Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 351 Figure 9: Correlation of the elongation at fracture A f decrease due to exposure to the EXCO solution as well as to 3.5 wt. % NaCl solution. rates than conventional yield stress for both, EXCO and 3.5 wt. % NaCl solutions as well as for artificial surface notches. In addition, it was shown that a ductile fracture mechanism is evident from the 45˚ slope of the fracture surface for up to 4 h for the specimens exposed to EXCO solution and even after 720 h for the specimens exposed to NaCl solution; however, for higher exposure times to EXCO solution, the fracture path seems to follow the surface pits. Another interesting finding of the study is that the more aggressive environment (EXCO) results in higher decrease of the conventional yield stress, especially for the short exposure times. Higher exposure times to 3.5 wt. % NaCl solution are needed for the same degradation of R p0.2% as for the EXCO solution, e.g. approximately 99 % normalized R p0.2% decrease was noticed after 1 h of exposure to EXCO solution while the same decrease is evident after 168 h of exposure to NaCl solution. The specimens exposed to exfoliation corrosion exhibited higher conventional yield stress R p0.2% degradation than the specimens with the machined artificial surface notches, especially for the short exposure times and low depth of notches. However, the highest notch depth of 0.50 mm resulted in the same normalized decrease, approximately 80 %, as for the highest exposure time. An empirical coefficient was introduced for the correlation between the corrosion-induced tensile ductility degradation with the equivalent artificially induced surface notches. Three cases were investigated: (a) EXCO exposure with artificial notches, (b) EXCO exposure with 3.5 wt. % NaCl exposure and (c) 3.5 wt. % NaCl exposure with artificial notches. Higher correlation regarding the ductility decrease was noticed for the short exposure times, where the slight pitting formation as well as hydrogen embrittlement for the case of EXCO solution are the dominant degradation mechanisms, and low-depth notches for all investigated cases. The best correlation between exposure to EXCO solution and artificial surface notches was found by using m = 20 for the short exposure times where the main degradation mechanisms are the pitting formation along with hydrogen embrittlement; thus, a total notch depth of 0.10 mm corresponds to 2 h of exfoliation corrosion with regard to the same tensile ductility degradation. For the case of correlation between 3.5 wt. % NaCl solution and artificial notches, the coefficient value that better simulates the corrosion-induced ductility decrease was found to be m = 500 for exposure times less than 400 h, where the incubation of pits is the dominant degradation mech- anism; hence, 100 μm surface notch depth results in the same elongation at fracture decrease as for 50 h of exposure to 3.5 wt. % NaCl solution. For the case of correlation between exposure to EXCO and 3.5 wt. % NaCl solutions, it is proposed that 1 h EXCO exposure is equivalent to 92 h exposure to NaCl solution regarding tensile elongation at fracture decrease. R EFERENCES [1] Menan, F., Henaff, G. (2009). Influence of frequency and exposure to a saline solution on the corrosion fatigue crack growth behavior of the aluminum alloy 2024, Int. J. Fatigue, 31, pp. 1684-1695. DOI: 10.1016/j.ijfatigue.2009. 02.033 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 2 4 6 8 10 12 14 16 18 Exposure time to NaCl solution [hours] Elongation at fracture A f [%] Aluminium alloy 2024-T3, t = 3.2 mm Experimental results of exposure to NaCl solution Results of the empirical correlation exposure to NaCl - EXCO solution n = 92