PSI - Issue 38

Sara Eliasson et al. / Procedia Structural Integrity 38 (2022) 631–639 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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Fig. 6. S-N plot for the fatigue test series.

4.2. Failure Modes The failure mode for each test was evaluated. A selection of different failure modes encountered from the fatigue testing are presented in Fig. 7 (edge splitting, longitudinal failure, inside tab failure, and failure in the gauge length). A common failure mode was number three in Fig. 7, edge failure inside tab. This failure mode will generally underestimate the fatigue life, since this failure mode occurs inside the tab and the specimen could have been affected by other factors, e.g. the clamping pressure, stress concentrations or adhesive failure.

Fig. 7. Different failure modes from fatigue testing at 80 % of UTS, (1) L/S I T/B; (2) SGM; (3) LI T/B; (4) LGM (ASTM International, 2017).

4.3. Load Level 80 % of UTS For further understanding of the damage additional specimens were run at 80 % of UTS since this load level had a high scatter, but at the same time indicated to be the fatigue limit. The fatigue specimens from the load level 80 % of UTS failed earliest at 7 880 cycles and at the same time could endure more than five million cycles (Fig. 6). The stiffness degradation of four specimens is presented in Fig. 8. The three specimens AT0_4, AT0_6, and AT0_7 had a similar behaviour where the stiffness degradation curve flattens out after 2 – 6 % stiffness loss. Whilst the fourth specimen, AT0_10, had a much higher stiffness degradation with up to 15 %. This specimen also had a large jump in the stiffness degradation and strain at 20 000 cycles. Looking at the strain development over time (Fig. 10) and comparing this to the different failure modes, the specimen with the highest stiffness degradation was also the specimen with the highest average strain and the shorter fatigue life. This was also the only specimen were the failure occurred in the gauge length. The specimen AT0_6, was a runout and the average strain for this specimen was the lowest, and it had a stable development of the stiffness degradation.