Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 349 conversion of the total depth of the surface notches in equivalent exposure time to corrosion solution, an empirical coef- ficient was devised. The value of this coefficient was selected such as to ‘tailor’ the equivalent ductility decrease curve of the surface notches in order to take approximate values with the experimental ductility decrease curve of the pre-corroded specimens. The empirical correlation factor m in [h/mm] was calculated based on the following equation to correlate the available elongation at fracture test results as: equivalent notch depth (mm) = ௘௫௣௢௦௨௥௘ ௧௜௠௘ ሺ௛ሻ ௠ . (1) Fig.7 shows the correlation of the elongation at fracture decrease induced by the exposure to EXCO solution as well as by the presence of the surface notches. The best calculation results were found by using the value m = 20 for the empirical coefficient and the simulation of the ductility decrease curve of the investigated AA2024-T3 specimens for the short expo- sure times, where the synergy of pitting formation and hydrogen embrittlement is the dominant degradation mechanism. By using this coefficient value, it is obvious that the results of the artificial notch depths are very close to the experimental values for the short corrosion exposure times and up to 2 h. It seems that the total notch depth of 0.10 mm corresponds to 2 h of exfoliation corrosion regarding the elongation at fracture decrease. On the other hand, the empirical coefficient that better simulates the tensile ductility decrease for the long exposure times, where the exfoliation of the corroded surfaces along with hydrogen embrittlement are the responsible mechanisms for the elongation at fracture decrease, was found to be approximate m = 100. For instance, a total depth of 150 μm surface notch results in the same A f decrease as for 15 h of exposure to exfoliation corrosion solution. Summarizing the available test results, the factor m takes values as: m EXCO to notch = ൜ 20, 0 ൏ ൏ 4 h, 100, 16 ൏ ൏ 48 h, ୮୧୲୲୧୬୥ ୟ୬ୢ ୦୷ୢ୰୭୥ୣ୬ ୣ୫୰୧୲୲୪ୣ୫ୣ୬୲ ୣ୶୤୭୪୧ୟ୲୧୭୬ . (2) Figure 7: Correlation of the elongation at fracture A f decrease due to exposure to the EXCO solution as well as to the presence of the artificial surface notches. Fig.8 shows the correlation of elongation at fracture decrease resulting from the exposure to 3.5 wt. % NaCl solution, along with the respective results from the artificial surface notches. It can be noticed that the residual elongation at fracture of the two different cases is well correlated for the short exposure times by using a coefficient value m = 500. This means that 50 h of exposure to 3.5 wt. % NaCl solution results in the same elongation at fracture decrease as for 0.1 mm surface notch depth. The pitting corrosion mechanism is responsible for the ductility decrease for the short exposure times. However, a different value of this coefficient should be used, m = 15000, in order to obtain good correlation for the long exposure times and high notch depths, wherein the effect of micro-cracks formation due to pit growth and coalescence is responsible for the A f degradation. Hence, the following equation can be drawn by the findings of the results of the experimental protocols: m NaCl to notch = ൜ 500, 0 ൏ ൏ 400 ℎ, 15.000, 2000 ൏ ൏ 8000 ℎ ୧୬ୡ୳ୠୟ୲୧୭୬ ୭୤ ୮୧୲ୱ ୮୧୲ ୥୰୭୵୲୦ . (3) 0 10 20 30 40 50 60 70 0 2 4 6 8 10 12 14 16 18 m = 20 Exposure time to EXCO solution [hours] Elogation at fracture A f [%] Aluminium alloy 2024-T3, t = 3.2 mm Exposure to EXCO solution Experimental results of exposure to EXCO solution Results of the empirical correlation total depth of the notches with exposure time to EXCO solution m = 100