PSI - Issue 13

Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 584–589 M. Iordachescu, M. de Abreu, A. Valiente/ Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Today, pre or post-tensioning tendon-cable systems using multiple strands are incorporated into a vast array of structures such as bridges, buildings, cryogenic liquefied natural gas (LNG) tanks, dams, nuclear power stations, retaining walls, tunnels, and wind towers (Dywidag, 2017 and BBR VT, 2009). The strands are made from 7 individual high-strength, cold-drawn eutectoid wires, 6 helically wound outer the central straight one (EN ISO 15630-3). Essentially, they carry tensile loads, but transverse loads from different sources may appear due to construction tolerances and misalignments, wind loads or temperature changes. These are generally accompanied by bending stresses in the anchorage systems or in their couplers used to form arc-shapes along the prestressing length. In both situations, the bending stresses might be of the same order of magnitude as the axial tensioning stresses and the results in reduced fatigue strength of strands (Cullimore, 1972, McTyer and Evans, 2017, Dywidag, 2017, BBR VT, 2009). Regarding this issue, additional data is required not only for structural design purposes but also for development and certification of new generations of prestressing wires. This paper gives new insights on the failure behaviour of two types of high-strength, heavily cold-drawn duplex stainless steel wires when simultaneously subjected to static transverse and longitudinal loadings, with the latter ones being of a fully tensile or cyclic nature. These are potential candidates for pre or post-stressing purposes, assuring similar strength levels, higher damage tolerance and significantly higher resistance to stress corrosion than the eutectoid steels wires (Valiente and Iordachescu, 2012, Iordachescu et al, 2015). The experiments were performed with a specially designed device to control a locally applied transversal compression during the wire loading in simple or cyclic tension. Thus, the results concerning the static bi-axial loading of the duplex stainless steel wires could be compared with those of currently used prestressing eutectoid wires: on this empirical basis, a unique fracture criterion is proposed. Scanning electron microscopy (SEM) was used to determine the corresponding failure macromechanisms of the tested wires as well as the differences in the fatigue damage when static transverse loading was combined with a longitudinal cyclic one.

Nomenclature DSS

high-alloyed duplex stainless steel wire prestressing eutectoid steel wire

ES

LDS

low-alloyed duplex stainless steel (lean duplex) wire

P

tensile load

P 0 P m

maximum load in simple tension

tensile bearing capacity under transverse loading maximum applied tensile fatigue load

P Fmax

Q transverse, constant compression load T-QL tensile test under transverse loading F-QL fatigue test under transverse loading

2. Wire materials and testing methods

The studied high-strength wires, of 4 mm diameter, were manufactured by cold drawing from two distinct classes of duplex stainless steels: the first a low-alloyed (LDS) and the second a high-alloyed (DSS) one. In addition, a current prestressing eutectoid steel wire (ES), of the same diameter, was used in the experiments for comparative purposes. Table 1 summarizes their mechanical properties. These were experimentally obtained by tensile testing wire samples of 350 mm length, at room temperature. The tests were performed with a 200 kN servo-hydraulic machine by using a constant crosshead speed of 1 mm/min, with the elongations being measured on a 12.5 mm gauge length with a conventional clip-on extensometer. The data were further used as a reference for evaluating the tensile bearing capacity of wires under static bi-axial loading. The first testing method employed in this study, namely the tensile test under transverse loading (here referred as T-QL), was aimed at assessing the sensitivity to transverse loading induced by the contact of a tensioned wire with a rigid boundary. In this view, a compression load was perpendicularly applied on the wire surface and held constant