Fatigue behaviour of bare and pre-corroded magnesium alloy AZ31 A.N. Chamos, Sp.G. Pantelakis * , V. Spiliadis Laboratory of Technology & Strength of Materials, Department of Mechanical Engineering and Aeronautics, University of Patras, Panepistimioupolis Rion, 26500 Patras, Greece article info Article history: Received 28 January 2010 Accepted 16 April 2010 Available online 22 April 2010 Keywords: Fatigue Crack initiation Corrosion attack Fracture mode abstract Constant amplitude fatigue tests have been performed using smooth specimens of a rolled AZ31 magne- sium alloy in order to assess the fatigue behaviour of the material. The tests were periodically interrupted and replicas were taken from the surface of the specimens in order to reveal crack initiation and early crack propagation. Based on the derived S–N curve a very high stress sensitivity of the fatigue life can be concluded; it may be attributed to the inability of the material to accumulate fatigue damage in terms of cyclic plasticity at the early stage of fatigue. Fatigue cracks initiate already after few fatigue cycles between strain incompatibility points (e.g. grain boundaries) due to difficulties in satisfying the von Mises criterion. The initiation and propagation mechanisms of the fatigue cracks are characterized as cleavage. Furthermore, the corrosion susceptibility of the material has been investigated in a salt spray environment. It becomes evident that the presence of corrosion damage, in terms of corrosion pitting, results in the development of stress concentration, facilitating essentially the initiation and propagation of fatigue cracks. Thus, the fatigue limit is reduced to 50% of the respective value of the un-corroded material. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Magnesium is the lightest structural engineering metal, and therefore, particularly attractive for structural applications where weight saving is of importance. Improvements in mechanical prop- erties, corrosion resistance [1,2] and the development of advanced manufacturing processes have led to increased interest in magne- sium alloys for automotive and aerospace applications [3–5]. One of the most popular wrought alloys amongst them is the AZ31 al- loy; it is usually supplied as a sheet material with small thickness. Both, processing technologies and tensile properties of the AZ31 al- loy have been extensively investigated in the literature, e.g. [6–10]. Major problems limiting the use of magnesium alloys in structural applications are the high corrosion susceptibility, due to the fact that magnesium is at the active end of the galvanic series [11], and the poor damage tolerance behaviour as compared to other structural alloys like aluminum and titanium. The studies concerning the fatigue behaviour of AZ alloys are rather limited, e.g. [12–14]. In all cases the characteristic smooth transition from low to high cycle fatigue regime was observed indi- cating a high stress sensitivity of the fatigue life. For the case of the AZ31 alloy [12,13] it has been shown that fatigue cracks initiate at an early stage of the fatigue process, independent of the stress amplitude, mainly at the interfaces between the Mg matrix and the existing intermetallic phases. As a consequence of the early fatigue crack initiation fatigue life consists almost solely of fatigue crack growth phase. In the same works it has been also observed that fatigue crack growth resistance of the AZ31 alloy is inferior when compared to other structural alloys. In [14], the effect of Mn content on the fatigue behaviour of AZ61 alloy has been eval- uated. Fatigue strength of high Mn content (0.4%) alloy was lower than that of low Mn content alloy (0.164%). Intermetallic Al–Mn inclusions were observed in high Mn content alloy which served as stress concentration site and then enhanced fatigue crack nucle- ation. Yet, these investigations remain limited and further work is needed to understand the fatigue mechanism of magnesium alloys. On the other hand, corrosion susceptibility is considered as a major problem of magnesium alloys. For the AZ31 alloy, the per- formed studies to assess the effect of corrosion on the mechanical behaviour remain up to day very limited and mainly focus on the stress corrosion cracking and corrosion fatigue behaviour, e.g. [15,16]. More specifically, the investigation performed by Blawert et al. [15] deals with the stress corrosion cracking behaviour of one of the most popular magnesium alloys, the AZ31 alloy, in var- ious solutions, as well as the tensile behaviour after exposure in a 0.01 M NaCl solution. The results show that the alloy is susceptible to SCC in the various solutions, namely distilled water and different NaCl solutions, as the time to fracture, the elongation to fracture and the reduction of the area of the specimens decreased dramat- ically compared to those tested in air. In [16], the corrosion fatigue characteristics of an extruded AZ31 alloy have been evaluated in a sodium chloride solution. At the initial stage of the corrosion fatigue process, corrosion pits initiate by preferentially corrosive 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.04.031 * Corresponding author. Tel.: +30 2610 969498; fax: +30 2610 997190. E-mail address: pantelak@mech.upatras.gr (Sp.G. Pantelakis). Materials and Design 31 (2010) 4130–4137 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes