PSI - Issue 28

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

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peripheral wires testing the braiding induced deformation at strand forming, the influence that the non-simultaneous breaking of all the strand wires can have on these results, and the existent differences in plasticization and strain hardening prior to testing. Fig. 4c and the detail presented in Fig. 4d illustrate the acquired curvature of the wires when conforming the strand which was also noted in the 12.5 mm cage length of the resistive extensometer used in the tensile test. Consequently, and despite the precautions adopted to induce the tensile load on the specimens by means of articulated clamps (no moment transmitting) the test was not a simple tensile test but a tensile-bending test. The results are presented in Fig. 4d, as stress vs. percentage elongation curves. In the four tests shown in the figure, the resistive extensometer was positioned on the wire generatrix of maximum concavity; three tests were made on peripheral strand-wires and one on the central wire. As may be seen in Fig. 4d, the maximum stress of all the tested samples is quite similar (1860 ± 20 MPa) and offers the certainty of considering it as being the tensile strength of steel wires, since it is reached by plastic instability when the specimen curvature has vanished and the stress state in the wire cross-sections was still uniform. The elastic modulus of the cold-drawn eutectoid steel from which the prestressing strands are manufactured of 205 GPa (prEN 10138-2, 2009) is perfectly compatible with the unloading lines slopes shown in Fig. 4d, and recorded during testing because the wire plastic instability occurred outside the extensometer gage. In contrast, the elastic load slopes are appreciably lower, of the order of 150 GPa, with the exception of that of the central wire, of 200 GPa. The peripheral wires bending explains their higher deformability. The percentage elongation under maximum load exceeds in all cases that of 3.5% required by standards as ductility condition (EHE, 2008), though dispersion suggests that two of the specimens have lost part of their ductility before testing, by previous plastic deformation and strain hardening at strands failure. The central strand-wire belongs to this group, which, given that it is generally less exposed to external agents, suggests that the strand failure occurred due to an environmentally assisted damage processes. The other two wires would have experienced a different failure sequence, because a previous localized damage process could have caused their failure under service loads. Table 1 presents the mechanical properties of the tested wires; they exceed the minimum values required by the current regulations in force (EHE, 2008), despite the service time.

Fig. 4. Images of: a) wires from a strand breaking end; b) un-corroded wire-strand section, far for failure; c) selected wire-samples for the mechanical tests, after the sheath removing, and d) stress vs. percentage elongation curves of the strand-wires after 30 years of service.

Table 1. Mechanical properties of the tested wires Elastic modulus, E [GPa] Yield strength, R p0.2 , [MPa]

Tensile strength, R m , [MPa]

Percentage elongation at maximum load, A 5 [%]

Steel wire

205

1720

1860

> 3.5

3. The effect of surface damage on stress corrosion resistance of the strand-wires The test, referred to as to SCC-CL and generally employed for the study of stress corrosion cracking resistance of prestressing steel-wires consists of maintaining a wire sample exposed to the FIP medium (20% aqueous solution of