PSI - Issue 39

Jesús Toribio et al. / Procedia Structural Integrity 39 (2022) 722–725 Author name / Procedia Structural Integrity 00 (2021) 000–000

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1. Introduction The role of microstructure in fracture behaviour of materials is out of doubt. In the case of pearlitic steels, the microstructural features governing fracture performance can be analyzed in a multi-scale approach, since both pearlitic colonies (first microstructural level) and pearlitic lamellae (second microstructural level) could be relevant. In randomly oriented pearlitic microstructures, the prior austenite grain size was shown to be the microstructural parameter governing the fracture process (Park and Bernstein, 1979; Alexander and Bernstein, 1982). However, when pearlitic steels are heavily cold drawn to produce prestressing steel used in prestressed concrete, such a manufacturing procedure 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 producing, macroscopically, anisotropic fracture behaviour (strength anisotropy) and crack path deflection. This paper analyzes the necessity of stress triaxiality (constraint) in the form of cleavage stress (in addition to the microstructutral anisotropy) to produce macroscopic crack path deflection and macroscopically anisotropic fracture behaviour in cold drawn pearlitic steels. 2. Experimental programme Material was a cold drawn pearlitic steel (commercial prestressing steel). Fig. 1 offers the two microstructural levels (pearlite colonies and Fe/Fe 3 C lamellae), showing the orientation in the cold drawing (wire axis) direction.

Fig. 1. Orientation of pearlitic colonies (left) and ferrite/cementite lamellae (right) in a direction quasi-parallel to the wire axis or cold drawing direction (represented by the vertical side of the micrographs) in heavily cold drawn pearlitic steel.

Mechanical properties of the material are: Young modulus E = 205 GPa, yield strength σ Y = 1.57 GPa, ultimate tensile strength (UTS) σ R = 1.84 GPa, strain at UTS ε R = 0.053, fracture toughness in transverse direction K IC (0º) = 152.1 MPa·m 1/2 and fracture toughness in axial direction K IC (90º) = 54.2 MPa·m 1/2 . Samples taken from the steel wires (7 mm diameter) were axially pre-cracked by fatigue in air and subjected to increasing loading up to final fracture, although some tests were interrupted to perform a fracto-metallographic analysis to evaluate the crack paths. Fig. 2 shows a fracto-metallographic section of the crack path just before final fracture and the fractographic aspect of the vertical crack path (deflected an angle of 90º in relation to the initial crack propagation direction in mode I), a signal of strength anisotropy or macroscopically anisotropic fracture behaviour as a consequence of the microstructural anisotropy. The fractographic aspect resembles cleavage appearance, i.e., unstable (brittle) fracture. However, it is not conventional cleavage, but a sort of oriented and enlarged cleavage , its enlargement and orientation being along the wire axis or cold drawing direction.