Pseudoelastic Response Research Articles

Thermodynamically Guided Improvement of Fe-Mn-Al-Ni Shape-Memory Alloys.

A microstructural informed thermodynamic model is utilized to tailor the pseudoelastic performance of a series of Fe-Mn-Al-Ni shape-memory alloys. Following this approach, the influence of the stability and the amount of the B2-ordered precipitates on the stability of the austenitic state and the pseudoelastic response is revealed. This is assessed by a combination of complementary nanoindentation measurements and incremental-strain tests under compressive loading. Based on these investigations, the applicability of the proposed models for the prediction of shape-memory capabilities of Fe-Mn-Al-Ni alloys is confirmed. Eventually, these thermodynamic considerations enable the guided enhancement of functional properties in this alloy system through the direct design of alloy compositions. The procedure proposed renders a significant advancement in the field of shape-memory alloys.

Laser additive manufactured NiTi-based bioinspired helicoidal structure with excellent pseudoelasticity and energy absorption capacity

Functionality-driven biomimetic additive manufacturing (AM), as a research hotspot of 3D printing field, is providing unprecedented opportunities for developing next-generation functional materials in mechanical, electrical, optical and biomedical engineering. In this work, we developed a laser AM-ed NiTi-based helicoidal structure with excellent pseudoelasticity and energy absorption capacity, inspired by the biological architecture in the exoskeleton of the crab's claw. The rotation angle θ corresponding to the misorientation of rods in adjacent layers, as the key structure parameter, was involved into the investigations on the forming quality, pseudoelastic response and fracture behavior of the laser AM-ed helicoidal structures. The results showed that an increase in θ could improve the forming accuracy of rods but decrease the porosity due to the formation of more closed pores filled with unmelted powder particles. These pores formed by the helically stacked rods had sharp edges that could trigger earlier stress concentration and phase transformation or even strain hardening during the loading process. In views of more uniform stress distribution, the helicoidal structure with θ = 36° exhibited the largest recoverable strain of 4.32 %. Regarding the deformation and fracture behavior, it was found that the helicoidal structures with small θ were inclined to exhibit an auxetic deformation behavior and a sluggish fracture process, while the ones with larger θ were prone to a buckling deformation mode and a sudden failure. Furthermore, all helicoidal structures presented excellent energy absorption properties in terms of both unit volume and unit mass values, which could be attributed to the cross-scale hierarchical structures.

An Explicit Modeling for the Pseudoelastic Response of Porous SMA Thick-Walled Cylinders under Internal and External Pressure

Shape-memory alloy (SMA) structures, either dense or porous, are widely used because of their functional characteristics arising from shape-memory effect and pseudoelasticity. Hence, the precise modeling of these materials is essential due to design issues. In this study, the solution algorithm based on an explicit phenomenological constitutive model is investigated to analyze pressurized cylinders. The proposed method is extracted following return mapping scheme to find the evolution of phase transformation in these structures. The formulation is validated by comparing uniaxial tension–compression curves during loading, unloading and thermal actuation against published literature. By the aid of this model, assuming elastic and phase transformation strains, a comprehensive study is presented to inspect stress, strain and displacement fields along the thickness of internally and externally pressurized thick-walled cylinders. To this end, the governing nonlinear equation in elastic–inelastic zones is discretized by the efficient method of generalized differential quadrature (GDQ) and the resultant relations are solved numerically. The influences of internal and external pressure on stress distribution along the cylinder wall during loading–unloading for different temperatures including phase transformation and saturation states are discussed. Also, the closed-form solution for critical pressure at the onset of the phase transformation is studied.

