PSI - Issue 77

João Nunes et al. / Procedia Structural Integrity 77 (2026) 593–600

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Joa˜o Nunes et al / Structural Integrity Procedia 00 (2026) 000–000

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membrane expansion when the hydrogen fuel cell is turned on. These measured values are relatively low during short-term operation, ranging from 0 to 20 µε . The red line shows the strain values before applying temperature compensation. Despite the relatively low values, the results indicate that the shafts are a suitable location for assessing loosening in the hydrogen cell tightening system.

3.2. Hydrogen Cell Loosening

To evaluate the adequacy of DIC to assess the loosening of the tightening system, and since the strains measured with the strain gauges were relatively low, the bolts were deliberately loosened and tightened to four di ff erent positions to assess whether DIC could detect the resulting shaft deformation, as shown in Figure 4 a). This procedure aimed to replicate the conditions reported to occur in this type of cell after prolonged functioning. After importing the image dataset into VIC-3D 9, a subset size of 27 pixels and a step size of 7 pixels were selected for the DIC analysis. Rigid body motion was removed, and the incremental correlation method with zero-normalised squared di ff erences was applied to minimise the influence of lighting variations. Figure 4 b) presents the principal Lagrange strain obtained after VIC-3D 9 software analysis.

Fig. 4. a) and b): two tightening positions provoked during experiments and c) Principal Lagrange strain during provoked loosening.

The principal Lagrange strain and strain gauge measurements obtained during provoked loosening tests were then compared, showing very similar results, as presented in Figure 5.

Fig. 5. Comparison of e1 (Principal) Lagrange Strain from DIC measurements (red) and Strain Gauge Measurements (blue) being number 1: test beginning, number 2: first bolt position, number 3: second bolt position and number 4: measurements after bolt tightening