Issue 50

A. Tijani et alii, Frattura ed Integrità Strutturale, 50 (2019) 141-148; DOI: 10.3221/IGF-ESIS.50.13 142 I NTRODUCTION lot of civil engineering structures use wire ropes [1] like suspended bridges, cable stayed bridges, prestressed concrete structures or cableways. Being involved in the structure of a large part of road and rail networks, these elements are subject to mechanical, thermal and environmental solicitations. Thus, regular monitoring and maintenance are mandatory in order to avoid any consequent economic and social disagreement due to the closure of the supported structure. The main function of a wire rope in an engineering structure is to transmit the traction load from the deck to the piles (suspended and cable stayed bridges) or to compress the concrete (prestressed structures). According to their use, wire ropes are exposed to variable loading levels in more or less aggressive environment. These cables are made of drawn steel wires which have a very high tensile strength and a carbon content close to the eutectoid [2]. Wire drawing is a process of wire transformation which consists in hardening the machine wire and then improving their mechanical characteristics by reducing their diameters when passing through dies. Thus, wire ropes have high yield strength and can support high axial loads. Thanks to their multi-component nature, the cable can continue to ensure its function in spite of the breakage of one or more wires. Indeed, in a multi-stranded cable, a broken wire has the ability to re-anchor by friction and to recover its load capacity [3]. Moreover, their flexibility makes it possible to store them on coils, which facilitates their implementation on structures, but also promotes their use in lifting gear thanks to the possibility of passage through pulleys. During their service life, broken wires appear on the wire ropes. In that respect, most codes and regulations use the number of broken wires as a criterion for quantifying the degradation level [4]. Moreover, regarding the low alloy steel composition and the environmental conditions (marine atmosphere, pollution, rain, etc.), the cables have an "ordinary" predisposition to corrosion [5]. So far, it is still difficult to quantify the impact of corrosion on the state of the structure and its safety of use. Drawn steel corrosion has been investigated in conditions simulating the wire rope loading in prestressed concrete structures (stress corrosion) [6] and in bridges (fretting-fatigue corrosion) [7]. The Fatigue strength reduction due to corrosion was studied in the case of bolted joins using an accelerated process of the marine-industrial environment [8]. Fretting fatigue of wire ropes used in coal mines has been studied in four different corrosive medias [9]. This work was carried out at the scale of the wire. Other research focused on the breakage of wires in wire ropes and their impact on the cable behavior. Beltrán and De Vico used numerical modeling for the study of wire ropes behavior with asymmetric damage [10,11].Wang et Al [12] investigated the wire failure mechanisms by acoustic emission, Torkar and Arzensi [13] investigated the failure of a crane wire rope. The main reasons were fatigue and lack of inspection. This work is a contribution to the methods of predicting the wire ropes behavior and life time. The influence of corrosion is investigated at the strand’s scale. The choice of carrying out our study on the strand’s scale is justified by the following points: - The strand is representative of the wire rope. This is, in fact, a specific case of wire rope construction (strand wire). The results obtained for a strand can therefore be extended to the case of the cable; - Taking into account the evolution by layers of the corrosion [14], it is necessary to have data not only on the corrosion of the constitutive wires but also on that of the strands. The proposed method is based on accelerated corrosion by immersing strands in sulfuric acid for different lengths of time. Besides, damage estimation is obtained by only static tensile tests. M ATERIAL AND METHODS he wire rope used for this research is manufactured by Bridon, of type 19*7 (1 * 7 + 6 * 7 + 12 * 7), rotation resistant, 10 mm diameter, with independent wire rope core (IWRC), right hand lay and preformed. General characteristics of the cable are given on table 1. Specimens preparation Samples are prepared in accordance with the ISO standard 6892 [15] relative to tensile tests on metal materials. Strands are extracted from the external layer of the wire rope cut beforehand to a length of 300 mm (200 mm according to the ISO standard over 100 mm required for mooring). This operation is carried out with particular care so as not to damage the specimens. Then, in the same way, wires are extracted from the strands. A T