A gradient regularized model for shape memory alloys

In this work a transformation strain gradient enhancement is introduced into a phenomenological constitutive model for the pseudoelastic behavior of shape memory alloys. The constitutive model is able to capture several unique features of the constitutive response of these materials during the transformation between austenite and martensite during the pseudoelastic response. These features include the asymmetry in the initial transformation stresses in tension versus compression, the asymmetry in the transformation strains in tension and compression, and finally the asymmetry in the hardening behavior in tension and compression. In fact, experiments have shown that untrained NiTi exhibits hardening during its transformation in compression, but softening for tensile loading. It is this softening behavior that motivates the need for the introduction of the transformation strain gradient into the constitutive modeling. Transformation strain gradient effects are introduced via a phase variable that describes the extent of transformation. The free energy of the material then depends on gradients of the phase variable, which introduces a material length scale into the theory. The governing equation for the phase variable is developed from a microforce balance and continuum thermodynamics analysis. The model is implemented in the commercial finite element software Abaqus through user defined subroutines and several numerical simulations are performed to illustrate the model response and lack of numerical mesh-dependency of the results.

Effects of V addition on microstructure and pseudoelastic response in Fe–Mn–Al–Ni alloys

Avoiding quench cracks and controlling the formation process of NiAl precipitates are essential to regulate the microstructure and pseudoelastic properties of Fe–Mn–Al–Ni shape memory alloys (SMAs). In this study, we investigated the effect of 1.5 at.%V addition on the quench sensitivity of Fe–Mn–Al–Ni SMAs and the precipitation behavior of NiAl precipitates in air-cooled Fe–Mn–Al–Ni–V SMAs. It is found that the V addition significantly inhibits the precipitation process of non-transforming γ-phase. Polycrystalline sample of bamboo structure was prepared by abnormal grain growth and tested by incremental strain cycling tensile test at room temperature. The polycrystalline Fe–Mn–Al–Ni–V SMAs obtain fully reversible thermoelastic martensitic transformation without quenching, which also prevents the formation of quenching cracks. Single crystal samples were further prepared by directional recrystallization. High number-density and relatively large-size coherent β-NiAl precipitates were obtained to strengthen the matrix in the single crystal without aging, resulting in large stress hysteresis and irrecoverable strain. After aging Fe–Mn–Al–Ni–V single crystal for 3 h at 200 °C, both the stress hysteresis and irrecoverable strain decreased, thereby achieving good pseudoelasticity in Fe–Mn–Al–Ni–V SMAs.

Taming the Pseudoelastic Response of Nitinol Using Ion Implantation

Implantation of Ni50.5Ti49.5 wire with 30 MeV Ni6+ ions at doses (< 0.1 DPA) typically smaller than employed in the literature is shown to systematically alter the pseudoelastic response, with extrema in Berkovich nanoindentation load (+50%), hysteresis (−60%), and recoverable displacement (−19%) occurring at ∼ 3.6 μm below the implantation surface. These extraordinary values are attributed to ∼ 10 to 20 nm amorphous clusters that constrain the stress-induced B2-B19′ phase transformation. This is substantiated by phase field simulations of crystalline-amorphous composites and molecular dynamics simulations of crystalline-vacancy cluster composites showing the spatial refinement of martensite caused by nm-scale defects. The results suggest that ion implantation may potentially expand the processing and performance space for NiTi, by creating amorphous defects at smaller length scales than dislocation substructures produced by conventional deformation processing.

Nonlinear Static Response of Low-Moderate Ductile Chevron Concentrically Braced Frames Designed According to Eurocode 8

Steel frames equipped with chevron bracing (Λ-CBF) are usually less ductile than other steel systems. Therefore, in many cases, it can be convenient to design Λ-CBF to exploit their stiffness and resistance to enforce a pseudo-elastic seismic response of the building in low to moderate seismic zones. In current EC8, the rules for moderate Λ-CBF are the same as those for high ductile frames, thus potentially leading to massive, over-resistant and uneconomic systems. In the next version of EC8 new rules have been set to design moderate ductile Λ-CBF, aiming to enhance the ease of use of the code as well as to obtain less expensive structures. The new rules of the updated EC8 are based on local requirements and elastic calculation without any plastic analysis. This paper discusses these rules that are numerically investigated by means of nonlinear static analyses on a set of 8-storey steel frames designed for different seismic intensities. The performed analyses show that the frames designed according to the updated EC8 exhibit moderate ductility, preventing damage to brace-intercepted beams and reducing ductility demand on braces under compression.

