PSI - Issue 68

R.J. Mostert et al. / Procedia Structural Integrity 68 (2025) 351–357 R.J. Mostert et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 2. The influence of pre-stretching and EXCO exposure on tensile properties of (a) AA2198-T8 and (b) AA2024-T3 samples.

Pretorius et al. (2021) showed the negative effects of EXCO exposure on the crack-extension resistance behaviour of AA2024 -T3, as indicated in Fig. 3. The fracture toughness curves following EXCO exposure were generally lower than that of the baseline, and the critical K R -value was also reduced ( K c = 87.3 vs 93.0 MPam 0.5 ). Pretorius (2024) furthermore found that a hydrogen removal heat treatment resulted in a full restoration of the crack extension critical values and removal of most of the diffusible hydrogen content, as measured by Thermal Desorption Spectrometry (TDS). After the heat treatment, the first TDS peak was observed to be at ~ 425 °C. These results led to the conclusion that the degradation of the bulk mechanical properties observed was due to HE. The average diffusible H-values are reported in Table 1 and the TDS plots are shown in Fig. 4.

Table 1. Total diffusible hydrogen content of samples, average, as determined by TDS at a ramp rate of 50 °C/ min. Sample description H Diff, ppm Baseline (unexposed) 0,41 EXCO exposed 4,81 EXCO exposed and heat treated 2,01

Fig. 3. The influence of EXCO exposure on crack extension resistance curves. Hollow symbols are the exposed samples, solid are as-received (unexposed). The fractography was performed on both the C(T) and the tensile samples. For the C(T) samples and on the primary fracture surface, three distinct zones could be identified (Fig. 5), the fatigue pre-cracking zone (Zone (i)), the stable crack extension zone (Zone (ii)), and the final fracture zone (Zone (iii)). On the adjacent flat surface, numerous shallow secondary cracks were observed, and this region was labelled as Zone (iv).