PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2438–2443 Jesús Toribio / Procedia Structural Integrity 00 (2020) 000–000

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2. Microstructural anisotropy Manufacturing of prestressing steel wires by progressive (multi-step) cold drawing of pearlitic steels induces microstructural anisotropy ( orientation ) in the material in the matter of both pearlitic colonies and ferrite/cementite lamellae (Toribio and Ovejero, 1997, 1998a, 1998b, 1998c). 3. Strength anisotropy The aforesaid microstructural orientation produces a material with strength anisotropy. As a consequence, crack deflection (or deviation from the original mode I propagation direction) arises and mixed mode propagation takes place with a strong component of mode II and a deflection angle close to 90º (Toribio, 2002). Thus the fracture toughness becomes a property dependent on the crack path (and the subsequent deflection angle), so that the concept of directional fracture toughness could be introduced, and evaluated from the fracture mechanics experiments on cold drawn pearlitic wires, as discussed by Toribio (2002). Fig. 1 (Toribio, 2002) plots such a property as a function of the number of drawing steps undergone by the pearlitic steel, i.e., by its degree of cold drawing. It is seen that slightly drawn steels (including the hot rolled base material that is not cold drawn at all) behave isotropically with a unique fracture toughness. On the other hand, heavily cold drawn pearlitic steels (with oriented pearlite microstructure) behave anisotropically, so that the fracture toughness for mode I propagation (breaking the strongest links) K IC (0º) is quite higher than the fracture toughness for a 90º deflection angle K IC (90ª), i.e., for longitudinal splitting or delamination .

Fig. 1. Directional fracture toughness of the progressively cold drawn pearlitic steels (Toribio, 2002).

4. Anisotropy of hydrogen embrittlement (HE) 4.1. Constant strain tests (CST)

With regard to hydrogen embrittlement (HE) behaviour of pearlitic steels, one of many acceptable techniques used to determine the rate of hydrogen assisted cracking (HAC) slow crack growth in high strength steels is the performance of constant strain tests (CST). In the paper by Toribio and Lancha (1998), tests were carried out on three-point bend prismatic pre-cracked specimens. Samples were subjected to constant strain tests in a cell containing an aqueous solution of 1 g/l calcium hydroxide plus 0.1 g/l sodium chloride (pH=12.5). All tests were carried out under potentiostatic control, and the electrochemical potential was E = –1200 mV SCE, associated with a phenomenon of HAC, especially dangerous from the structural engineering point of view since it produces catastrophic failure without previous notice. Both the load on the sample and the crack length were measured during the test. Strain was increased step by step to have different points of the curve crack growth rate vs. stress intensity factor (Toribio and Lancha, 1998).