Impact Energy Absorption Analysis of Shape Memory Hybrid Composites

Shape memory hybrid composites are hybrid structures with fiber-reinforced-polymer matrix materials. Shape memory wires due to shape memory/super-elastic properties exhibit a pseudo-elastic response with good damping/energy absorption capability. It is expected that the addition of shape memory wires in the glass-fiber-reinforced-polymer matrix composite (GFRP) will improve their mechanical and impact resistant properties. Stainless-steel wires are also expected to improve the impact resistance properties of GFRPs. In this research work, we investigated the effect of addition of shape memory wires and stainless-steel wires on the impact resistance properties of the GFRP and compared our results with conventional GFRPs. Super-elastic shape memory alloy wires and stainless-steel wires were fabricated as meshes and composites were fabricated by the hand-layup process followed by vacuum bagging and the compression molding setup. The shape-memory-alloy-wires-reinforced GFRP showed maximum impact strength followed by stainless-steel-wires-reinforced GFRPs and then conventional GFRPs. The effect of the energy absorption capability of super-elastic NiTi wires owing to their energy hysteresis was attributed to stress-induced martensitic transformation in the isothermal regime above the austenite transformation temperature. The smart shape memory wires and stainless-steel-wires-based hybrid composites were found to improve the impact strength by 13% and 4%, respectively, as compared to the unreinforced GFRPs. The shape-memory-reinforced hybrid composite also dominated in specific strength as compared to stainless-steel-wires-reinforced GFRPs and conventional GFRPs.

A discrete particle model study of the effect of temperature and geometry on the pseudoelastic response of shape memory alloys

In this paper, the effects of temperature and geometry on the hysteresis behavior of shape memory alloy (SMA) under stress induced transformations are studied. We used a discrete model referred to as the constitutively informed particle dynamics (CPD), for the analysis. The CPD approach uses a multibody interaction derived from a polyconvex free energy appropriate for phase transforming materials. This approach is able to describe evolution of the phases in a sharp interface paradigm. Using such an approach allows discrete element models to describe complex material behavior such as phase transformations which were hitherto not possible. The temperature dependent pseudoelastic response is studied under uniaxial tension. The effect of temperature on this stress induced transformation is studied at four different temperatures. The results show an increase in hysteresis at higher temperatures. Next, the effect of geometry on hysteresis is studied in a tapered bar geometry. Tapered bars show hardening in the stress–strain curve. The influence of temperature and specimen geometry on the pseudoelastic stress–strain response would be essential to design and develop MEMS (Micro-Electro-Mechanical Systems) applications using these classes of smart materials.

The effect of heat treatment on pseudoelastic behavior of spark plasma sintered NiTi

Porous NiTi shape memory alloys are considered as artificial bio-materials for possible use in implant applications. Their extraordinary pseudoelastic response under uniaxial loading through phase change needs further analysis. The objective of this work is to show the effect of heat treatment on the pseudoelastic behavior of NiTi SMA produced by spark plasma sintering (SPS). NiTi samples were prepared from pre-alloyed Ti-50.7 at.% Ni powders of 100-150 μm average diameter, and subjected to a wide range of homogenization and/or aging at different temperatures and duration. Transformation characteristics of heat treated samples were obtained from DSC, their phase composition was identified using X-ray diffraction (XRD), and checked with scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Instrumented micro-indentation was carried out to measure the hardness altered by aging. Uniaxial compression tests were performed on heat treated samples, and their pseudoelastic behavior was shown on the stress–strain diagrams; the results showed a variation in pseudoelasticity. In general, Ni concentration of the matrix resulted in partial-pseudoelastic behavior of samples. Homogenization prior to aging improved the pseudoelasticity of the SPSed NiTi. The pseudoelastic response of the samples that have not shown the expected full pseudoelastic behavior was enhanced by applying a considerably long period of homogenization and aging.

