Steel wires are widely used in various industrial applications. Hence, the drawing process and the resulting mechanical properties are of significant scientific and industrial importance. In this investigation the focus is on pearlitic steel wires with relatively high drawing strains and resulting high ultimate tensile strengths up to several GPa.
The fracture behavior was investigated for two different wires with a drawing strain of 3.10 and 6.52 with a diameter of about 100 µm and 20 µm, respectively. The resulting ultimate tensile strength varies between 4 and 7 GPa [1]. The fracture toughness was measured with crack propagation direction in drawing direction and perpendicular to it. To test the fracture toughness of the samples in drawing direction micro-bending beams were fabricated utilizing a focused ion beam (FIB). For investigating the direction perpendicular to the drawing direction, the wires were notched with a FIB and tested under tensile and bending loading. The fracture toughness experiments for both directions were performed in-situ in the SEM. In addition, some samples of the perpendicular direction were tested ex-situ as well.
The results of the fracture experiments show a strong anisotropy of the fracture behavior. It was revealed that in drawing direction the wires show a significantly lower fracture toughness than perpendicular to it. This is further supported by the in-situ and ex-situ bending experiments of the second testing direction, where the crack kinks into the drawing direction.



[1] Y. Li, D. Raabe, M. Herbig, P.-P. Choi, S. Goto, A. Kostka, H. Yarita, C. Borchers and R. Kirchheim, Segregation Stabilizes Nanocrystalline Bulk Steel with Near Theoretical Strength, Phys. Rev. Lett. 113 (2014) 106104.