PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 206–213 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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1. Introduction High-strength prestressing steel wires (ultimate tensile strength > 1500 MPa), usually cold-drawn pearlitic steels (i.e., high-strength cold-drawn eutectoid pearlitic steel wires) are widely used as the most resistant constituents of prestressed concrete structures in civil engineering construction. Prestressing wires are usually made of eutectoid pearlitic steel that is heavily cold drawn during the manufacture process to create elevated tensile properties. These high-strength wires, once subjected to elevated tensile stresses in service, remain in this condition forever, usually working in hostile environments, e.g., due to atmospheric humidity. All that makes them sensible to the presence of environmentally-assisted surface cracks, thereby producing the degradation phenomenon known as stress corrosion cracking (SCC) or, generally, environmentally assisted cracking (EAC). There is general agreement that for a wide range of electrochemical conditions (pH and potential), hydrogen can be present in the fracture process zone and may be the main cause of damage or degradation of material properties In addition, the differences between the local (crack-tip) electrochemical environment and the global (bulk) external conditions promote the hydrogen presence in the near-tip area (Brown et al., 1969). In the important case of prestressing steel wires, there is general agreement that HE plays a fundamental role in the environmental cracking of such a steel ( Bergsma et al., 1978 ), leading in many cases to loss of structural integrity and catastrophic failure of engineering structures ( Vehovar et al., 1998 ), with higher risk in the case of cathodic protection or over-protection (Isecke and Mietz 1993). The International Federation for Prestressing FIP ( Fédération Internationale de la Précontrainte ) has proposed a test – the Ammonium Thiocyanate Test (ATT) – to evaluate the HE susceptibility of prestressing steels (FIP-78, 1981). In spite of some objections to this standard corrosion test method of prestressing steel under tension in a solution of NH 4 SCN, it is still the best suited to steel control and acceptance. Thus, any contribution to a better understanding of the meaning of the ATT, and particularly of the influence of stress level on the time to failure, should be welcome, both from the scientific point of view and for practical and economic reasons. In this regard, important scientific papers were published by Toribio and Elices (1991), Takai et al. (1994; 1995), Toribio (1997; 2000), Toribio and Kharin (2006), Ichiba et al. (2015) and Iwanaga et al. (2022). In this paper, the influence of internal residual stress distribution on the HE susceptibility of prestressing steels is analyzed through the ATT (FIP-78, 1981). Residual stresses are introduced in the steel during the manufacturing process, so they can be considered an intrinsic characteristic of the material, and represent a variable for the design of any steel structure. The final aim of this research is to obtain curves representing applied stress versus time to rupture of the high strength steels, as a function of the different residual stress distributions in the wires. These curves allow a prediction of the life of the material in a hydrogen environment. Procedures such as surface rolling, useful to extend the life of the steel wires, are also beneficial. In the case of high-strength cold-drawn eutectoid pretressing steel wires, residual stresses generated by the cold drawing process itself, i.e., manufacturing-induced residual stresses, are known to be capable of influencing the process of HE of the wires, thereby affecting the prestressing steel wire lives in the ATT solution and producing scatter in the results, such a scatter being dependent on the level of externally-applied stress, as analyzed and discussed in detail elsewhere (Toribio and Elices, 1991; Toribio, 1997; Toribio, 2000; Toribio and Kharin, 2006). This paper revisits previous research of the author on the topic of the influence of residual stresses on the HE of high-strength cold-drawn eutectoid pearlitic steel wires, on the basis of analyzing a wide set of experimental results on the ATT and formulating a numerical model of stress-assisted diffusion of hydrogen in the pearlitic steel wire, together with a fracture criterion to estimate the wire life in the ATT solution, such a life being clearly influenced by the residual stress distribution in the wire after manufacturing. 2. Problem statement Within the framework of damage tolerance analyses, the fracture mechanics approach to the HE phenomenon tries to ensure the structural integrity of the whole engineering construction while it is operative, as well as to