PSI - Issue 39

Jesús Toribio et al. / Procedia Structural Integrity 39 (2022) 560–563 Author name / Procedia Structural Integrity 00 (2021) 000–000

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1. Introduction In pearlite microstructures, the interlamellar spacing is the parameter governing fatigue crack growth because the ferrite/cementite interfaces block dislocation movement (Gray et al ., 1985), while the pearlite colony size controls fatigue thresholds (Ravichandran, 1991). With regard to the Paris regime, in cold drawn pearlitic steel the Paris law shifts to lower values as the drawing degree increases, so that he fatigue crack growth resistance increases with cold drawing (Toribio and Toledano, 1999). The presence of oriented pearlite colonies and lamellae promotes a tortuous crack path with crack deflections and bifurcations (Korda et al ., 2006a; 2006b) and delaying fatigue crack growth. This paper analyzes fatigue macro- and micro-crack paths (multi-scale approach) in progressively cold drawn pearlitic steels, with special emphasis on the role of the drawing-induced microstructural evolution. 2. Experimental programme The materials used were progressively drawn pearlitic steels with eutectoid chemical composition. Cold drawing affects the microstructural arrangement in the form of a slenderizing of the colonies, decrease of interlamellar spacing and orientation in the direction of cold drawing (wire axis) of both the colonies and the lamellae (Toribio and Ovejero, 1997, 1998a, 1998b, 1998c), i.e., microstructural anisotropy, as shown in Fig. 1.

Fig. 1. Microstructures of the hot rolled bar (left) and the cold drawn wire (right) in longitudinal sections. Vertical side of the micrograph is always parallel to the wire axis or drawing direction, whereas horizontal side is associated with the radial direction of the cylinders.

Cold drawing improves the conventional mechanical properties of the steel. Both the yield strength and the ultimate tensile stress (UTS) increase with cold drawing. The stress-strain curves of the steels are plotted in Fig. 2.

Fig. 2. Stress-strain curves of the different steels.

Tensile fatigue tests on round wires were performed step by step under load control, the load being constant in a step and decreasing from one to another step, with an R factor equal to zero, and a frequency of 10 Hz. Tests were interrupted and a fracto-metallographic analysis was performed to examine the fatigue crack path after polishing.