PSI - Issue 28

Mihaela Iordachescu et al. / Procedia Structural Integrity 28 (2020) 39–44 Mihaela Iordachescu et al./ / Structural Integrity Procedia 00 (2019) 000–000

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2. Microstructure, surface state and mechanical characteristics of the strands wires after 30 years of service The samples supplied for the analysis of the service failure and for pertinent laboratory tests were strands sections, polyethylene sheathed, of approximately 3 m length, having at one of the ends the ruptures given in Fig. 2a and Fig. 2b. The microstructural analysis of the broken strand steel was made on metallographic samples extracted from the strand end protected by the polyethylene sheath. Fig. 2c shows the wires microstructure in the axial direction consisting of axially oriented fine pearlite colonies. In contrast, in transverse direction (Fig. 2d) the pearlite colonies are randomly oriented and the cementite lamellas are considerably smaller in length or when larger, have a curled form (Toribio J. and Valiente A., 2006). These differences in the microstructure, induced by cold drawing, are indicators of mechanical anisotropy, particularly of fracture resistance, which determines the failure mechanism of these high-strength eutectoid wires (Iordachescu M. et al., 2015).

Fig. 2. a, b) Location of the strands failures with respect to the polyethylene sheaths on the tie-down cables; c, d) Wire microstructure: c) in axial direction; d) in transverse direction

Fig. 3. EDX images showing: a) the Zn-coating in the wire´s cross section in the sheath-protected area of a broken strand; b,c) wires surface in the unprotected areas showing distinct states of cracking and dissolution of the Zn layer. The surface analysis of the wires in areas protected by the polyethylene sheath shows that they still preserve the Zn coating generated by galvanizing, as a consumable barrier against pitting corrosion. The results of the EDX analysis acquired on a wire sample cut in transverse direction are shown in Fig. 3a in which the zinc layer (in red) of about 60 µm thick, despite its porosity, continues to protect the wires steel (in green) in the sheathed strands. The porosity may be due to the partial dissolution of Zn in the poorly aerated environment inside the sheath over the 30 years of service. The situation changes significantly in the unprotected areas in the proximity of strands failure. The EDX analysis of the wire surfaces in these areas (Fig. 3b and Fig. 3c) reveal damage levels in the Zn-coating that reach its complete dissolution. Fig. 3b shows the main differences in the Zn-layer appearance on the wires surface, namely in the outer zone of the strand and in the contact with other wires: the first, of rough appearance is due to both the galvanizing process and the oxidation and formation of porosities by zinc dissolution up to the corrosion pits formation, while the second is smoother with transverse microcracks as a result of the combined action of the wires contact, the applied tensile load and the environment aggressivity that promote hydrogen-assisted corrosion (Iordachescu M. et al., 2018). Fig. 3c shows a higher damage level in the zinc coating up to its full dissolution in the strand-wire outer surface together with the iron oxide formation in the unprotected regions of the wire steel. The corrosion state of the steel wires in the proximity of the service failures can be seen in Fig. 2a, Fig. 2b and Fig. 4a. These images strongly contrast with that of Fig. 4c, which shows at macroscale the wire samples cut from the sheathed part of the strands (Fig. 4b), far away from the failure zone, selected for the laboratory experiments. The mechanical properties of the wires after 30 years of service were obtained by tensile testing of undamaged wire samples of 235 mm length (Fig 4c). The interpretation of these tests requires to be taken into account in case of