Material Model For Shape Memory Alloys Research Articles

Development of a constitutive model considering functional fatigue and pre-stretch in shape memory alloy wires

Shape memory alloy (SMA) wires integrate functionality into composites and encourage the design and creation of adaptable smart structures. Upon the application of cyclic loading, however, the functional properties of SMA wires can decrease significantly resulting in functional fatigue. To simulate this cyclic behavior, an SMA material model with logarithmically evolving and temperature dependent functional fatigue is developed. Pre-stretch is also considered to simulate structures with embedded pre-stretched SMA wires. The material model is fit to a set of tests (constant strain, isobaric, isothermal tension, isothermal cyclic tension) performed on SMA wires. The fit demonstrates an excellent agreement with the experimental results with minor discrepancies. The simulations show the capabilities of the model to evolve the residual strains, residual stresses, hysteresis and hardening in the presence of cyclic loading. Additionally, the isobaric actuation behavior and the reorientation response of the model are validated using experiments from the literature. The fitted and validated model is then used to simulate the response of a rubber specimen with two embedded SMA wires subjected to cyclic thermal loading. The simulated rubber specimen illustrates the loss of functionality after the applied cyclic loading and highlights the importance of not exceeding design temperature loads. Furthermore, the complex thermomechanical loading in the SMA wires displays the importance of including temperature or stress dependence for defining functional fatigue.

Everting of tubular net structures based on Shape Memory Alloys

Foldable technical structures can be used to provide a temporary additional volume. In nature, a tubular folding application is the extension of the snail eye. The present study uses this approach. A transformation into a technical application is only successful if the high eversion loads are considered. The study aims to develop a method to realize such structures based on a metallic wire net structure. The tube consists of Shape Memory Alloy (SMA) pieces connected by developed crimp elements produced by additive manufacturing. Additive manufacturing was used to provide a fast and effective way to make several crimp element geometries available to define a preferred variant. In a preliminary building study, the printing parameters for crimp element production were improved. The tubular structure development is assisted by numerical simulation of the eversion process. The SMA material model parameters were identified with experimental tension tests. A feasible way to connect additively manufactured crimp elements with SMA wire was found within a joining method study. Tension tests of the connections protect against pull-out failure. The eversion process was investigated using a high-speed camera system. Multiple eversion of the developed structure is possible without failure.

Finite-element modelling of NiTi shape-memory wires for morphing aerofoils

AbstractThis paper presents the development and implementation of a user-defined material (UMAT) model for NiTi Shape-Memory Alloy (SMA) wires for use in LS-DYNA commercial explicit finite-element analysis software. The UMAT focusses on the Shape-Memory Effect (SME), which could be used for actuation of aerostructural components. The actuation of a fundamental structure consisting of an SMA wire connected in series with a linear spring was studied first. The SMA thermomechanical behaviour obtained from the finite-element simulation was compared with that obtained from the analytical solution in MATLAB. A further comparison is presented for an SMA-actuated cantilever beam, showing excellent agreement in terms of the SMA stress and strain as well as the tip deflection of the cantilever beam. A mesh sensitivity study on the SMA wire indicated that one beam element was adequate to accurately predict the SMA thermomechanical behaviour. An analysis of several key parameters showed that, to achieve a high recovery strain, the stiffness of the actuated structure should be minimised while the cross-sectional area of the SMA wire should be maximised. The actuation of an SMA wire under a constant stress/load was also analysed. The SMA material model was finally applied to the design of morphing aluminium and composite aerofoils consisting of corrugated sections, resulting in the prediction of reasonably large trailing-edge deflections (7.8–65.9 mm).

Target shape optimization of functionally graded shape memory alloy compliant mechanisms

This article focuses on the design optimization of shape memory alloy compliant mechanisms with functionally graded properties to achieve a user-defined target shape. The functional grading is approximated by allowing the geometry and the modulus of elasticity of each zone to vary. The superelastic phenomenon has been taken into account using a standard nonlinear shape memory alloy material model with linear region of higher modulus of elasticity and a superelastic region with much lower modulus of elasticity. A large deflection beam model is integrated with a multi-objective evolutionary algorithm for constrained optimization of the structure’s mechanical properties and geometry. Examples illustrate the trade-offs between the objectives of minimizing shape error, maximum stress, and volume. It is observed that in the optimized designs, the elastic modulus and the geometry work together in regions where large flexibility is required to achieve the target shape.

