PSI - Issue 10

N. Siskou et al. / Procedia Structural Integrity 10 (2018) 79–84 N. Siskou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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3

of the side-surfaces, i.e., thickness, was also performed) to be corroded. After the corrosion exposure the corroded specimens were immediately cleaned according to ASTM G34 and then were subjected to tensile testing in an INSTRON 8801 loading frame.

4. Results and discussion

One of the main goals of the present article is to investigate the tensile mechanical behavior of PA condition of AA2024 after being exposed to corrosive solution for different times. Alexopoulos and Papanikos (2008) have shown that the high corrosion exposure times result in the formation of corrosion-induced surface pits that act as stress con centrators; they have a profound effect on the mechanical properties degradation and especially on tensile ductility. Different corrosion exposure times were selected to corrode the reference material (without ageing at the T3 condition) as well as the artificially aged material (PA condition); their effect on the typical tensile flow curves can be seen in Figs 1a and 1b for the T3 and peak-ageing conditions, respectively. It is evident from the Fig.1b that the artificial ageing condition has an essential effect on the stress - strain flow curve with the yield stress and ultimate tensile strength to increase (from 320 MPa to approximate 420 MPa) and the elongation at fracture to be essentially reduced (from approximately 19 % to 9 %). Regarding the corrosion behaviour, it can be noticed that it has an essential effect on the mechanical properties degradation and especially on tensile ductility for the specimens in T3 condition. Both, conventional yield stress and elongation at fracture are decreasing with increasing exposure time to the EXCO solution. An essential elongation at fracture decrease is evident even after the very short corrosion exposure times, e.g. 0.5 h where no surface deterioration exists, that was attributed to the hydrogen embrittlement phenomenon according to Alexopoulos et al. (2016). On the contrary, it was observed that neither yield stress nor elongation at fracture exhibit significant degradation due to corrosion exposure for PA specimens, even after the highest exposure time, e.g. 24 h.

without Exco

500

400

0,5 h Exco

4 h Exco

24 h Exco

2 h Exco

100 Axial nominal stress σ [MPa] 200 300

Aluminum alloy 2024-Τ3, t = 3.2 mm, L rolling direction Exposure to EXCO solution on large surfaces

0

0,00

0,05

0,10

0,15

0,20

Axial nominal strain ε [-]

(a) (b) Fig. 1. Typical experimental tensile flow curves of corroded aluminum alloys (a) T3 condition and (b) peak-ageing condition for different exposure times to exfoliation corrosion solution. Images of the specimen surfaces in PA condition can be seen in Fig.2, when exposed for various corrosion exposure times to the exfoliation corrosion solution. Figs.2(a-c) show the corroded surfaces when exposed for 0.5 h, 4 h and 24 h, respectively, while Figs.2(d-f) show the same surfaces of the specimens after the tensile test and fracture (on the same image scale). The surface pitting formation is limited for the very short exposure times, e.g. 0.5 h as can be seen in Figs.2(a,d). Additionaly, the high plastic deformation is evident in the fracture surface of this specimen, Fig.2b. However, with increasing exposure time the pitting density tend to increase, e.g. Fig.2b and Fig.2c. However, it can be noticed that the fracture mechanism and path do not essentially change with increasing exposure time in PA condition, which is in accordance with the tensile tests results.