Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 344 E XPERIMENTAL PROCEDURE he material used was a wrought aluminum alloy 2024-T3 which was received in sheet form of 3.2 mm nominal thickness. The weight percentage chemical composition of the alloy is 4.35 % Cu, 1.50 % Mg, 0.64 % Mn, 0.50 % Si, 0.50 % Fe, 0.25 % Zn, 0.10 % Cr, 0.15 % Ti and Al rem. Tensile specimens were machined from the material sheet according to ASTM E8 specification with 12.5 mm x 50 mm being the reduced cross section of the specimen. All the specimens were cut parallel to the longitudinal (L) rolling direction of the material. Reference specimens were tensile tested according to ASTM E8 specification, while two series of tensile specimens were exposed for various times to different laboratory corrosion environment, namely exfoliation corrosion (hereafter called EXCO solution) and 3.5 wt. % sodium chloride (hereafter called NaCl solution) according to the specifications ASTM G34 and G44, respectively. The EXCO solution consisted of the following chemicals diluted in 1 l distilled water; sodium chloride (4.0 M NaCl), potassium nitrate (0.5 M KNO 3 ) and nitric acid (0.1 M HNO 3 ). The results of the EXCO solution exposure have been performed and reported in a previous article of the authors [33]. The concentration of the NaCl solution consisted of 3.5 g NaCl for each 96.5 ml of water. The solution volume was calculated per exposure area of the specimens and it was constant for all specimens. In both experimental procedures, the specimens were cleaned with alcohol prior to corrosion exposure according to specification ASTM G1. Additionally, the specimens were masked with appropriate insulating PVC tape in order to be exposed only at the reduced surface area of approximate 55 mm in length. The experimental procedure was carried out in laboratory environmental conditions and at room temperature. According to the literature [26], corrosion damage and hydrogen embrittlement is evident on the large surfaces of the tensile specimen’s gauge length and not so intense on the side surfaces. Machining of the artificial notches was decided to be performed on the large surfaces of the tensile specimen, namely vertical to the loading axis. A drawing of the specimen with manufactured two surface notches can be seen in Fig.1. The two artificial notches facing one the other (on the same vertical level) can be well seen in the Figure as well as the maximum depth of 0.5 mm per notch. This notch depth per surface was the maximum depth of attack of corrosion products (pits and cracks) generated after approximating 72 h exposure time in exfoliation corrosion solution. In different specimens, notches with smaller depths were manufactured (ranging from 0.1 till 0.5 mm) to incrementally simulate the corrosion surface damage on AA2024-T3 as well as the specimen’s residual tensile strength and tensile ductility after the corrosion exposure. Tensile tests were carried out in a servo-hydraulic Instron 8801 100 kN testing machine according to ASTM E8M specification, with a constant deformation rate of 3.3 x 10 -4 sec. An Instron extensometer 50 mm ± 10 mm maximum travel was attached to the specimen’s gauge length before the tensile test. A data logger was used during all tensile tests to store the values of load, displacement and axial strain in a computer. To get representative average values of the tensile properties, at least three tensile tests have been carried out per each test series. Evaluated properties were the conventional yield stress R p0.2% (0.2 % proof stress), ultimate tensile strength R m and elongation at fracture A f . Figure 1: Sketch of the tensile specimen with machined two (upper and lower) surface notches on the large surfaces. R ESULTS AND DISCUSSION Surface characterization of pre-corroded specimens he exposure of AA2024 specimens to the corrosive environment (EXCO or 3.5 wt. % NaCl solution in the present study) results in the deterioration of the surface of the specimens due to the nucleation of corrosion- induced surface pits, as can be seen in Fig.2. The depicted corroded area has dimensions of 12.5 mm x 55 mm being T T