Issue 29

M. Marino, Frattura ed Integrità Strutturale, 29 (2014) 96-110; DOI: 10.3221/IGF-ESIS.29.10 96 Focussed on: Computational Mechanics and Mechanics of Materials in Italy An ideal model for stress-induced martensitic transformations in shape-memory alloys Michele Marino Department of Civil Engineering and Computer Science Engineering (DICII), Università degli Studi di Roma Tor Vergata, via del Politecnico 1, 00133 Roma – Italy m.marino@ing.uniroma2.it A BSTRACT . In this paper, a novel model for stress-induced martensitic transformations in shape-memory alloys is proposed. Accordingly, the constitutive pseudo-elastic behavior of these materials is described. The model accounts for the possible co-existence of austenitic/martensitic phases and for asymmetric response in tension and compression (both for transformation and stiffness properties). The model is developed under the assumption of ideal behavior during martensitic transformation, and the predicted response is governed by few parameters, standard in the context of shape-memory alloys' constitutive models, that can be straightforwardly identified from experimental data. Moreover, proposed modeling framework opens to the investigation on the effects of non-linear transformation lines in phase diagrams and of temperature-dependent transformation strains. K EYWORDS . Shape-memory alloys; Pseudo-elastic behaviour; Constitutive modelling; Stress-induced martensitic transformation. I NTRODUCTION hape-memory alloys (SMAs) are special materials endowed of fascinating properties thanks to possible rearrangements of their thermoelastic lattice microstructure, generally referred to as phase transformations. In fact, objects made of those materials, when significantly deformed, regain their original geometric configuration during heating (one-way shape-memory effect) or, at higher temperature, in the unloading phase (pseudo-elasticity). The intriguing SMA thermomechanical properties allow to design smart structures, opening to innovative applications in many engineering fields. Proposed designs based on such materials range from aeronautic/mechanical applications (e.g., adaptive smart wings and actuators) and telecommunication devices (e.g., deployment and control mechanisms of satellites and antennas), to biomedical (e.g., self-expanding stents, orthodontic wires, and prostheses) and civil applications (e.g., devices for passive, active and semi-active controls of civil structures) [1, 2]. As proved by the recent wide literature in the field [1, 3-5], there is a great need of constitutive models able to reproduce SMAs behavior, including a refined description of phase transformation mechanisms. In order to be effectively employed in practical applications, models should be characterized by parameters whose values have to be easily identified from well-established experimental procedures. Moreover, since there exists a number of different materials with shape- memory and pseudoelastic properties, models should be as flexible as possible in order to be adapted to the wide range of very different thermomechanical features shown through experiments. Finally, models should be formulated within a consistent theoretical framework that, in the respect of thermodynamics laws, might be implemented in feasible numerical algorithms for computational analyses. S