Assessing the performance characteristics and clinical forces in simulated shape memory bone staple surgical procedure: The significance of SMA material model

This work is focused on the detailed computer simulation of the key stages involved in a shape memory alloy (SMA) osteosynthesis bone stapling procedure. To this end, a recently developed three-dimensional constitutive SMA material model was characterized from test data of three simple uniaxial-isothermal-tension experiments for powder metallurgically processed nickel-rich NiTi (PM/NiTi-P) material. The calibrated model was subsequently used under the complex, thermomechanical loading conditions involved in the surgical procedure using the body-temperature-activated PM/NiTi-P bone staple. Our aim here is to assess the immediate and post-surgical performance characteristics of the stapling operation using the material model. From this study: (1) it was found that adequate compressive forces were developed by the PM/NiTi-P bone staple, with the tendency of this force to even increase under sustained thermal loading due to the intrinsic “inverse relaxation phenomena” in the SMA material, (2) the simulation results correlated well with those from experimental measurements, (3) the body-temperature-activated PM/NiTi-P staple was proved to be clinically viable, providing a stable clamping force needed for speedy coaptation of the fractured bones, and (4) these realistic assessments crucially depend on the use of suitable and comprehensive SMA material models.

Finite Element Model of Shape Memory Alloy Incorporating Drucker-Prager Model

The unique features of shape memory alloys (SMA), including pseudoelasticity and shape memory effect, give SMAs a wide application in aeronautical, biomedical, and structural engineering. These features stimulate the interest in the development of constitutive models. In this paper, a 3D finite element model of shape memory alloy material model has been developed to incorporate the Drucker – Prager model in order to describe the asymmetry of SMA under tension and compression. This paper also takes into account the variation of Young’s modulus of the austenite and the martensite. The development and implementation of a robust integration algorithm is presented. The provided numerical simulation demonstrates its capabilities. Further studies should be performed to seek quantitative fitting with experimental results. DOI: http://dx.doi.org/10.11591/telkomnika.v11i7.2841 Full Text: PDF

The cyclic and evolutionary response to approach the attraction loops under stress controlled isothermal conditions for a multi-mechanism based multi-axial SMA model

The main focus in the present work has been on studying the evolutionary responses of Shape Memory Alloy (SMA) materials under isothermal, cyclic loading conditions. To this end, predictions of a recently-developed SMA material model by the authors is used here to carry out the qualitative comparisons to some of the available experimental results in the literature for SMA material responses under different uniaxial and multi-axial conditions of stress-control, both in the pseudoelastic and the pseudoplastic regimes. In the formulation of this model, the significant roles played by the internal state variables, underlying the inelastic mechanisms in the model to regulate the material’s evolutionary response under extended cycles, are emphasized. The results presented have led to a number of important conclusions. First, the evolutionary character for pseudoelasticity is markedly different from that occurring in the pseudoplastic regime. Second, the virgin material response under minor loop cycles is dramatically different from its pre-cycled counterpart for which a prior major loop was established. Third, the careful selection of the loading-control variable such as magnitudes of the mean stress and stress amplitude plays a major role in dictating the amount of strain and detailed shapes of the saturated stress–strain loop achieved, as well as the number of cycles required to reach the saturation. Lastly, multi-axial load cycling under conditions of combined tension/compression/shear leads to a faster approach to the saturated states compared to the uniaxial load condition.

Calibration of a three-dimensional multimechanism shape memory alloy material model for the prediction of the cyclic “attraction” character in binary NiTi alloys

As typically utilized in applications, a particular shape memory alloy device or component operates under a large number of thermomechanical cycles, hence, the importance of accounting for the cyclic behavior characteristics in modeling and characterization of these systems. To this end, the present work is focused on the characterization of the evolutionary, cyclic behavior of binary 55NiTi (having a moderately-high transformation temperature range). In this study, an extensive set of test data from recent cyclic, isobaric, tension tests was used. Furthermore, for the calibration and characterization of this material, a newly developed, multiaxial, material-modeling framework was implemented. In this framework, multiple, inelastic mechanisms are used to regulate the partitioning of energy dissipation and storage governing the evolutionary thermomechanical response.

형상기억합금의 개선된 구성적 모델

Abstract Shape memory alloys(SMAs) exhibit pseudoelastic behavior, characterized by the recovery of an original shape even after severe deformation, during loading and unloading within appropriate temperature regimes. The distinctive mechanical behavior is associated with stress-induced transformation of austenite to martensite during loading and reverse transformation to austenite upon unloading. To develop a material model for SMAs, it is imperative to consider the difference in moduli of active phases. For example, the Young’s modulus of the martensite is one-third to one half of that of the austenite. The model proposed herein is a modification of the one proposed recently by Ho[17]. The prediction of the behavior of SMAs during unloading before the onset of reverse transformation was improved by introducing a new internal state variable incorporating the variation of the elastic modulus. Key Words : Shape Memory Alloy, Pseudoelasticity, Constitutive Model, Elastic Modulus