Comprehensive modeling of shape memory alloys for actuation of large-scale structures.

摘要

Inspired by universe, the design of smart systems and structures that can repair themselves have always been one of the ambitions of engineers and researchers. Throughout the history of science, metals and alloys have played an important role in the advancement of engineering, science & technology. With the emerging new class of materials, Shape Memory Alloys (SMAs) are being viewed as an alloy for a new era. The good performance of SMAs in commercial applications (e.g. Boeing adaptive chevron, Mars Sojourner Rover Actuator, Biomedical Stents etc.) has been well established.;SMAs are primarily known for their shape recovery characteristics that occur as a result of the transformation between two phases: Austenite and Martensite. These materials undergo a diffusionless, thermoelastic, martensitic phase transformation, and can recover strains as large as 8%. However, the true potential of these materials has not yet been realized in practical applications. A lack of sufficient experimental information and unified approaches to constitutive modeling has made it difficult for designers to incorporate these vastly unique materials in practical ways. In general, the constitutive modeling approaches utilized by various research groups around the world have been based on the classical plasticity theories. Also, most of the models reported in the literature are either one dimensional, and/or extended in ad-hoc ways to capture three dimensional aspects. These models are prepared with an eye towards fitting a specific set of experimental data (i.e. a specific set of SMA features). The most common feature of SMAs of practical importance is the evolution with cycles that has almost always been neglected by the researchers.;In the current research, a new, fully general, completely three dimensional, multimechanism based, viscoelastoplastic, unified SMA constitutive model has been developed as an extension of previously formulated constitutive theory by Saleeb et al. for high temperature Ti-alloys. The success of the present unified model in describing all the salient features of SMAs (uniaxial, multiaxial, coupled thermomechanical, evolution with cycles, rate effects, etc.) is attributed to the careful partitioning of energy (into storage and dissipation) through multiplicity of viscoelastoplastic mechanisms, and to the strict adherence to the well established mathematical and thermodynamical requirements of convexity, associativity, normality, etc. It will be shown that the present thermodynamical framework is able to capture all the known features, as well as the evolutionary characteristics, of SMAs.;The present unified SMA constitutive model has been fully integrated with a commercial, large-scale, finite element package. For the demonstration of its capability and efficiency for boundary value problems, an application of SMAs as actuators for wing morphing (shape control) of a conceptual aircraft wing will be presented. Unlike other SMA actuation mechanisms, where only uniaxial response and one-way shape memory effects have been utilized, the current design concept focuses on the complete three dimensional description of actuation under biased, thermal-cycling conditions, and has led to an interesting concept for future aircraft wings.