Effect of volume fraction and unit cell size on manufacturability and compressive behaviors of Ni-Ti triply periodic minimal surface lattices

Ni-Ti shape memory alloys with triply periodic minimal surface (TPMS) lattice structures have aroused increasing interests due to their great application potential in bone tissue engineering. In this study, the effect of volume fraction and unit cell size on the manufacturability, compressive mechanical properties, and shape memory behaviors after compression of Ni-Ti Gyroid TPMS lattice structures, which were fabricated by laser powder bed fusion ( L -PBF), were systematically investigated. A mathematical model was established to analyze the geometric influence factors on manufacturing fidelity of Ni-Ti Gyroid TPMS lattice structures fabricated by L -PBF. It was revealed that the manufacturing fidelity decreased as the volume fraction and unit cell size decreased due to the aggravation of powder adhesion caused by the increase of specific surface area and overhanging rate of struts. Three power-law equations based on the Gibson-Ashby models were established to describe the dependences of compressive modulus, nominal yield strength, and ultimate strength of Ni-Ti Gyroid TPMS lattice structures fabricated by L -PBF on the volume fraction and unit cell size. All the above mechanical properties showed positive dependences on the volume fraction but negative dependences on the unit cell size. Both the volume fraction and unit cell size had little influence on the martensitic transformation temperatures as well as the total shape recovery ratio of Ni-Ti Gyroid TPMS lattice structures prepared by L -PBF. All the as-built Ni-Ti lattices exhibited high total shape recovery ratios of above 96.5% after 8% compressive deformation at room temperature (RT). But noteworthily, as the volume fraction increased, the pseudoelastic response (i.e., the recovered strain during unloading) rapidly decreased, followingly the recovered strain during heating increased. This was ascribed to the enhanced stability of stress-induced martensite at RT caused by the increased actual strain of local struts at the same overall compressive deformation in the Ni-Ti lattices with higher volume fractions. In contrast, the Ni-Ti lattices with different unit cell sizes exhibited similar strain distribution and consequently similar deformation recovery behaviors. • Manufacturing fidelity decreases as volume fraction and unit cell size decrease. • Mechanical properties show positive dependences on volume fraction. • Mechanical properties exhibit negative dependences on unit cell size. • Both volume fraction and unit cell size hardly affect the total recovered strain. • Superelastic response decreases as volume fraction increases.

Pseudoelastic PP and PSSP modeling in stratified media

Many modeling techniques have been developed to find the acoustic and elastic responses of a stack of plane layers to a plane spectral wave. For an elastic medium bounded above by an acoustic half-space, the acoustic wave propagator matrix modeling method can be modified to model pseudoelastic PP arrivals and PSSP arrivals. PP arrivals propagate as pure longitudinal (P) waves in the layers, whereas PSSP arrivals propagate as shear (S) waves in the elastic part of the model. A simple modification of the pseudoelastic PP response modeling scheme allows modeling of primary P reflections. A primary reflection event involves just one reflection in the plane stratified model and thus excludes internal multiples. The propagator modeling scheme is formulated in the frequency-horizontal slowness domain. By applying inverse Fourier transforms over the frequency and horizontal wavenumbers, where the wavenumber is the horizontal slowness divided by the frequency, modeled seismograms are computed and displayed in the time-space domain. By applying an inverse Fourier transform over the frequency for selected horizontal slowness components, the computed seismograms can be shown in the intercept time-horizontal slowness ([Formula: see text]) domain. When the source wavelet is unity for frequencies of interest, the [Formula: see text] domain seismograms become plane-wave Green’s function seismograms. The [Formula: see text]-traces of the Green’s function primary P-wave seismograms accumulate with increasing time band-limited step functions weighted by reflection strengths.

