PSI - Issue 28

Dimitris Georgoulis et al. / Procedia Structural Integrity 28 (2020) 2297–2303 Author name / Structural Integrity Procedia 00 (2019) 000–000

2300

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3.2. Analysis of the experimental results One of the main goals of the present article is to investigate and compare the tensile mechanical behaviour of AA2024-T3 and AA2198-T8 after being exposed to corrosive solution. Tensile specimens of both alloys were exposed for different times to 3.5 wt. % NaCl solution. It is well known that for higher exposure times to corrosion solution, corrosion induced surface pits are formed that act as surface notches; they have profound effect on the ductility degradation of the specimen as they act as stress concentrators and raisers. Different corrosion exposure times were selected to corrode the materials and their effect on the typical tensile flow curves can be seen in Figs. 2a and 2b for AA2024-T3 and AA2198-T8, respectively. The tensile test results for AA2024 were in detail reported by Alexopoulos et al. (2019). No essential stress decrease due to corrosion exposure was noticed for AA2024 specimens even for the highest exposure time, e.g. 4200 h. On the contrary, elongation at fracture exhibited a significant degradation even for the very short exposure times such as 6 h. It is worth mentioning that higher ductility degradation was observed at the time range between 6 and 168 h, where pitting incubation takes place, as well as in the time range of 720 and 2160 h, probably because of the change in the degradation mechanism, e.g. pit growth and coalescence. The same corrosion behaviour was noticed for AA2198-T8 specimens where no essential decrease in the yield stress is evident with increasing exposure time while a considerable degradation of elongation at fracture was evident even for the short exposure times to the corrosive solution. The decrease in tensile ductility is attributed to the hydrogen embrittlement effect for the early stages of corrosion exposure since no surface deterioration exists.

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168 h

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Reference

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100 Axial nominal stress σ [MPa] 4200 h 200 300

720 h

4320 h 2160 h

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100 Axial nominal stress [MPa]

AA2198-T8, t = 3.2 mm Exposure time to 3.5% wt . NaCl solution

AA2024- T3, t = 3,2 mm, Exposure time to 3.5 % wt. NaCl solution

0 2 4 6 8 10 12 14 16 18 20 22 0

0 2 4 6 8 10 12 14 16 18 20 22 0

Axial nominal strain [%]

Axial nominal strain ε [%]

(a) (b) Fig. 2: Typical experimental tensile curves of pre-corroded (a) AA2024-T3 and (b) AA2198-T8 for different exposure times to 3.5 w.t % NaCl solution.

3.3. Effect on mechanical properties

3.3.1. Conventional yield stress Fig. 3 shows the experimental results of AA2024-T3 and AA2198-T8 as average values and respective standard deviation derived from three specimens each material. It should be mentioned that conventional yield stress R p0.2% was calculated based on the nominal cross-section of the specimens and without taking into account the effective thickness of the specimens. The available experimental test results were simply interpolated with the aid of a B-Spline curve (eye-catch) in order to roughly assess the corrosion exposure effect on the mechanical properties of the two aluminium alloys tested. The corrosion exposure time range was divided into two distinct regimes, namely short exposure times and long exposure times, with different degradation rates and corrosion behaviour. AA2024-T3, represented by red