Functionally Graded Shape Memory Alloy Research Articles

Research and simulation on the thermal mechanical properties of functionally graded shape memory alloy composite beam considering tension-compression asymmetry

Considering the tension-compression asymmetry of Shape Memory Alloy (SMA), the tension and compression constitutive equations of the Functionally Graded Shape Memory Alloy (FG-SMA) beam are established based on the simplified constitutive relation of SMA. The influences of the power exponent, temperature, tension-compression asymmetry coefficient, and strain (along the length direction of the beam) on the martensite volume fraction and stress of the beam are analyzed, and the correctness of the theory is verified by comparing the results of theory with finite element analysis. The research shows that when the height of the beam is constant, the power exponent, temperature, tension-compression asymmetry coefficient are proportional (inversely proportional) to the stress (martensite volume fraction) of the cross-section, and the strain is proportional to both the stress and martensite volume fraction of the cross-section. When the other conditions are fixed, compared with compression, the martensite volume fraction (stress) under tension is larger (slightly less) at the same layer cross-section position.

Finite Element Method of Functionally Graded Shape Memory Alloy Based on UMAT

Functionally graded shape memory alloy (FG-SMA) is widely used in practical engineering regions due to it possessing the excellent properties of both FG material and SMA material. In this paper, the incremental constitutive equation of SMA was established by using the concept of a shape memory factor. On this basis, the secondary development function of the ABAQUS software 2023 was used to write the user-defined material subroutine (UMAT). The phase transformation and mechanical behavior of transverse and axial FG NiTi SMA cantilever beams under concentrated load at free ends were numerically simulated by discrete modeling. Numerical results show that the stress and shape memory factor were distributed asymmetrically along the thickness direction of the transverse FG-SMA cantilever beam, while the stress and the shape memory factor distributed symmetrically along the thickness direction of the cross section of the axial FG-SMA cantilever beam. The bearing capacity of the axial FG-SMA cantilever beam is stronger than the SMA homogeneous cantilever beam, but weaker than the transverse FG-SMA cantilever beam. The load-bearing capacity of the transverse FG-SMA cantilever beam is twice that of the axial FG-SMA cantilever beam under the same functionally graded parameters and deflection conditions. The discrete modeling method of FG-SMA beams proposed in this paper can simulate the phase transformation and mechanical behavior of an FG-SMA beam well, which provides a reference for the practical application and numerical calculation of FG-SMA structures.

On size-dependent bending behaviors of shape memory alloy microbeams via nonlocal strain gradient theory

This paper focuses on the size-dependent behaviors of functionally graded shape memory alloy (FG-SMA) microbeams based on the Bernoulli-Euler beam theory. It is taken into consideration that material properties, such as austenitic elastic modulus, martensitic elastic modulus and critical transformation stresses vary continuously along the longitudinal direction. According to the simplified linear shape memory alloy (SMA) constitutive equations and nonlocal strain gradient theory, the mechanical model was established via the principle of virtual work. Employing the Galerkin method, the governing differential equations were numerically solved. The functionally graded effect, nonlocal effect and size effect of the mechanical behaviors of the FG-SMA microbeam were numerically simulated and discussed. Results indicate that the mechanical behaviors of FG-SMA microbeams are distinctly size-dependent only when the ratio of material length scale parameter to the microbeam height is small enough. Both the increments of material nonlocal parameter and ratio of material length-scale parameter to the microbeam height all make the FG-SMA microbeam become softer. However, the stiffness increases with the increment of FG parameter. The FG parameter plays an important role in controlling the transverse deformation of the FG-SMA microbeam. This work can provide a theoretical basis for the design and application of FG-SMA microstructures.

A theoretical study on asymmetric bending for functionally graded shape memory alloy beam

Bending is an attractive mode of operation and application of functionally graded shape memory alloy (FG-SMA). For decades, numerical methods, such as the finite element method, have been used to model the mechanical behavior of these alloys. In this paper, combining with the critical stress–temperature relationship and stress–strain relationship of shape memory alloy, it researched the mechanical behavior of FG-SMA beam under thermal and mechanical loads with the macroscopic constitutive model. The split-step method was adopted to analyze the process of FG-SMA phase transformation in the loading process. The relationship between stress distribution and three factors that included tension–compression asymmetry coefficient, temperature and power exponent were obtained, and the nonlinear controlling equations of FG-SMA beam were acquired. The influence of tension–compression asymmetry coefficient, temperature and power exponent on deflection, curvature and cross section stress, neutral axis and phase boundary were obtained by solving the equations. It was found that the phase transformation of FG-SMA beam became more and more difficult as the increase in temperature and power exponent. Furthermore, the tension–compression asymmetry coefficient had a great influence on the compression side phase boundary, but it had a little influence on the tension side phase boundary.

On thermomechanical behaviors of the functional graded shape memory alloy composite for jet engine chevron

This study presents a theoretical work for a novel adaptive jet engine chevron concept based upon embedding the functionally graded shape memory alloy actuators in a composite laminate, termed a functionally graded shape memory alloy actuator composite. The constitutive models of the functionally graded shape memory alloy actuator composites including the monolayer shape memory alloy composites and multilayer shape memory alloy composites with different volume fractions of the shape memory alloy were first given. An example using such models was discussed on a published finite element work on a shape memory alloy hybrid composite jet engine chevron concept to prove the validity of the theoretical work. The thermomechanical behaviors of the functionally graded shape memory alloy actuator composite with different volume fractions of the shape memory alloy subjected to the thermal loading were then discussed using the obtained constitutive model. The tip deflections of the jet engine chevron with different embedding patterns of the shape memory alloy were finally obtained.

