Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 348 Effect on tensile ductility The results of the residual elongation at fracture A f values can be seen in Fig.6 for the three investigated cases, namely the EXCO exposure, the NaCl exposure as well as the artificial notches. The results are presented in the form of normalized decrease from the initial value of A f with double X-axis for comparison reasons. Corrosion exposure has a deleterious effect on tensile ductility (elongation at fracture in the present case) even from the short exposure times to both investigated corrosive solutions. The normalized decrease of elongation at fracture A f is almost the same for both solutions at short exposure times where the pitting formation remains limited; approximately 75 % remaining percentage of the initial mech- anical property was observed after only 1 h of exposure to EXCO solution while the same decrease was noticed for the case of exposure to 3.5 wt. % NaCl solution after 48 h (Fig.6a). The same was observed for 2 h at EXCO and 168 h at 3.5 wt. % NaCl with a normalized decrease value of 68 %. Nevertheless, higher exposure times are needed for the 3.5 wt. % NaCl solution in order to result in the same degradation percentage with the respective specimens exposed to EXCO solution. It should also be taken into consideration that the hydrogen embrittlement phenomenon takes place in this case, especially at the short exposure times in EXCO solution. The respective results for the comparison between corrosion exposure to EXCO solution and artificial surface notches are presented in Fig.6b. It is evident that the normalized A f decrease is not linear proportional to the notch depth nor to the exposure time increase in the reduced cross-section. An essential A f decrease was observed even for the low-depth notches, e.g. approximately 70 % normalized decrease for 0.10 mm, as well as for the short exposure times such as 75 % normalized decrease after 1 h of exposure. After 12 h of exposure to exfoliation corrosion solution, the A f decrease seems to reach a plateau value and further corrosion exposure did not decrease the tensile ductility considerably while for the increasing notch depth it decreases continuously to extremely low values such as 6 % for the 0.50 mm notch depth; this is evidence of maximum depth of attack of the surface corrosion-induced cracks [30]. It is well accepted that the ductility decrease from the short corrosion exposure times can be attributed to the hydrogen embrittlement phenomenon. (a) (b) Figure 6: Normalized decrease of elongation at fracture A f values for the various exposure times to the EXCO solution of AA2024-T3 compared with (a) 3.5 wt. % NaCl solution and (b) notch depth. Correlation of corrosion exposure times to notches regarding ductility degradation As shown in the previous two sections, the increase of the surface notch depth decreases the tensile elongation at fracture (ductility) of AA2024-T3 and to a lesser extent the conventional yield stress. From the experimental tensile test results of pre-corroded 2024-T3 specimens, an essential decrease in tensile ductility was noticed even for the short corrosion exposure times, for both investigated corrosive solutions as well as for small artificial notch depth values. Hence, it is obvious that the empirical correlation between the problem of corrosion-induced degradation and the equivalent problem with artificial surface notches should be assessed through the residual tensile ductility property. The experimental results of the residual elongation at fracture A f of the pre-corroded in EXCO solution tensile specimens of AA2024-T3 can be seen in Fig.7. In the same figure, the exponential decrease curve fitting was plotted as well. For the 0 4 8 12 16 20 24 0 20 40 60 80 100 Aluminum alloy 2024-Τ3 t = 3.2 mm, direction L Exposure time to NaCl solution [hours] Normalized decrease of elongation at fracture A f [%] Exposure time to EXCO solution [hours] EXCO Solution 0 700 1400 2100 2800 3500 4200 3.5 wt. %NaCl solution 0 4 8 12 16 20 24 0 20 40 60 80 100 Normalized decrease of elongation at fracture A f [%] Exposure time to EXCO solution [hours] EXCO solution 0,0 0,1 0,2 0,3 0,4 0,5 Aluminum alloy 2024-Τ3 t = 3.2 mm, direction L Notch depth [mm] artificial notches