Session: 01-03 Shape Memory Alloys 2

Paper Number: 171435

171435 - An Updated Empirical Model for the Fracture Toughness of a Ni-Mn-Ga Magnetic Shape Memory Alloy 

Ni-Mn-Ga is a magnetic shape memory alloy (MSMA) that experiences the shape memory effect due to either the magnetic field-induced or mechanical stress-induced material’s microstructure reorientation. Most commonly, the alloy is used in applications that do not heat up the material above the martensite to austenite transformation temperature, hence leveraging on the martensite reorientation to achieve desired application outcomes. To date, the MSMA based applications load the alloy mostly in compression, although applications in which the alloy would be loaded in tension have been proposed. 

While the fracture mechanics of thermally activated shape memory alloys (SMAs) have been investigated to some extent [22, 23, 24, 25, 26], the understanding of the effect of thermo-mechanical loading on the mechanisms governing their fracture mechanics is still limited [27]. The literature [28, 29, 30] reports that stresses in the SMAs’ crack tip region result in a localized microstructure transformation at the crack tip, from twinned into detwinned martensite. This results in material toughening and contributes to crack growth resistance in SMAs. These limited findings cannot be extrapolated to MSMAs, because i) the MSMAs deployed in engineering applications are single crystals while SMAs are polycrystals, and ii) the SMAs exhibit a phase transformation during operation while MSMAs exhibit microstructure reorientation; these differences impact the fracture mechanics mechanisms of the two alloys.

The motivation for this study comes from reported findings that magnetic field loading facilitates crack growth and reduces the fatigue life of MSMAs [1]. Correspondingly, while designing tensile applications of the alloy, one has to be able to predict the fracture toughness of the alloy, both as a function of mechanical stress and of magnetic field. This study provides an updates empirical relationship that allows the evaluation of Ni-Mn-Ga alloy's fracture energy/fracture toughness under magnetomechanical loading conditions. The fracture energy was evaluated through Vickers microindentation experiments that allowed the investigation of crack nucleation and growth under magneto-mechanical loading conditions. The length of the cracks and the dimensions of the microindentation impressions were used to develop a semi-empirical relationship that predicts the fracture energy of the alloy as a function of magneto-mechanical loading conditions. The results confirm that the magnetic field (particularly the transversely applied magnetic field) facilitates crack growth, decreasing the fracture energy, while axial compressive stress impedes crack growth, increasing the fracture energy of the alloy. The proposed empirical relationship helps identify the magneto-mechanical loading conditions that are least conducive to fracture initiation and growth in MSMAs.

This presentation will discuss the experimental approach, the underlying hypotheses for the developed empirical model, and will discuss the model predictions in relation to the anticipated behavior of the alloy under the considered loading conditions.

Presenting Author: Constantin Ciocanel Northern Arizona University

Presenting Author Biography: Cornel Ciocanel is a Professor of Mechanical Engineering whose areas of expertise are magnetic shape memory alloys and multifunctional composites with power storage capabilities.