hydrostatic stress point at the fracture instant coincides with the size of the region affected by the hydrogen at the microscopical level. Therefore, the theoretical distance over which the stress field favours hydrogen diffusion is close to the real physical dimension of the hydrogen-assisted micro-damage (HAMD) region detectable by scanning electron microscopy as a special topography ( tearing topography surface or TTS). Since the axial and the hydrostatic stresses have their maxima approximately at the same material point, the depth of the maximum tensile (or hydrostatic) stress should be a relevant microstructure scale to mark the prospective fracture loci in HAC. This idea is consistent with the previous conclusion based on micromechanics of HAC after fractographic analysis of the broken specimens. From the chemical point of view, the results of stress-and-strain assisted hydrogen diffusion computations show that hydrogen accumulation in the vicinity of the crack tip also depends on previous cyclic loading, the effects being clearly detectable in the prospective fracture loci associated with the maximum tensile (and hydrostatic) stress or, accordingly, with the HAMD/TTS region. Acknowledgements The author wishes to kindly and sincerely acknowledge the continuous and long-standing financial support to his scientific research in the field of fracture & structural integrity provided by the following Spanish Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002-01831), Ministry for Education and Science (MEC; Grant BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008-06810), Ministry for Economy and Competitiveness (MINECO; Grant BIA2011- 27870) and Junta de Castilla y León (JCyL; Grants SA067A05, SA111A07, SA039A08 and SA132G18). The results presented in this paper are based on previous publications. The author warmly and sincerely acknowledges the co- authors of such papers, especially Dr. Ana María Lancha (Polytechnic Univer sity of Madrid and CIEMAT) with regard to experimental aspects of the paper and D r. Viktor Kharin (University of La Coruña and University of Salamanca) in the matter of numerical issues of the article. In addition, acknowledgement is given to two companies: E MESA T REFILERÍA (La Coruña. Spain) and T REFILERÍAS Q UIJANO (Los Corrales de Buelna, Santander, Spain) for providing pearlitic steel for the experiments. References Costa, J.E., Thompson, A.W., 1982. Hydrogen Cracking in Nominally Pearlitic 1045 Steel. Metallurgical Transactions 13A, 1315 - 1318. Thompson, A.W., Chesnutt, J.C., 1979. Identification of a Fracture Mode: The Tearing Topography Surface. Metallurgical Transactions 10A, 1193 - 1196 Toribio, J., 1997. Fracture Mechanics Approach to Hydrogen Assisted Microdamage in Eutectoid Steel. Metallurgical and Materials Transactions 28A, 191 - 197. Toribio, J., 1998. Role of Crack-Tip Residual Stresses in Stress Corrosion Behaviour of Prestressing Steel. Construction and Building Materials 12, 283-287. Toribio, J., 2012. Time - Dependent Triaxiality Effects on Hydrogen - assisted Micro - damage Evolution in Pearlitic Steel. ISIJ International 52, 228 233. Toribio, J., Kharin, V., 1997a. K - Dominance Condition in Hydrogen Assisted Cracking: The Role of the Far Field. Fatigue and Fracture of Engineering Materials and Structures 20, 729 - 745.