PSI - Issue 78

Annarita Palmiotta et al. / Procedia Structural Integrity 78 (2026) 489–496

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Fig. 2. USFP geometrical configurations (Source: Coniglio et al., 2025)

USDDs were tested during an experimental campaign conducted at SISLab, the Engineering Structural Laboratory of the University of Basilicata (Potenza, South Italy). It was subjected to cyclic tests, carried out in a quasi-static regime with a displacement velocity v= 5 mm/s, having an increasing horizontal displacement according to the EN 15129 2018. Results on experimental tests and a numerical model based on the USDD are documented in the works of Braga et al. (2019) and Coniglio et al. (2025). Specifically, 3 groups of cycles were applied, having the maximum displacement equal to 25%, 50% and 100% of the horizontal displacement imposed (equal to 333,33 mm) as shown in Fig. 3a. Fig. 3b illustrates the experimental results in terms of force-displacement imposed. It is evident that within each group of cycles, the force-displacement curve shows a negative slope in the first and third quadrants. This slope decreases as the number of cycles increases, probably due to steel hardening. a b

Fig. 3. (a) time-history displacement imposed during experimental tests; (b) cyclic results (Source: Coniglio et al., 2025)

3. FEM model validated with experimental tests In Midas FEA NX, the numerical model was developed with the Construction Stages (CSs) method. This model incorporates non-linearities from both the USFPs stainless steel and contact and geometric elements, by not assuming small displacements. It is worth noting that the numerical model considered for this research incorporates all construction stages of the USFPs, including bending phases (Fig. 4), obtained by applying the rigid mandrel on the two undeformed USFPs (Fig. 4a). This ensures that the initial conditions of the USFPs at the beginning of the tests are accurately represented. Shell elements are used for modelling the mandrel and USFPs while the steel mold is modelled through two pinned supports that allow the outer plate to slide only in the horizontal direction when bending is applied. Moreover, contact conditions are assigned to the shell surfaces to inhibit penetration between the elements during the analysis, while considering only the impact friction between them. Following the mandrel's action, a horizontal shift occurs during an overbending phase (along the Y-axis, Fig. 4b). This ensures that once the mandrel is removed, the material springs back, leaving the straight sections of the UFSPs parallel, as in Fig. 4c.