PSI - Issue 68

Louka Eleftheria-Sotiria et al. / Procedia Structural Integrity 68 (2025) 894–900 Louka Eleftheria-Sotiria et al. / Structural Integrity Procedia 00 (2025) 000–000

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according to ASTM G34 specification, and then subjected to tensile testing. At least three (3) specimens from each case were used for the reproducibility of the results and to obtain reliable average values. 2.3. Evaluation Tensile tests were carried out in a servo hydraulic Instron 100 kN testing machine according to the ASTM E8 specification ( ASTM International, 2009) and the crosshead displacement rate was kept constant and equal to 0.7 mm/min. An external Instron extensometer with 50 mm ± 10 mm maximum travel was attached at the reduced cross-section gauge length of the specimens. A data logger was used during all tensile tests and the values of load, displacement and axial strain were recorded and stored in a computer. More than three specimens were tested in each different case to get reliable average data. The pre-corroded rectangular specimens (10 mm × 20 mm × 3.2 mm) were cut with the aid of a microtome to get several cross-sections for the investigation of the corrosion products’ (e.g., pits, micro-cracks, exfoliated surfaces) morphology and geometrical characteristics both in the specimens’ surfaces and in specimens’ interior. More than three cross-sections of each specimen were mounted in epoxy matrix, surface ground up to 1200 grit, polished with 0.25 μm paste down and then examined in a light optical microscope in order to measure the width and depth of corrosion-induced micro-cracks as well as their distribution on the specimens’ surface. A statistical analysis was then performed to reveal their increasing trend with increasing exposure time. 3. Statistical analysis of corrosion products Fig. 1 shows the classification of the depth of attack (depth of micro-cracks) for the two different investigated ageing tempers of AA2198. Calculation of the depth of attack is of major importance for the assessment of materials degradation since it was found in previous publications of the authors, e.g. , (Alexopoulos, Charalampidou, Skarvelis, & Kourkoulis, 2017) and (Charalampidou, Dietzel, Zheludkevich, Kourkoulis, & Alexopoulos, 2021), to significantly contribute to the elongation at fracture A f decrease, and consequently to ductility degradation. Higher depth of attack values was noticed for higher exposure times (i.e., 24 h) in both tempers of AA2198, with the maximum depth of attack for 24 h exceeding 200 μm. Nevertheless, it can be noticed that the average depth of attack for T8 temper is lower than in T3. More corrosion-induced cracks of lower depth were noticed in T8 temper at both exposure times, e.g., approximately 60 % probability percentage of appearance noticed in the range 0 - 40 μm after 2 h EXCO exposure and more than 40 % in the range 81 - 120 μm after 24 h EXCO exposure while approximately 40% probability percentage of appearance of cracks with depth higher than 200 μm was observed for the specimens of T3 temper after 24 h EXCO exposure.

Fig. 1. Classes of depth of corrosion attack for (a) T3 and (b) T8 temper of AA2198 for two different exposure times.