Alexandros Solomou

Phenomenological modeling of induced transformation anisotropy in shape memory alloy actuators

This paper considers new extensions to a three-dimensional constitutive model originally developed by Lagoudas and co-workers. The proposed model accurately and robustly captures the highly anisotropic transformation strain generation and recovery observed in actuator components that have been subjected to common material processing and training methods. A constant back stress tensor is introduced into the model, which is implemented in an exact form for simple tension/torsion loading as well as into a commercial finite element code to perform a 3-D analysis of a Shape Memory Alloy (SMA) torque tube actuator subjected to different loading schemes. Numerical correlations between predicted and available experimental results demonstrate the accuracy of the model.

A coupled layered thermomechanical shape memory alloy beam element with enhanced higher order temperature field approximations

This article describes the development and validation of a new thermomechanically coupled multi-layered shape memory alloy beam finite element. The finite element is formulated, assuming coupled equilibrium equations for the mechanical and thermal problems. The constitutive shape memory alloy model of Lagoudas and coworkers is implemented in the formulation. Multi-field kinematic hypotheses are proposed, combining a first-order shear displacement field with a sixth-order polynomial temperature field through the thickness of the beam, enabling adequate representation of the temperature and phase transformation profiles due to rapid thermal loading, uneven thermal loading, and boundary conditions and multi-layered configurations with variable thermal properties. The non-linear transient discretized equations of motion of the shape memory alloy beam are synthesized and solved using the Newton–Raphson method with an implicit time integration scheme. Numerical results illustrate the time response of uniform and bi-layered NiTi beams under various thermomechanical loads predicted by the developed finite element. Correlations of the beam element predictions with those of plane stress two-dimensional finite element shape memory alloy models demonstrate excellent agreement in the calculated displacement, temperature, and phase transformation fields. Additionally, the developed beam finite element yields computationally fast simulations providing an effective tool for the design and simulation of rod, beam, and strip shape memory alloy actuators and active structures.

A coupled thermomechanical beam finite element for the simulation of shape memory alloy actuators

The proposed article describes the development of a new beam finite element for the coupled thermomechanical analysis of shape memory alloy actuators. The element is formulated, assuming coupled equilibrium equations for the thermoelastic stresses and thermal loads. Displacements and temperature are treated as internal degrees of freedom giving the ability to predict the coupled thermal–displacement response of a shape memory alloy beam. The constitutive shape memory alloy model of Lagoudas and coworkers is implemented in the formulation. A generalized beam theory is formulated assuming shear deformation with a cubic temperature field through the thickness. The new element is capable to simulate heat transfer phenomena, electric Joule heating as direct input, and heat convection effects. The coupling between mechanical and thermal equilibrium equations due to endothermic/exothermic martensitic transformation procedures is also included. Numerical results and evaluations of the developed beam element are presented for the thermomechanical response of shape memory alloy wire actuators and an adaptive strip subject to various types of applied thermal loading and heat convection conditions. The effect of coupling terms on the prediction of shape memory alloy actuator response is also evaluated.

Airfoil morphing based on SMA actuation technology

Purpose – This study aims to develop an innovative actuator for improving the performance of future aircraft, by adapting the airfoil shape according to the flight conditions. The flap’s camber of a civil regional transportation aircraft’s trailing edge actuated and morphed with the use of shape memory alloys (SMA) actuator technology, instead of the conventional split flap mechanism is studied. Design/methodology/approach – For the flap’s members sizing an efficient methodology is utilised based on finite element (FE) stress analysis combined to analytically formulated design criteria. A mechanical simulation within an FE approach simulated the performance of the moving rib, integrating both aerodynamic loads and SMA phenomenology, implementing Lagouda’s constitutive model. Aim of this numerical simulation is to provide guidelines for further development of the flap. A three-dimensional assembly of the flap is constructed to produce manufacturing drawing and to ensure that during its morphing no interfer...

Full-Field Micromechanics of Precipitated Shape Memory Alloys

A full-field micromechanics approach is developed to predict the effective thermomechanical response of precipitation-hardened near-equiatomic Ni-rich NiTi alloys on the basis of composition and heat treatment. The microscale-informed model takes into account the structural effects of the precipitates (precipitate volume fraction, elastic properties, and coherency stresses due to the lattice mismatch between the precipitates and the matrix) on the reversible martensitic transformation under load as well as the chemical effects resulting from the Ni depletion of the matrix during precipitate growth. The post-aging thermomechanical response is predicted based on finite element simulations on representative microstructures, using the response of the solutionized material and time–temperature–martensitic transformation temperature maps. The predictions are compared with experiments for materials of different initial compositions and heat treatments and reasonably good agreement is demonstrated. The proposed methodology can be in principle extended to predict the post-aging thermomechanical response of other shape memory alloy systems as well.

