PSI - Issue 82

Celalettin Baykara et al. / Procedia Structural Integrity 82 (2026) 206–212 C. Baykara et al./ Structural Integrity Procedia 00 (2026) 000–000

209

4

3. Results and discussion Fatigue test results for single-lap joints with adhesive thicknesses of 1 mm (in Figure 3), 2 mm (in Figure 4), and 3 mm (in Figure 5) were obtained under three force levels: 0.5 kN, 0.45 kN, and 0.4 kN. At the highest load level of 0.5 kN, all adhesive thicknesses showed relatively low fatigue lives (<8000 cycles). Among them, the 2 mm adhesive exhibited the highest resistance (7,673 cycles), slightly outperforming both the thinner (1 mm) and thicker (3 mm) joints. This suggests that a moderate thickness offers better stress distribution and less severe peel stress concentration at the lap edges, which is consistent with the numerical results presented by Metehri et al. (2024) and the fatigue models of Beber et al (2017). At 0.4 kN, fatigue performance increased significantly for the 1 mm and 2 mm joints, with both reaching approximately 32,000 cycles to failure. The similar performance of these two indicates that both adhesive thicknesses are capable of handling moderate cyclic stresses effectively, though likely through different mechanisms—stiffness in thin layers vs. stress absorption in moderately thick ones.

Fig. 3. Fatigue test results for 1 mm adhesive thickness, (a) 0.5 kN load to 6723 cycles, (b) 0.45 kN load to 20274 cycles, (c) 0.4 kN load to 32802 cycles.

Fig. 4. Fatigue test results for 2 mm adhesive thickness, (a) 0.5 kN load to 7673 cycles, (b) 0.4 kN load to 32417 cycles.

In contrast, the 3 mm bond-line exhibited substantially lower fatigue life (15,609 cycles at 0.4 kN). At this load level, the adhesive layer nearly detached entirely from the aluminum substrate, indicating severe interfacial degradation. This poor performance is due to the fact that as the thickness of the polyurethane-based adhesive increases, its flexibility also increases, leading to cohesive failure of the adhesive. This results in higher shear deformation, local damage accumulation, and inadequate stress transfer. These findings are consistent with previous studies (Heidarpour et al., 2018; Biscaia et al., 2024) showing that excessive adhesive thickness can compromise fatigue durability due to interfacial failure and internal strain localization.