Experimental Mechanics (2014) 54:493–499 DOI 10.1007/s11340-013-9812-8 BRIEF TECHNICAL NOTE Fracture Strains, Damage Mechanisms and Anisotropy in a Magnesium Alloy Across a Range of Stress Triaxialities B. Kondori · A.A. Benzerga Received: 10 July 2013 / Accepted: 18 September 2013 / Published online: 25 October 2013 © Society for Experimental Mechanics 2013 Abstract We report on the effect of stress-state triaxiality on damage accumulation leading to fracture at ambient tem- perature in magnesium alloy AZ31. We find that the strain to failure is weakly sensitive to triaxiality for the conditions investigated, at variance with the behavior of most alloy sys- tems. Using plastic anisotropy measurements, post-mortem fractography and transverse cross-sectioning of specimens at incipient cracking, we discuss the contributions of plastic anisotropy, shear failure and coalescence-controlled crack- ing to limiting the net effect of stress triaxiality. Keywords Ductility · Stress state · Twinning · Interrupted tests · Microcrack coalescence Introduction Magnesium alloys are prime candidates for lightweight structural applications but suffer from poor formability and low (post-necking) ductility at room temperature. Much is known about the yielding and deformation behavior of Mg alloys, including their tension–compression asymme- try, plastic flow anisotropy and propensity for twinning under favorable loading conditions [1–3]. However, lit- tle research has been devoted to damage and fracture [4–6]. While microstructural manipulations, through alloy- ing, grain refinement or texture design [7–9], hold the B. Kondori · A.A. Benzerga () Materials Science & Engineering Department, Texas A&M University, College Station, TX 77843, USA e-mail: benzerga@tamu.edu B. Kondori · A.A. Benzerga Department of Aerospace Engineering, Texas A&M University, College Station, TX 77843, USA promise of developing strong, damage-resistant alloys, there currently is limited understanding of damage and fracture mechanisms in these materials. The failure mode of rect- angular prismatic, initially smooth specimens appears to be shear-like in both tension and compression [4, 8]. Voids are occasionally observed at cracked particles and twinned grains. Contraction {10 ¯ 11} twins and {10 ¯ 11}–{10 ¯ 12} dou- ble twins have been identified as favorable initiation sites of microcracks [4, 6, 10] while correlations have been inferred between elongation and the type and morphology of dispersoid particles [8, 11]. It is well established that stress state triaxiality plays an important role in the ductile fracture of polycrystals [12] and single crystals [13] but the cited literature on damage and fracture in Mg alloys has been restricted to uniaxial loading conditions. In this paper, we investigate the effect of triaxial- ity on the room-temperature ductility of AZ31B (Mg–3Al– 1Zn mass%). Because load triaxiality affects the relative importance of activated deformation systems, we character- ize the plastic anisotropy and its evolution. We exclusively use round specimens so as to deconvolute structural effects from intrinsic anisotropy. In addition, we investigate dam- age initiation sites and their dependence upon triaxiality by means of optical and scanning electron microscopy. Methods The material used is from a 32 mm thick AZ31B rolled plate supplied by Magnesium Elektron. The principal direc- tions are denoted L (rolling), T (transverse) and S (short- transverse). Examination of the microstructure in optical microscopy (Fig. 1(a)) reveals a dual grain size distribu- tion with mean values of about 12μm and 3–4μm, which is likely the result of dynamic recrystallization during hot