PSI - Issue 33

Jesús Toribio et al. / Procedia Structural Integrity 33 (2021) 1219–1224 Jesús Toribio / Procedia Structural Integrity 00 (2021) 000–000

1220

2

1. Introduction

Cold drawn pearlitic steel is a structural material widely used in wire form as a component of prestressed concrete structures in civil engineering. The manufacture process to produce cold drawn pearlitic steel wires is made by progressive plastic deformation from the initial hot rolled bar (free of plastic strain because it is not cold drawn at all) to the final commercial product (prestressing steel wire which has undergone a heavy drawing process). From the point of view of mechanical behaviour the cold drawing process produces an increase of yield stress and ultimate tensile strength of the steel by means of a strain hardening mechanism (Borchers and Kircheim, 2016). With regard to microstructural evolution, in the scientific literature there are important classical analyses (Embury and Fisher, 1966; Langford, 1977; Ridley, 1984). More recently, it has been shown that the cold drawing process produces a progressive orientation of the pearlite colony (first microstructural level) with its main axis approaching the wire axis or cold drawing direction, and a slenderising of the colony itself, as described elsewhere (Toribio and Ovejero, 1997; 1998a). In the matter of the pearlitic lamellar microstructure or second microstructural level, cold drawing produces both a decrease of interlamellar spacing and an orientation of the plates in the cold drawing direction (Toribio and Ovejero, 1998b; 1998c). The consequences of the afore-said microstructural arrangement is an anisotropic fracture behaviour of the most heavily cold drawn steels (Toribio, 2018; Toribio and Ayaso, 2020). This sort of anisotropic fracture is associated with mixed mode propagation and delamination cracks (Tanaka, 2016; Toribio, 2018; Toribio and Ayaso, 2020). In this paper a study of the markedly anisotropic fracture behaviour of the most heavily drawn wires is carried out to elucidate the important role of the cleavage hoop stress in promoting the anisotropic fracture.

2. Experimental programme

The materials used in the fracture test were eutectoid steel wires with different degree of plastic strain, from the initial hot rolled bar (not cold drawn at all) to the final commercial product (prestressing steel wire). Table 1 shows the chemical composition of the steel and Table 2 the mechanical properties of the different materials. The steels were named with the numbers 0 to 6, this number is an indication of the cold drawing chain’s step of each sample (and so the plastic strain level). Thus the 0 number is for the initial hot rolled bar and 6 is for the final product.

Table 1 . Chemical composition of the steel.

C

Mn

Si

P

S

Al

Cr

V

0.80

0.69

0.23

0.012

0.009

0.004

0.265

0.06

Table 2. Mechanical properties of the steels.

Steel

D (mm)

D i /D 0

E (GPa)

σ Y (GPa)

σ R (GPa)

0 1 2 3 4 5 6

12.00 10.80

1

197.4 201.4 203.5 197.3 196.7 202.4 198.8

0.686 1.100 1.157 1.212 1.239 1.271 1.506

1.175 1.294 1.347 1.509 1.521 1.526 1.762

0.9

9.75 8.90 8.15 7.50 7.00

0.82 0.74 0.68 0.63 0.58