PSI - Issue 69

Diego Scaccabarozzi et al. / Procedia Structural Integrity 69 (2025) 80–88

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times for every sample, replacing each time the samples. This methodology provided a robust dataset to assess the reproducibility of the damping measurement. The loss factor ( ) was calculated using the half-power bandwidth method. This method determines the damping capacity based on the resonance curve (Lee et al., 2008). Physically, the loss factor reflects how quickly the system loses energy during an oscillation. Having a higher loss factor means the system dissipates more energy (stronger damping), which leads to a broader resonance peak. Furthermore, a low loss factor indicates less energy loss and a sharper resonance. This makes mandatory in evaluating the damping characteristics of materials or mechanical structures under dynamic loading. The loss factor can be obtained by: = ! ! "! " ! # (1) where: • # and $ are the frequencies at 3 dB below the resonance peak amplitude. • % is the resonance frequency at the peak amplitude. Previous frequencies shall be obtained from the measured Frequency Response Function (FRF), derived from the acquired stimulus and response of the tested specimens. 3. Results Differential scanning calorimetry Fig. 4 shows the evolution of the martensitic transformation in the explored thermal treatment conditions and in the as-built condition. The as-built sample exhibits a single-stage transformation, both upon heating and cooling, whereas the heat-treated ones present a double stage upon cooling (from austenite to R phase and from R phase to martensite). It may be observed that both heat treatments caused a slight shift of transformations to higher temperatures (A f is about 18 °C in as-built condition and 28 °C in T500-5’ condition). In details, it is well known that LPBF process promotes residual stresses, due to the fast cooling during the rapid solidification; such residual stresses could alter the martensitic transformation, as it can occurs in plastic deformation, where the phase transformation can be tuned at different temperatures and also completely suppressed under high levels of deformation degrees [Miller, 2001], [Biffi, 2019]. The measured transformation temperatures of the Nitinol samples in the investigated conditions are listed in Table 2. DSC scans show that the predominant phase present at room temperature is austenite in all the studied conditions, because the peak of the phase transformation lies below room temperature. Therefore, under limited loading conditions, it can be affirmed that further damping evaluation would be carried out for the austenitic phase. Conversely, the increase of treatment time did not induce a considerable peak shift, but rather produced a slight sharpening of said peaks.

Fig. 4. DSC scan of the Nitinol samples in as-built and heat-treated conditions.