PSI - Issue 78

Chiara Miglietta et al. / Procedia Structural Integrity 78 (2026) 309–316

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During the monotonic test of the specimen S2, test ID T2_M_1, lower loads were involved than in the previous test. As reported in the load-displacement curve (Figure 6(a)), a maximum load of about 10 kN was reached at a displacement of 100 mm. The hardening effect was observed at a displacement of approximately 60 mm, probably due to the loss of orthogonality between the applied load and the pipe’s longitudinal axis, which could cause a change in the stress distribution.

Fig. 6. Results of the monotonic test of S2: (a) load-displacement curve; (b) deformed configuration.

Strain gauges recorded the deformations of the plate near the weld in both upper and lower positions. As depicted in Figure 7(c), the lower part of the plate is deformed by tensile deformation, while the upper part is initially subjected to compression and then, as the deformation increases, to tensile deformation. This behaviour is reported in Figures 7(a) and (b). The upper strain gauge recorded compressive deformations up to a load of approximately 4 kN. Thereafter, a reversal of the sign of the strain was noted, due to the occurrence of local buckling phenomena. The red dotted line corresponds to the yielding deformation of the plate, ε y ; it yielded at a load of about 2 kN. On the other hand, the lower strain gauge recorded tensile deformations, and the lower region of the plate was found to yield at a load of approximately 2 kN. Figure 7(c) shows the deformed configuration at a final upward displacement of 100 mm.

Fig. 7. Deformations of S2 recorded by strain gauges: (a) upper strain gauge; (b) lower strain gauge; (c) detail of deformed configuration.

Regarding the monotonic test of specimen S3, test ID T3_M_1, the load-displacement curve in Figure 8(a) didn’t exhibit a peak load due to the achievement of the maximum capacity of the testing machine. The change in slope, at a load of approximately 27 kN, indicates the occurrence of local buckling phenomena. Second-order resisting mechanisms were responsible for the hardening effect observed in the second part of the curve. Figure 8(b) shows the deformed configuration at the end of the test, with an upward displacement of approximately 67 mm and a corresponding load of around 80 kN.