PSI - Issue 5

Jesús Toribio et al. / Procedia Structural Integrity 5 (2017) 1446–1453 Jesús Toribio / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Materials behavior is influenced by the previous mechanical history (load, stress or strain trajectory) and by the surrounding physico-chemical environment, so that the ideas proposed by the Dutch philosopher Baruch de Spinoza about the hetero-determined nature of human being are totally adequate in this matter. In the same sense, the Spanish philosopher José Ortega y Gasset coine d the very famous sentence “Yo soy yo y mi circunstancia “ (translated into English as “I am I and my circumstance”) which is the key -stone of his philosophy of human life. On the basis of this, and paraphrasing José Ortega y Gasset, the author of this paper (Toribio, 1992a; 2000) established the innovative idea that “the material is itself and its circumstance” (Toribio, 2000) emphasizing the evolution of material’s performance throughout the service life, due to the double and varying influence of the previous mechanical loading history and the surrounding physico-chemical environment. The key-idea was launched in Spanish (“el material es él y su circunstancia”) by Toribio (1992a) when he reached the Full Professorship in the Spanish University (BOE, 1991; 1992). In a review paper of his previous scientific research work, Toribio (2000) re-formulated the idea in English (“the material is itself and its circumstance”). In this philosophical way of reasoning, it is clear that a given material does not have a behavior per se , but can exhibit one or another depending on the circumstance (in the form of mechanical loading history and also of physico chemical environment). In the paper by Toribio (2000) hydrogen plays the role of circumstance , showing the effect of history (of hydrogenation versus cracking) from the viewpoint of fracture mechanics, damage tolerance and microstructural integrity, demonstrating the important role of previous history (the yesterday influences the today, and the latter does the same on the tomorrow), recalling the words of the Spanish major poet Antonio Machado “hoy es siempre todavia”, and perhaps T.S. Elliot’s “and all is always now” (Toribio, 2000). This paper deals with hydrogen effects on cold-drawn pearlitic steel wires . These materials are usually denoted as prestressing steel wires (Toribio, 1992b) and, due to their good mechanical properties in civil engineering, they can be considered as high-performance structural materials (Toribio, 2006) because exhibit good mechanical properties associated with an extremely high yield strength (  Y ) and ultimate tensile strength (  R ). In addition, Toribio (2006) pointed out how they behave as micro-composites , so that their oriented lamellar microstructure influences their fracture behaviour due to their inherent anisotropy induced by the manufacturing process in the form of multi-step (progressive) cold drawing (Toribio, 2004). In the case of prestressing steel wires, the first role of circumstance (in the sense proposed by Ortega y Gasset) is played by the manufacture process by increasing cold drawing during which a strain hardening mechanism is activated, so that the cumulative plastic strain produces an increase of yield strength and ultimate tensile strength (UTS), as recently described by Toribio (2017). Such a yield strength improvement is related to the decrease of interlamellar spacing after cold drawing, although a Hall-Petch relationship cannot be fitted in the case of oriented pearlitic microstructures, as shown by Toribio (2004). However, in the case of cold-drawn pearlitic steels with oriented pearlite, an innovative (modified) Hall-Petch equation in the form of that proposed by Embury and Fisher (1966) has been formulated by Toribio et al. (2014). In the matter of the UTS, it is very high and only limited by cleavage fracture ( cleavage limited strength ), as explained by Gil-Sevillano (1986). In this paper, the second role of circumstance (in the sense proposed by Ortega y Gasset) is played by hydrogen, reviewing previous work of the author about hydrogen degradation (HD) or hydrogen embrittlement (HE) or, at the micro-level, hydrogen-assisted micro-damage (HAMD) of progressively cold-drawn pearlitic steels. Aktough the classical term of HE is used, the concept of HD is preferred, as used by the European Structural Integrity Society (ESIS) in its TC10 Committee of Environmentally Assisted Cracking (EAC) together with a Sub-Committee on HD (Toribio and Nykyforchyn, 2017a), organizing a special symposium on EAC & HD during the 2 nd International Conference on Structural Integrity (ICSI 2017) held in Madeira (Toribio and Nykyforchyn, 2017b).

2. Progressive (multi-step) cold drawing of pearlitic steels

Manufacture of commercial prestressing steels is made by progressive (multi-step) cold drawing of a previously hot-rolled bar with pearlitic microstructure. Fig. 1a shows two views of the real cold drawing chain in the factory, Fig.