Issue 50

N. Alexopoulos et alii, Frattura ed Integrità Strutturale, 50 (2019) 342-353; DOI: 10.3221/IGF-ESIS.50.29 343 problems in maintenance and repair of aircraft structures is corrosion. The possibility that corrosion will interact with other forms of damage, e.g. fatigue cracks, impact etc. can result in significant loss of the structural integrity and may lead to fatal consequences, e.g. the Aloha Airlines accident . The synergetic interaction of corrosion and fatigue has a deteriorating effect on the mechanical performance of aeronautical aluminum alloys, mainly because of accelerated crack propagation [1]. The major damage mechanism on the corroded surface affecting the integrity of aircraft structures is the formation of pitting, e.g. [2-4]. Corrosion-induced pits initiate on the surface due to chemical or physical heterogeneities such as inter- metallic particles, dislocations or mechanical damage and flaws, e.g. [5,6]. Wrought aluminum alloys used in aircraft applications, contain numerous intermetallic particles increasing substantially the mechanical properties (yield stress, fatigue crack growth etc.), nevertheless they play a pivotal role in the nucleation of pitting, e.g. [7,8]. Aluminum alloy 2024, is highly used in the aircraft industry due to its improved mechanical properties and high damage tolerance capability, nevertheless it shows high susceptibility to corrosion attack due to its microstructure [9-11]. Corrosion involves several electrochemical mechanisms; in acidified solutions, the basic anodic reaction is the metal dissolution while the cathodic reactions are oxygen and hydrogen reduction resulting from aluminum ion hydrolysis [2]. Dealloying of S - type (Al 2 CuMg) particles, that are the most common intermetallic phases in 2xxx aluminum alloys, leads to Cu-rich remnants within the clusters [12] that switch the anode reaction to the alloy matrix adjacent to the particle and eventually to the grain boundaries; thus, it assists the formation of sub-surface micro-cracks [13-15]. Cracking formation generally starts at local defects such as microstructural features inside the material, surface features such as notches or in-service damage pro- cess such as corrosion (e.g. pitting) that act as stress concentrators [16,17]. Accumulated corrosion damage can be noticed on aging aircrafts due to corrosion-induced embrittlement mechanisms. Hydrogen embrittlement phenomenon along with corrosion of aluminum alloys can lead to the rapid failure of the materials. Hydrogen is usually produced by surface corrosion reactions and afterwards diffuses into the material and is trapped at preferential sites [18] as shown in [19,20]. It can be adsorbed at crack tips or notches or diffuse ahead of cracks [21] that embrittles the material below the crack tip. To face the corrosion-induced structural degradation issue, available data usually refer to accelerated laboratory tests. Many researchers focused on the development of damage functions to account for the corrosion assessment on the mechanical properties, e.g. [21-23]. The most common accelerated corrosion test used for the aluminum alloys of the 2xxx and 7xxx alloy series is the exfoliation corrosion (EXCO) test according to ASTM G34. It has been reported that 24 h exposure of the aluminum alloy 2024-T4 to the EXCO solution corresponds to nearly 6 years of natural exposure of the same structural element regarding its surface exfoliation [24]. Various mechanical tests had been carried out on AA2024-T3 to assess the effect of the corrosion damage on the material’s structural integrity. Tensile and fatigue mechanical tests had been carried out in pre-corroded material, resulting to the mechanical properties degradation [25,26]. Corrosion of AA2024 was found to result in a moderate decrease of the strength properties (yield stress and ultimate tensile strength) with a significant reduction of tensile ductility [27,28]. According to Alexopoulos and Papanikos [29] the cross-sectional area of AA2024 specimens which was supposed to be unaffected by micro-cracks (referred as ‘effective thickness’), decreases exponentially with increasing exposure time to EXCO solution due to the crack propagation mechanism that leads to higher corrosion penetration into the material. Corrosion exposure was found to have a negative effect on yield stress mainly due to the cross-sectional decrease at higher exposure times as well as on tensile ductility decrease due to the combination of hydrogen embrittlement in the low exposure times and decrease of the cross-section for the higher exposure times [30]. The reduction of the load carrying cross section of the specimens as well as the notch effects caused by pitting formation and exfoliated areas are sufficient to explain the moderate reduction of tensile strength properties. The corrosion problem includes several degradation mechanisms and the damage could be described and analyzed as the sum of several parameters downgrading the mechanical properties. In order to better interpret the corrosion-induced damage, various mechanisms involved in the corrosion process should be taken into consideration; they may depend on material, temper, corrosive environment and exposure time. The aggressiveness of the corrosive environment is a significant parameter influencing corrosion damage evolution as well as the underlying corrosion mechanism. Recent investigations [31] indicate that in-service obtained corrosion damage correlates well to the one caused by a 3.5 wt. % NaCl solution, with pitting density, depth and shape evolving with exposure time. Vasco et al. [32] performed correlations between corrosion damage from accelerated corrosion tests of varying aggressiveness by accounting for both, the metallographic features of corrosion damage and the mechanical properties of the corroded material. Correlations regarding geometrical metallographic features were found under dominance of pitting corrosion and up to 8 hours in EXCO solution; higher variations after the occurrence of pit coalescence and transition to dominance of exfoliation corrosion were presented. The aim of the present work is to simulate the real corrosion-induced degradation of the mechanical properties due to cor- rosion surface pits and possible hydrogen embrittlement and to correlate it with the equivalent degradation by the artificially induced surface notches. Moreover, a comparison between the effects of the aggressiveness of the corrosion environment on the mechanical behavior of AA2024 specimens subjected to EXCO and 3.5 wt.% NaCl solution will be investigated.