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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In spite of these difficulties, an empirical treatment of the phenomenon is possible. To analyse the mixed mode propagation, the crack growth angle  was measured as a function of the crack length increment  a, the former evaluated in relation to the initial crack propagation direction (perpendicular to the wire axis), and the latter measured just from the point at which the crack leaves the initial crack growth direction in mode I and initiates the mixed mode path. Fig. 3 shows a plot of crack growth rate as a function of the direction.

Fig. 3. Crack growth rate as a function of the propagation direction (Toribio and Lancha, 1998).

For angles  ≈ 0º the crack growth rate is da/dt ≈ 3x10 –7 m/s, very similar to that obtained in the hot rolled bar, cf. Fig. 3 (in any direction since the latter has isotropic stress-corrosion behaviour). For angles between  ≈ 40º and  ≈ 90º the crack growth rate is da/dt ≈ 3x10 –4 m/s, i.e., three orders of magnitude higher than that for  ≈ 0º. Thus, although the properties of the hot rolled bar and the cold drawn wire are similar when measured in transversal direction, in longitudinal direction the cold drawn wire exhibits a dramatic reduction of resistance in the cathodic

regime of cracking (HE or HAC). 4.2. Slow strain rate tests (SSRT)

The HAC behaviour of progressively cold drawn pearlitic steels was studied by slow strain rate tests (SSRT) in transversely precracked rods by Toribio and Ovejero (1999, 2001, 2007). Fig. 4 shows the evolution of the fracture profile in HAC as the degree of cold drawing increases.

Fig. 4. Fracture profile of the different steels under HAC conditions (Toribio and Ovejero, 2007).

In the first steps of cold drawing (specimens 0 and 1) the crack growth develops in mode I in both fatigue precracking and HAC. In steels 2 and 3 there is a slight deflection in the hydrogen-assisted crack. The mode I propagation distance associated with subcritical crack growth by HAC only appears in the first stages of cold drawing, since in heavily drawn steels crack deflection takes place just at the fatigue precrack and thus the mode I propagation distance is zero