13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- (non cold drawn at all) and, secondly, as a commercial prestressing steel wire which has undergone seven cold drawing steps up to reaching a cumulative plastic strain εP=1.6 and a posterior stress-relieving treatment to eliminate, or at least diminish, residual stresses. Steel was supplied in form of wires with circular section, the diameter ranging respectively between 11 and 5 mm for the hot rolled bar and the prestressing steel wire. Cold drawing produces a clear improvement of conventional mechanical properties (Table 1) obtained from a standard tension test: both the yield strength (σY) and the ultimate tensile strength (UTS, σR) increase with cold drawing, while the Young’s modulus (E) remains constant and the strain at UTS (εR) decreases with it. Table 1. Mechanical properties of the material in both conditions, i.e. as a hot rolled bar and as a prestressing steel (cold drawn) wire Steel E (GPa) σY (MPa) σR (MPa) εR Hot rolled bar 202 700 1220 0.078 Prestressing steel wire 209 1480 1820 0.060 Cold drawing also improves the fatigue and fracture behaviour of eutectoid steel. The fracture toughness KIC was obtained by means of fracture test on precracked wires under tensile loading, as described in [10]. The KIC value increases from 53 MPa·m1/2 in the hot rolled bar to 137 MPa·m1/2 (for θ=0º, fracture toughness in the transverse direction) in the prestressing steel wire, where cold drawing also induces an important strength anisotropy with a directional fracture toughness whose value is dependent on the particular axis of analysis. Values C and m (constants of the Paris law) were obtained by means of tensile fatigue tests, as explained in ref. [11]. The m coefficient in the Paris law (slope of the line) is the same for the two steels and rounds the value 3, whereas the C parameter decreases with cold drawing, changing form 5.3·10-12 in the hot rolled bar to 4.1·10-12 in the cold drawn wire (units for C and m are the adequate to measure da/dN in m/cycle and ΔK in MPa·m1/2). 2.2. Test Procedure Wöhler fatigue tests were performed under tensile load control with constant Δσ, sinusoidal wave shape, frequency of 10 Hz, R-ratio R=0 and a maximum stress lower than the yield stress σY (some S-N tests were performed under a stress range of about half the yield strength). The specimens were in the form of 30 cm long bars of circular cross section and the same diameter as the supplied wires. A total number of 20 tests were performed. Fracture surfaces were analyzed by scanning electron microscopy (SEM). 3. Experimental Results 3.1. Microstructure Figs. 1 and 2 show the microstructure of both steel forms, hot rolled bar and prestressing steel wire, in both transverse and longitudinal section, where the horizontal side of the micrograph corresponds to the radial direction and the vertical side is associated with the axial direction in the longitudinal cut and with the circumferential one in the transverse cut. Cold drawing produces important microstructural changes in the pearlitic steel [12, 13] in the form of slenderizing of pearlitic colonies, decreases of interlamellar spacing of pearlite and progressive orientation with cold drawing of both colonies and lamellae. Thus, the transverse section (Fig. 1) shows that the lamellae evolve towards a structure with increased packing closeness while at the same time adopting a curved appearance (curling phenomenon) from the very beginning of the cold drawing
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