Predicting the constitutive response of precipitation hardened NiTiHf

Current efforts towards the identification of suitable processing parameters of shape memory alloys (SMAs) that enhance their actuation performance, are based on semi-empirical approaches. This is largely due to a lack of models able to predict the macro-mechanical response of SMAs as function of given composition and the temperature and time of an imposed heat treatment. The present work aims for the development of multi-field Finite Element (FE) based models, for the NiTiHf SMA material system, adequate to address these challenges and able to simulate materials macro-mechanical response including transformation strain, hysteresis and transformation temperatures. Representative Volume Elements (RVEs) with periodic geometry and boundary conditions are used to model materials microstructure. Randomly placed precipitates are considered in the NiTiHF matrix, while eigenstrains corresponding to the lattice mismatch between the precipitates and the matrix are introduced in order to model the residual stress and strain fields. The Hf concentration field is taken into consideration in addition to the displacement field in order to capture the Hf diffusion process through the adoption of Fickian diffusion law. To this end the composition of NiTiHf in the vicinity of the precipitates is computed thus resulting in substantial SMA transformation temperature shifts. The developed framework is validated based on correlations with experimental results.

Micromechanical Modeling of Precipitation Hardened NiTiHf

Actuation response of NiTiHf high temperature SMAs can be enhanced by means of suitable heat treatment on the material through precipitation hardening. Heat treatments can be chosen carefully to improve the performance of the NiTiHf SMAs in order to meet the requirements of targeted applications to design more robust and efficient high temperature solid-state actuator systems. The present work aims to develop a novel approach to model and predict the behavior of heat-treated NiTiHf SMAs. The predictions of the thermomechanical response of NiTiHf SMAs are based on Representative Volume Elements (RVEs). The precipitated NiTiHf SMA is modeled as a composite consist of of thermo-elastic non-transforming precipitates and a polycrystalline SMA matrix. The structural effect of precipitates and the effect of Hf-concentration gradient resulted from Hf depletion during precipitation are included. The composition distribution resulting from the elemental depletion and the transformation temperature distributions in the SMA matrix are related. In the present work, these relations are developed from experimental measurements on several NiTiHf compositions. Thermo-mechanical responses of Ni50.3Ti29.7Hf20 heat-treated at 500°C for 48h at different loading conditions are predicted and the correlations with experimental results demonstrate the validity of the proposed framework.

Constitutive response of precipitation hardened Ni-Ti-Hf shape memory alloys through micromechanical modeling

Shape memory alloys (SMAs) are unique materials with the ability to generate and recover moderate to large inelastic deformations. Due to their aforementioned ability, SMAs are suitable for applications in aerospace, oil and gas and automotive industries, where compact actuators with high actuation energy density are required. The current work presents a modeling framework that links the heat treatment of SMAs with their effective response and aims to accelerate the discovery of new high temperature SMAs with optimal performance. Thus a finite element based, multi-field micromechanical framework is developed to capture the constitutive response of precipitation hardened Ni-Ti-Hf SMAs. A representative volume element of precipitated polycrystalline SMAs is considered which contains randomly distributed non-overlapping precipitates, while periodic boundary and geometric conditions are maintained. The SMA matrix is assumed to behave isotropic as a result of random texture while the precipitates are considered as linear elastic solids. The effect of the lattice mismatch between the precipitates and the matrix, and the effect of the Ni and Hf depletion during precipitation on the thermo-mechanical response of the material are taken into consideration. The Fickian diffusion law is used to predict the Ni and Hf concentration field in the vicinity of the precipitates, which results in substantial SMA transformation temperature shifts. Finally, the predictive capability of the developed framework is assessed through correlations with experimental results.

Fracture toughness of shape memory alloy actuators: effect of transformation-induced plasticity

Numerical analysis of static cracks in a plane strain center-cracked infinite medium shape memory alloy (SMA) panel subjected to cyclic thermal variations and a constant mechanical load is conducted using the finite element method. In solid-state SMA actuators, permanent changes in the materials microstructure in the form of dislocations are caused during cyclic thermomechanical loading, leading to macroscopic irreversible strains, known as transformation induced plastic (TRIP) strains. The influence of these accumulated TRIP strains on mechanical fields close to the crack tip is investigated in the present paper. Virtual crack growth technique (VCCT) in ABAQUS FEA suite is employed to calculate the crack tip energy release rate and crack is assumed to be stationary (or static) so that the crack tip energy release rate never reaches the material specific critical value. Increase in the crack tip energy release rate is observed during cooling and its relationship with accumulation of TRIP due to cyclic transformation is studied.

Development of SMA Actuated Morphing Airfoil for Wind Turbine Load Alleviation

Wind turbine rotor upscaling has entered a range of rotor diameters where the blade structure cannot sustain the increased aerodynamic loads without novel load alleviation concepts. Research on load alleviation using morphing blade sections is presented. Antagonistic shape memory alloy (SMA) actuators are implemented to deflect the section trailing edge (TE) to target shapes and target time-series relating TE movement with changes in lift coefficient. Challenges encountered by the complex thermomechanical response of morphing section and the enhancement of SMA transient response to achieve frequencies meaningful for aerodynamic load alleviation are addressed. Using a recently developed finite element for SMA actuators [1], actuator configurations are considered for fast cooling and heating cycles. Numerical results quantify the attained ranges of TE angle movement, the moving time period and the developed stresses. Estimations of the attained variations of lift coefficient vs. time are also presented to assess the performance of the morphing section.