Issue 33

J. Toribio et alii, Frattura ed Integrità Strutturale, 33 (2015) 221-228; DOI: 10.3221/IGF-ESIS.33.28 221 Focussed on characterization of crack tip fields Crack tip fields and mixed mode fracture behaviour of progressively drawn pearlitic steel J. Toribio, B. González, J.C. Matos University of Salamanca toribio@usal.es A BSTRACT . This paper deals with the influence of the cold drawing process on the fracture behaviour of pearlitic steels. To this end, fracture tests under axial loading were performed on steel wires with different drawing degree (from a hot rolled bar to a commercial prestressing steel wire), transversely pre-cracked by fatigue, analyzing in detail the changes in fracture micromechanisms. The deflection angles of the fracture path were measured by longitudinal metallographic sections and the characteristic parameters of the load- displacement plot were related to different fracture events. Results allowed a calculation of critical stress intensity factors for different fracture angles and drawing degrees, thus evaluating the strength anisotropy and obtaining a sort of directional toughness. K EYWORDS . Progressively drawn pearlitic steel; Strength anisotropy; Crack path deflection; Mixed mode fracture; Directional toughness. I NTRODUCTION he micro-mechanisms taking place during the fracture process of metals and alloys, as well as the fracture toughness, change with the temperature, the loading rate, and the cold work [1-3]. In the particular case of pearlitic steel used to produce high-strength cold-drawn prestressing wires, the cold drawing process affects the phenomenon of the fracture, so that the most heavily drawn steels (undergoing severe plastic deformation) exhibit strength anisotropy, and a change in the crack propagation direction, which approaches the wire axis or drawing direction [4]. This leads to the calculation of two values of the angular fracture toughness, one in the radial direction and the other in the axial one, the first being much greater than the second for steels with severe plastic deformation [5-7]. In wires with axisymmetric notches, the degree of the fracture anisotropy also depends on the notch geometry [8]. The fracture surface of pearlitic steel presents a change in the fracture mechanism as the wire is drawn [6]. In the hot rolled wire the fracture is produced through cleavage, while in the early stages of cold drawing there appears fracture caused by growth and coalescence of microvoid and then cleavage. On the other hand, the heavily drawn steels exhibit, after the propagation of microvoids in mode I, a step at about 90º followed by a mixed propagation of microvoids and cleavage [4]. The fracture behaviour of pearlitic steel mainly depends on the size of the prior austenite grain [9-11]: the smaller the size of the grain, the greater the fracture toughness. If the pearlitic steel is cold drawn, then the pearlite colony, rather than the prior austenite grain, is the critical fracture unit determining the size of the cleavage facet [12]. This paper deals with the anisotropic fracture behaviour of eutectoid pearlitic steel wires with different degree of cold drawing (distinct level of strain hardening and diverse microstructural arrangements) exhibiting strength anisotropy. The analysis is focused on the crack path deflection angle and the directional toughness. T