DESARROLLO NACIONAL Y PLAN DE DESARROLLO DEPARTAMENTAL 1.12.1 OBJETIVOS DE DESARROLLO DEL MILENIO

The wires that make up the cables are manufactured from high carbon steel with nominal tensile strengths in the range of 1570 MPa to 1770 MPa. Their high tensile strength is attributed to the wire drawing process (i.e. cold working) and heat treatment processes, and the inclusion of small amounts of chromium, silicon and vanadium (Walton, 1996). The cold drawing process produces important micro- structural changes which influence the strength of the material (Kumeria et al 1990; and Parkins et al 1982). For example, the wires used to make up the cable types highlighted in Fig. 2.3 are typically worked from 12 mm diameter high carbon wire rods with ultimate tensile strengths in the range of 750 MPa to 1150 MPa (Corus High Carbon Wire Rod, 2002). The working process typically consists of a progressive pass of the wire rods through a series of conical dies with the diameter progressively becoming smaller with each pass until the desired diameter is achieved. Through each die the steel is plastically deformed. Following this, the wires are normally thermo- mechanically treated (typically annealed) to eliminate crystalline defects induced by the drawing process, resulting in a more stable structure with increased ductility. In the wire drawing industry, annealing is typically used to re-induce ductility after the elimination of the ductile micro-structural slip planes that had existed before cold drawing. The result of which is a softened and stress-relieved material.

In a study by Toribio and Ovejero (1998) the microstructure of high strength eutectoid steel used for civil engineering purposes in pre-stressed concrete was observed after each stage of a cold drawing process. Longitudinal and transverse sections were studied as depicted in Fig. 2.5.

Fig. 2.5 Placement and orientation of the longitudinal (L) and transverse (T) metallographic section (Toribio and Ovejero, 1998)

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Figures 2.6a (0) and 2.6b (0) (L and T) refer to the steel rod before drawing. Figures 2.6c (3) and 2.6d (3) (L and T) represent a drawn down rod after 3 passes through the dies and Figures. 2.6e (6) and 2.6f (6) represent the fully drawn rod down to wire. For the longitudinal cut section it was observed that before drawing, the microstructure of the rod had a randomly orientated microstructure with no preferential orientation angle with respect to the loading axis. In the following passes it was seen that the lamellae had a tenancy to align in the drawing direction resulting in a closer and more orientated packing resulting from the plastic deformation induced from drawing. In the transverse direction, the only variation was the shape of the lamellae. Toribio and Ovejero highlighted that the drawing process produced a compressive stress resulting in kinking of the pearlite plates, however the orientation angles was in the same range as the previous steps. Therefore after drawing the wire microstructure is considerably anisotropic.

(a) L (0) (b) T (0)

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(e) 6L (f) 6T

Fig. 2.6 Longitudinal and transverse micrographs high strength eutectoid steel after cold drawing (Toribio and Ovejero, 1998)

The Young’s Modulus (E) of the wires is approximately 190 MPa, however when the wires are spun into a cable the modulus of the whole cable is less than that of the individual wires and is dependent on the lay length and lay angle. The longer the lay length the nearer the modulus comes to a straight wire and vice-versa i.e. the straighter the wires the greater the axial stiffness of the cable (Walton, 1996). The lay length describes the distance after which a wire reappears at the same angular position along the longitudinal axis of the cable. It is different for each layer of wires as the diameter of each layer increases. The breaking strength of the cable is dependent on the lay length, being lower for shorter lays and greater for longer lays. For spiral strand cables the lay lengths normally range between 9 and 12 times the cable diameter. Within locked coil strands however the z-shaped wires require longer lay lengths due to their geometry and interlocking requirements and this is another factor why they have greater axial stiffness’s than spiral strands.

2.1.8 Strength

Uniaxial tensile strength tests on individual wires is normally carried out by the cable manufacturer to obtain an engineering stress-strain relation which is used to determine the basic mechanical properties such as the Young’s Modulus (E), yield strength σ0.2 and ultimate tensile strength (UTS) of the wire. A measure of wire ductility is also established from the wire elongation and reduction in cross-sectional area at failure. Typical test data provided by Bridon for both 1570 MPa and 1770 MPa strength wires

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of 5 mm diameter are listed in Table 2.1, where 0.2% proof stresses are given because from tests no obvious yielding points can be seen in the stress-strain relations.

Table 2.1 Typical mechanical properties of 1570 MPa and 1770 MPa strength wires (provided by Bridon, 2010 Appendix A)

Nominal Wire Strength (MPa) Wire Diameter (mm) Breaking Load (kN) 0.2 % Proof Stress σ 0.2 (MPa) Young’s Modulus (MPa) Ultimate Tensile Strength (MPa) 1570 5 32.66 1391 200.4 1664 32.7 1402 191.6 1665 32.73 1401 201.3 1667 33.95 1430 206.9 1729 34.01 1410 202.7 1732 34.01 1431 192.8 1732 34.07 1438 204.8 1735 34.11 1435 196.7 1737 34.06 1356 192.7 1735 34.14 1435 215 1739 1770 5 36.95 1558 195.3 1882 37.34 1611 200.4 1902 36.99 1606 202.4 1884 37.19 1595 188.4 1894 37.33 1604 199.6 1901

Some typical engineering stress-strain curves are shown in Fig. 2.7 for 1570 MPa and 1770 MPa strength wires also provided by Bridon.

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Fig. 2.7 Typical engineering stress-strain relations for 1570 MPa and 1770 MPa strength wires (provided by Bridon, 2010 Appendix A)

When deciding upon the allowable stress level, the effect of relaxation must also be taken into account (Tibert, 1999). Tests on steel wires show that the relaxation accelerates when the wire is held under a permanent stress larger than 50% of the ultimate tensile strength, therefore the stresses induced in the cable for permanent loading should not exceed 45% of the tensile strength (Gimsing, 1997).