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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1. Introduction and background In earlier work, it has been found that certain tensile and plain-stress fracture toughness properties of AA2024-T3 and AA2198-T8 alloys degrade during simulated atmospheric corrosion exposure, using the standard EXCO solution according to ASTM G34, e.g., Charalampidou et al. (2024). The latter alloy is a third-generation Al-Cu-Li alloy destined for a significant uptake in aeronautical applications, while the AA2024-T3 alloy has been widely used in these applications for some time. In addition to the bulk embrittlement, it had also been observed that shallow surface cracking could be observed following exposure after relatively low levels of plastic strain. These observations are of obvious concern, given the application of the alloys. It has been confirmed that the bulk embrittlement of the specimens was due to hydrogen embrittlement (HE), but the mechanism for the sub-surface cracking has not been confirmed. In the current work, the short-term EXCO exposure (2 hours) was again used. In addition, a number of pre-stretching levels were utilized in the laboratory, in order to simulate the industrial manufacturing process and gauge its impact on the observed embrittlement. The primary focus of this work was to use fractography and metallography of the baseline and HE specimens to illuminate the underlying mechanisms of bulk and surface embrittlement. 2. Experimental Procedure As shown in Fig. 1, commercially produced 3.2 mm thick sheet was used for the machining of both tensile and compact tension [C(T)] specimens. Some of the tensile samples, machined to the requirements of ASTM E8 standard, were subjected to simulated pre-stretching in uniaxial tension and at different levels of total elongation. Baseline (non pre-stretched) and pre-stretched tensile samples, as well as pre-cracked C(T) samples machined to the requirements of ASTM E561 for K r -testing, were suspended in the standard EXCO solution for two hours. All samples were tested at a slow displacement rate (5E-04 mm/s). A Zeiss 540 cross-beam FEG SEM system was used for the fractography and EDS analysis.

Fig. 1. Overview of the experimental procedure.

3. Experimental results From the work of Charalampidou et al. (2024), the influence of pre-stretching and EXCO exposure on the tensile properties are shown in Fig. 2. Accordingly, the effect of pre-stretching was generally positive on yield stress, but with strain hardening tapering off at high pre-strain levels. The EXCO exposure had a negative influence on both the yield stress and the elongation to fracture, with this effect being much more pronounced in the case of the AA2024 T3 alloy. The negative EXCO influence on the mechanical properties of AA2198-T8, especially on elongation to fracture, was at a maximum at low pre-stretching levels (1.5 %), and reduced with increasing pre-stretching levels. On the contrary, the pre-stretching had no significant influence on the EXCO behaviour of AA2024-T3.