Mechanics of Martensitic Phase transformation in shape memory alloys Experiments and modelling

Abstract

Thermally activated NiTi based Shape Memory Alloys (SMAs) are of tremendous practical interest as actuators in varied engineering applications owing to its characteristic Martensitic Phase Transformation (MPT). The MPT in thermally activated SMAs from a parent Austenite (A) to a product Martensite (M) structure, can be brought by a change in temperature or stress and is accompanied by reversible strain, typically between 5-8% whichis exploited in commercial applications. In practical applications, due to varied nature of thermomechanical loading, this phase transformation can be partial, incomplete, interrupted or discontinuous. While significant body of literature exists on phase evolution in kinetics, several aspects of kinetics still need focused study. Furthermore, this MPT is also accompanied by several effects like latent heat of transformation, changes in modulii, specific heat, thermal conductivity and electrical resistivity which gives rise to a coupled the thermal mechanical and electrical problem. A better experimental understanding of the kinetics of phase transformation and a modeling framework that can predict the response under realistic application scenarios would facilitate better design and efficient applications. With this motivation, this thesis attempts to highlight the SMA response in general and phase kinetics in particular and develop models with desired predictive capability.Different types of thermal cyclic loads are chosen to induce different types of partial or interrupted transformations. Arbitrary loads leading to partial or incomplete transformation in both forward and reverse loading directions is studied with series of experiments on wire form NiTi based SMAs. Type of loading that involves transformation interrupted or incomplete and the loading does not induce reverse transformation and the transformation resumes upon increased loading in subsequent cycles. As the transformation in such loading are arrested it is referred as Thermal Arrest Memory Effect (TAME). It is reported in the literature that TAME is associated only during the MA transformation, the present study brings out a case wherein the AM transformation also exhibits TAM. Experiment shows during TAME, the material remembers the loading condition at which the transformation was arrested. In contrast during the loading with partial loops a “memory” of the state corresponding to the maximum evolution of the product phase is present in the material. Furthermore, this study, shows that the memory is not perfect and transformation seem to resume at a loading level that is lesser than the previously applied load level. For a better understanding of the kinetics of phase transformation and a modeling framework that can predict the response under realistic application scenarios would facilitate better design and efficient applications. A relatively simple phenomenological model with memory parameter is proposed to capture the hysteretic response under arbitrary thermomechanical loading. Distance of a point on the load path, In the σ -T phase diagram is used as the memory parameter. To also incorporate the observed imperfection in memory and stabilization associated with it, another empirical material dependent parameter is introduced in the form of „stabilization parameter”. The proposed model with these two new parameters is shown to be able to capture the complex response of SMAs under arbitrary thermomechanical loading under 1-D loading. A 3-D SMA material model extended from the earlier developed 1-D model using the principles of generalized plasticity is formulated. This methodology is used to solve several case studies of practical relevance to highlight the capability of the proposed methodology to solve complex IBVPs. Numerical case studies have clearly highlighted that the proposed model can capture realistic SMA response under combined electro-thermo-mechanical loading. Furthermore, effect of conditions like crimping of wires, multi-axial state of stress and geometric complexities like notches on the phase inhomogeneity is clearly illustrated. The present work may be extended to assess the role of such phase inhomogeneities on the functional and mechanical fatigue of SMAs under arbitrary cyclic loading.