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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The hot rolled bar (not cold drawn at all) presented an isotropic HAC behaviour with mode I crack growth, i.e., with no changes in the crack propagation direction from the initial one perpendicular to the bar axis. The crack growth kinetics curve (da/dt-K I ) for this steel is plotted in Fig. 2. The threshold stress intensity factor is K th = 0.35 K 1C . For higher levels of K I there is a plateau (with da/dt  1.5x10 –7 m/s), then a sudden increase up to about 10 –3 m/s, a slightly sloped section and finally the catastrophic fracture. The actual plateau corresponds to a real sub critical stage of type II (with sub-critical crack growth rate) followed by a second pseudo-plateau (stage III) in which the crack growth rate can be considered as critical (near the final fracture). While the first plateau may be assumed to be environmentally (HE) governed, the second pseudo-plateau has a more defined mechanical nature, although a combined effect of hydrogen presence and stress concentration could exist.

Fig. 2. Crack growth kinetics curve (da/dt-K I ) for the hot rolled bar (Toribio and Lancha, 1998). With regard to the heavily cold drawn pearlitic steel wire, the most important experimental fact is the change in crack propagation direction from the initial one perpendicular to the wire axis. From a certain stage of crack growth, the crack becomes curved and approaches the cold drawing direction (wire axis and main average orientation of the pearlite lamellae), producing mixed mode propagation. This is a consequence of the manufacturing process (cold drawing), consistent with the lamellar structure of the steel and the oriented appearance of the pearlite lamellae in the direction of the drawing axis resulting in a highly anisotropic stress corrosion behaviour of the drawn steel, with the crack tending to grow along the path of minimum resistance to HAC. The problem of obtaining the crack growth kinetics curve of the cold drawn steel is extremely difficult in the framework of linear elastic fracture mechanics (LEFM) because of the aforesaid mixed mode propagation . Firstly, the stress intensity factor in mode I (K I ) cannot be obtained from simple analytical expressions since the crack is now curved, and the K I value would have to be computed for each particular crack path . Secondly, a crack inclined from the main loading direction is subjected to mixed mode loading so that a stress intensity factor in mode II (K II ) appears and in this case the crack growth rate would depend on both K I and K II , or any combination of them (i.e., in the form of energy release rate G for mixed mode propagation).