A new phenomenological constitutive model for shape memory alloys

A new thermomechanical constitutive modeling approach for shape memory alloys (SMAs) that undergo a martensite to austenite phase transformation and the associated pseudoelastic and shape-memory responses is presented. The novelty of this new formulation is that a single transformation surface is implemented in order to capture the forward and reverse phase transformations, as well as the reorientation and detwinning of the martensite phase. The framework is akin to the usual flow theory plasticity with kinematic hardening, however in addition to the transformation strain there is also a transformation entropy that is directly related to the martensite volume fraction appearing in prior theories. A transformation surface in effective stress and effective temperature space is introduced and an associated flow rule governs the evolution of the transformation strain and entropy. In order to capture the multitude of SMA behaviors, a transformation potential function is introduced in transformation strain and entropy space for the derivation of the back stresses and back temperatures that define the kinematic hardening behavior. It is this potential function that governs all of the important behaviors within the model. After the description of the general theory, specific forms for the transformation surface and the transformation potential are devised and results for the behaviors captured by the model are provided for a range of thermomechanical loadings. Finally, the model is implemented in the finite element method and applied to the pseudoelastic and shape memory effects of a beam in pure bending.

Implementation of visco-pseudo-elastic dampers for vibration reduction of sandwich shells using a large deformation FE technique

This study introduces a numerical method for investigating the appropriate arrangement of visco-pseudo-elastic dampers applicable to shell and plate structures under large deformation. These passive dampers are considered as discrete shape memory alloy (SMA) wires and viscoelastic layers. The SMA constitutive model is adopted and developed based on carried out experimental tests (i.e., calorimetry and tensile tests) on Ni-rich Nitinol alloy samples. Pseudoelastic, pseudoplastic, strain recovery and de-twinning phenomena are perceived experimentally. The presented model characterizes pseudoelastic response of shape memory alloys. The current finite element formulation is based on an incremental updated Lagrangian (UL) approach along with the Newmark's integration technique. The used sandwich element is capable of accurate modeling of the viscoelastic core damping behavior, as a result of independent rotation of element's layers. Also, the creep functions regarding to the viscoelastic constitutive model are estimated using Dirichlet-Prony series. Employing the state variables, the viscoelastic deferred strain is presented in a proper incremental form. A nonlinear FE program is developed to assess the proposed procedure. Obtained results demonstrate that quick vibration mitigation may be possible using visco-pseudo-elastic dampers while overcoming their individual drawbacks.

Modelling of minor hysteresis loop of shape memory alloy wire actuator and its application in self-sensing

Shape memory alloy (SMA) wire actuators offer large force and displacement capabilities out of the stress and temperature-dependent martensitic phase transformation. Among the various modelling approaches in the literature, the one proposed by Boyd and Lagoudas (1996), is mostly used in analysing SMA based components. In practice, partial actuation is often required and is enacted by aborting and subsequently resuming the phase transformation; exhibiting minor hysteresis response. The existing models are found to be inaccurate in simulating such partial transformation cases. In this study, a novel yield parameter, which depends on the martensite volume fraction, has been proposed; enabling the model of Boyd and Lagoudas to simulate minor hysteresis loop accurately. The corresponding model parameters are derived, satisfying certain practical aspects of the SMA response. Using the modified model, pseudoelastic responses are simulated for different partial loading cases and are compared against the same obtained from the existing models, illustrating its efficacy. Following the proposed model, the discrete form of the system dynamics of a SMA wire actuator is formulated. Using this, an extended Kalman filter (EKF) has been developed, to estimate the output of the system from the change in electrical resistance of the SMA wire during actuation. This approach eliminates the requirement of external sensors for feedback control. Using the developed EKF, the response of an SMA wire actuated system is estimated from the experimentally measured electrical resistance data of the SMA wire and are compared with the corresponding measured responses. The results reveal that a significant level of accuracy can be achieved with the proposed approach.

