PSI - Issue 17

Maricely De Abreu et al. / Procedia Structural Integrity 17 (2019) 618–623 M. De Abreu et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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helically wound outer the central straight one (prEN 10138-2, 2009 and FIB, 2005). The structural tendon-strands support high tensile loads, but they may also be subjected to contact transverse loads in the anchoring systems, couplers or guide deviators. These loads are transmitted to the strands wires and are added to those caused by their mutual contact by the strand tensioning. The undesired stresses that these transverse loads generate induce significant losses in the tensile bearing capacity and fatigue resistance of tendon-strands (Dywidag Systems, 2017 and BBR HiAm CONA, 2009). In recent years, numerous investigations regarding the fatigue resistance of eutectoid steel wires, from which high-strength strands for structural tendons are manufactured, have been carried out using Fracture Mechanics approaches. However, current research not only addresses the improvement of existing products in order to satisfy the current demand of the construction industry requiring the increase of the resistant capacity and durability of structural tendons, but also the development and assessment of new generation of high-strength wires, in particular that of cold-drawn duplex stainless steels, as potential candidates for prestressing. Thus, the references (Valiente A. Et al., 2005 and Iordachescu M. et al. 2015), compare the damage tolerance of transversely cracked duplex steel wires with that of conventional eutectoid ones, and for this purpose tensile rupture tests together with an elementary theoretical model of plastic collapse were used. The cold-drawing effects in each steel microstructure consistently explain the resulted differences, which favor the new generation of duplex steel wires. Similarly, the experimental data presented in (De Abreu M. et al., 2018) reveal that their stress corrosion resistance widely exceeds that of eutectoid wires. The objective of this work is to empirically determine the tensile fatigue life of high-strength wires used for the strands fabrication when simultaneously supporting concentrated static loads in transverse direction. Two types of cold-drawn wires, the first made of low-alloyed duplex stainless steel (LDS) and the second of eutectoid steel (ES) have been tested. The experiments have been carried out with a specially designed device that maintains constant a transverse compressive load locally applied on the wires while they are simultaneously subjected to cyclic tensile loading. To this end, a single stress range of 200 MPa was applied for being that required by the standards in force to assure the fatigue safe life of prestressing wires free from transverse loading (prEN 10138-2, 2009). Thus, a fatigue failure diagram showing the influence of the applied transverse load has been plotted from the obtained results. The diagram quantitatively describes the behavior of the two tested wires, shows the differences between them, and delimits the combinations of transverse and maximum tensile loads that produce their fatigue failure within the stress range of 200 MPa.

Nomenclature ES

high-strength eutectoid wire LDS high-strength lean duplex stainless steel wire P 0 tensile load in simple tension P max transverse compression load F-QL tensile fatigue test under local transverse loading Q

maximum tensile fatigue load under transverse loading

2. Materials and Testing Method

The studied high-strength wires of 4 mm diameter were manufactured by cold drawing from two distinct steel classes: the first belonging to a new family of low-alloyed duplex stainless steels known as "lean" duplex "(LDS), and the second a conventional eutectoid prestressing steel (ES). The microstructural features of the tested steel wires are presented elsewhere (Valiente A. Et al., 2005, Iordachescu M. et al. 2015 and De Abreu M. et al., 2018). Table 1 summarizes the mechanical properties at room temperature. These were determined from simple tensile tests, performed on samples of 350 mm length at a constant crosshead speed of 1mm/min on a 200 kN servo-hydraulic machine. A 12.5 mm resistive strain gauge was used for the strain measurement. In the fatigue tests under transverse load (F-QL) the wire samples were simultaneously subjected to axial cyclic tensile loading and to local static compression loading perpendicular to the wire axis. The testing device is shown in