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

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Results at 0.45 kN were only available for the 1 mm adhesive (20,274 cycles), indicating a notable improvement from the 0.5 kN case. The absence of data for the 2 mm and 3 mm joints at this load level limits direct comparison, but the consistent improvement in the 1 mm performance suggests a strong sensitivity to small reductions in load amplitude.

Fig. 5. Fatigue test results for 3 mm adhesive thickness, (a) 0.5 kN load to 7266 cycles, (b) 0.4 kN load to 15609 cycles.

To better illustrate the effect of adhesive thickness on fatigue life, a force–life (F–N) curve was plotted for each thickness level using the applied force values (converted to nominal shear stress) against the logarithm of the number of cycles to failure. The resulting F–N diagram (Figure 6) demonstrates that while all adhesive configurations exhibited typical downward fatigue trends with decreasing force, the 2 mm adhesive consistently occupied the upper envelope, indicating superior fatigue resistance at all comparable load levels.

Fig. 6. Force-fatigue life (F-N) curve.

In particular, at lower forces (0.4 kN), the 1 mm and 2 mm curves converged, suggesting similar durability under moderate loading. However, the 3 mm thickness curve showed a much steeper decline, reflecting poor performance under both moderate and high loads. This clearly visualizes the detrimental effect of excessive adhesive thickness on fatigue life. 4. Conclusions Based on fatigue testing of adhesively bonded DC01–aluminum single-lap joints under three load levels (0.5, 0.45, and 0.4 kN), the following conclusions can be drawn: • Adhesive thickness strongly influences fatigue behavior, particularly at moderate load levels.