Thermal mechanical behavior of a functionally graded shape memory alloy cylinder subject to pressure and graded temperature loads

Abstract

Study on behaviors of functionally graded shape memory alloy cylinder

For better controllability in actuations, it is desirable to create Functionally Graded Shape Memory Alloys (FG-SMAs) in the actuation direction. It can be achieved by applying different heat treatment processes to create the gradient along the radius of a SMA cylinder. Analytical solutions are derived to predict the macroscopic behaviors of such a functionally graded SMA cylinder. The Tresca yield criterion and linear hardening are used to describe the different phase transformations with different gradient parameters. The numerical results for an example of the model exhibit different pseudo-elastic behaviors from the non-gradient case, as well as a variational hysteresis loop for the transformation, providing a mechanism for easy actuation control. When the gradient disappears, the model can degenerate to the non-gradient case.

Functionally-graded shape memory alloy by diffusion annealing of palladium-coated NiTi plates

Diffusion annealing of palladium-coated Ti-Ni plates was performed at temperatures ranging from 900 °C to 1,000 °C, to accomplish a compositional gradient in Ti-rich, Ti-Ni shape memory alloys. The aim of this study was to increase the transformation temperatures and transformation temperature intervals. Palladium diffusion profiles were measured by energy dispersive spectroscopy, and the corresponding approximate diffusion coefficients of the annealed specimens were calculated. The Gaussian solution of Fick’s second law for the one-dimensional lattice diffusion of a tracer was used. The transformation behavior studies were performed by differential scanning calorimetry. It was depicted that annealed specimens show longer transformation intervals compared to the bare alloy. In addition, annealed specimens showed improved shape memory properties that were attributed to the lower amount of Ti2Ni precipitates in the diffusion layer. The shape memory behaviour of the samples was detected using micro-indentation at room temperature, followed by heating them above the austenite formation temperature to calculate the shape recovery ratio.

A theoretical model for functionally graded shape memory alloy cylinders subjected to internal pressure

Abstract

Theoretical analysis of a functionally graded shape memory alloy plate under graded temperature loading

ABSTRACTAs a new kind of functional material, functionally graded shape memory alloy (FG-SMA) possesses the excellent properties of both shape memory alloy (SMA) and functionally graded material (FGM). By combining the heat conduction theory with the theory of the mechanics of composite materials, a macro constitutive model, which can be used to describe the mechanical behavior of FG-SMA under graded temperature loading, is established. With this macro constitutive model, the thermo-mechanical properties of an FG-SMA plate, which is composed by ceramic and SMA and subjected to different surface temperature loads, are investigated. The theoretical results show good agreement with the existing data, which indicates that the macro constitutive model provided here is valid. The obtained results show that the martensite transformation does not always happen first at the top or bottom of the plate, and it is dependent on the surface temperatures. It can also be found that the stress decreases markedly due to the martensite transformation. This research can provide a basis for the design and in-depth investigation of FG-SMA material.

On Behaviors of Functionally Graded SMAs under Thermo-Mechanical Coupling

An analytical solution is obtained for the Functionally Graded Shape Memory Alloy (FG-SMA) composites subjected to thermo-mechanical coupling. Young's modulus and thermal expansion coefficient of the material are assumed to vary in different forms of power function through the thickness, with the Poisson's ratio being constant. An SMA constitutive model is combined with the averaging techniques of composite to determine the mechanical properties of the FG-SMA composites. Different phase transformation steps and the corresponding stress distributions through the thickness direction are given. The results show that the average stresses decrease as the transformations proceed.

On the transformation behavior of functionally graded SMA composites subjected to thermal loading

Abstract Shape Memory Alloy (SMA) composites are being used in an ever-expanding set of applications. For new applications, Functionally Graded (FG) SMA composites are being developed incorporating a wide variety of matrices. An analytical methodology combining averaging technique of composites and an SMA constitutive model is developed to determine the transformation properties of the FG–SMA composite. The results obtained from the analyses of such a composite show that after transformation the stress in the SMA composite is lower than in the case of pure elastic composite under the same thermal loading. This decrease in stress can result in an increase in temperature resistance and improved mechanical properties of SMA composites. This work will be explored through a parametric study to understand their influence on SMA composites design.

Theoretical Analysis of Functionally Graded Shape Memory Alloy Beam Subjected to Pure Bending

The functionally graded shape memory alloy(FG-SMA) is a new functional composite which possesses the excellent properties of both functionally graded material(FGM) and shape memory alloys(SMA).A macroscopic constitutive model of FG-SMA is established by using the mechanics of composite materials method and the existing SMA model.With this macro constitutive model,the mechanical behavior of a FG-SMA beam composed by elastic material A and SMA subjected to pure bending is investigated.The loading processes including elastic process and phase transformation process are discussed in detail and the analytical solutions are obtained.The distribution of stress on the cross section,the position of the neutral axis and the curvature-bending moment relation are discussed through numerical results.It can be learned that compared with common FGM,FG-SMA can decrease the maximum stress efficiently,preventing the destruction caused by oversize stress.This research can provide a base for the design and in-depth investigation of FG-SMA material.