Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 347 Effect on the conventional yield stress The experimental results of the conventional yield stress R p0.2% (nominal values) for the (a) exposure times to EXCO solution, (b) exposure times to 3.5 wt. % NaCl solution and (c) notch depths are presented in Fig.5. The results are pre- sented in decreasing normalized values (y-axis) from the initial value of the conventional yield stress for comparison pur- poses. The black circles and dotted line represent the experimental results and fitting line of exposure to EXCO solution in both figures, the orange triangles and line corresponds to the fitting results of exposure to 3.5 wt. % NaCl solution and the blue squares and dashed line to the respective results of notch depth. Regarding the comparison of conventional yield stress decrease between the two investigated corrosive environments, it is evident that the more aggressive solution (EXCO) leads to higher decrease of conventional yield stress up to 12 h of exposure. For the very short exposure times, the stress decrease seems to be almost the same for both investigated solutions; however specimens exposed to the mild corrosion solution (3.5 wt. % NaCl) needed more hours for the same degradation percentage, e.g. approximately 99 % normalized decrease of R p0.2% can be noticed after 168 h of exposure to NaCl solution as well as after 1 h of exposure to EXCO solution (Fig.5a). It is worth to mention that the 3.5 wt. % NaCl solution does not reveal any exfoliation of the corrosion attacked material surfaces which could represent a more pronounced notch effect reducing the specimens’ cross-sectional area. However, the decrease of conventional yield stress, remains limited for all the investigated corrosion exposure times of both investigated environments and does not exceed 40 % at maximum and for the investigated exposure times. The highest normalized decrease of R p0.2% was approximately 68 % for the specimens exposed to 3.5 wt. % NaCl solution and 79 % for the specimens exposed to EXCO solution. The comparison of the conventional yield stress decrease between the exposure to exfoliation corrosion solution and machined notch depths, is performed in Fig.5b. The specimens exposed to exfoliation corrosion exhibited higher conventional yield stress R p0.2% degradation than the specimens with the machined artificial surface notches up to 12 h of corrosion exposure as well as 0.3 mm notch depth. Corrosion exposure decreases the conventional yield stress with higher rates even from the short exposure times, e.g. 2 h where the pitting formation is limited, probably because of the hydrogen embrittlement phenomenon. No essential decrease of R p0.2% was noticed even after the notch depth of 0.30 mm, e.g. a 97 % normalized decrease on R p0.2% was observed. For the short exposure times and low notch depth values, the contribution of EXCO and notch depth to the normalized property decrease is almost the same, e.g. the 0.1 and 0.15 mm notch depths resulted approximately in the same decrease as for the 0.5 and 1 h of exposure to EXCO solution; however, after 2 h corrosion exposure a higher decrease rate is noticed because of the synergetic effect of pitting corrosion and hydrogen embrittlement. Nevertheless, the machined notch depth of 0.50 mm yielded the same normalized conventional yield stress decrease as for the highest exposure time to EXCO, with the normalized decrease, reaching almost 80 %. The decrease in the conventional yield stress with increasing notch depth is well accepted since high notch depth values tend to increase the plasticity induced in front of the notch tip; plastic region increases with increasing stress level and general yielding at the reduced cross-section occurs for lower applied force level, thus resulting in lower nominal stress level. (a) (b) Figure 5: Normalized decrease of conventional yield stress R p0.2% values for the various exposure times of AA2024-T3 to the EXCO solution compared with (a) 3.5 wt. % NaCl solution and (b) notch depths. 0 4 8 12 16 20 24 0,0 70 80 90 100 Aluminum alloy 2024-Τ3 t = 3.2 mm, direction L Normalized decrease of conventional yield stress R p0.2% [%] Exposure time to EXCO solution [hours] EXCO solution 0 700 1400 2100 2800 3500 4200 Exposure time to NaCl solution [hours] 3.5 wt . % NaCl solution 0 4 8 12 16 20 24 0,0 70 80 90 100 Exposure time to EXCO solution [hours] Normalized decrease of conventional yield stress R p0.2% [%] 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