PSI - Issue 84

L. Pisani et al. / Procedia Structural Integrity 84 (2026) 1144–1150 L. Pisani et al. / Structural Integrity Procedia 00 (2026) 000–000

1148

5

Figure 4 illustrates this behaviour for low carbon (rebar) steel and high strength harmonic steel at incremental stress. 3. Measurements on wires and strands by XRD One of the first experimental measurement test to verify the applicability of XRD to high strength prestressing steel wires has been performed at Politecnico di Torino in 2020. In evaluating XRD data, it is to be accounted for that the measurement of the stress state by XRD includes not only the value of the applied load but also the contribution of the stress arising from the wire manufacturing process. This stress, technically called Residual Stress, can be compressive or tensile and is to be subtracted from the overall XRD measured value. With reference to Table 1, at 0 kN applied load a tensile stress of +130 MPa was measured. This is due to the residual stress induced by wire production and, as mentioned above, is to be subtracted from the overall observed value, obtaining a result close to the applied stress as expected. The observed behaviour is repeatable and is systematically observed in experiments.

Fig 5: wire strand tensioned at incremental loads. (Politecnico di Torino, June 2020).

Table 1: Test results on 7 wire strand tensioned at incremental loads. Applied stress (MPa) XRD Measured stress (MPa)

XRD measured stress – residual (manufacturing) stress (MPa)

0

130 574 777 978

0

459 688 917

444 647 848

1146

1141

1011

Following the laboratory measurements, the technique has been used in several field applications, like Ponte di Brivio, whose results are reported in Table 2. The presence of a broken wire allowed a measurement at zero applied mechanical tension indicated as RS value that has been subtracted by the XRD average.