Elastocaloric properties of thermoplastic polyurethane

Very few studies have explored the elastocaloric effect of elastomers other than natural rubber (NR). The aim of the present article is thus to evaluate the elastocaloric properties of a thermoplastic polyurethane (TPU) in terms of microstructural characteristics and thermoelastic coupling. Calorimetric measurements showed two successive peaks at 240 K and 282 K, attributed to the crystallization and melting of soft segments, respectively. X-ray diffraction indicated that TPU exhibited a fully reversible strain-induced crystallization at room temperature. Thermomechanical experiments performed at different elongations revealed a minimum adiabatic temperature variation of about −8 K after retraction of a sample initially elongated at λ = 5. This is comparable to NR performances. However, for cycles carried out between λ = 1 and λ = 5, tensile stress/elongation curves showed a non-elastic behavior of TPU. A pseudo-elastic response was obtained for cyclic elongation when unloading was incomplete, in our case, when λ was between 3 and 5. The recorded peak-to-peak temperature variation decreased from 4.5 K to 3.3 K when the number of cycles was increased to 5000. Despite the fact that the issue of fatigue resistance for TPU needs to be addressed, this work opens new perspectives for studying the elastocaloric properties of various polyurethanes (whether crosslinked or thermoplastic) as well as other materials with a tendency for strain-induced crystallization, such as polychloroprene, hydrogenated acrylonitrile butadiene rubber, and others.

Dynamic mechanical behavior of different coral sand subjected to impact loading

The mechanical performance of coral sand exhibits significant variation due to the different physical properties of coral sand sampled from individual coral reefs. In this paper, a split Hopkinson pressure bar (SHPB) apparatus is used to conduct impact tests on two types of coral sand to investigate mechanical behavior. Using this approach, compressive stress-strain curves of the one-dimensional strain state are obtained, with strain rates ranging from 460 s−1 to 980 s−1. The results show that the internal porosity of particles is the main influence factor on strain rate dependency of coral sand subjected to impact loading. Various crushing patterns of the two coral sands will result in different strength performance and friction effects, directly creating variations in the dynamic mechanical properties of moist coral sand. Crushing patterns also have a significant influence on yielding stress and the bulk modulus of the pseudo-elastic response but have little effect on the bulk modulus after yielding. In this paper, the varying dynamic mechanical properties are analyzed on typical brittle coral sand by investigating the dominant crushing pattern of the two sand varieties. The conclusions obtained also provide insight into the strain rate dependency of quartz sand.

Finite strain constitutive modeling for shape memory alloys considering transformation-induced plasticity and two-way shape memory effect

This work presents a three-dimensional constitutive model for shape memory alloys considering the TRansformation-Induced Plasticity (TRIP) as well as the Two-Way Shape Memory Effect (TWSME) through a large deformation framework. The presented logarithmic strain based model is able to capture the large strains and rotations exhibited by SMAs under general thermomechanical cycling. By using the martensitic volume fraction, transformation strain, internal stress, and TRIP strain tensors as internal state variables, the model is capable to capture the stress-dependent TRIP generation when SMAs are subjected to a multiaxial stress state, as well as the TWSME for thermomechanically trained SMAs under load-free conditions. A detailed implementation procedure of the proposed model is presented through a user-defined material subroutine within a finite element framework allowing for solving different Boundary Value Problems (BVPs). Comprehensive instruction on calibrating the model parameters as well as the derivation of continuum tangent stiffness matrix are also provided. In the end, the simulated cyclic pseudoelastic and actuation responses by the presented model for a wide range of SMA material systems under both uniaxial and multiaxial stress states are compared against experimental results to validate the proposed modeling capabilities.

Tailoring the Microstructure in Polycrystalline Co–Ni–Ga High-Temperature Shape Memory Alloys by Hot Extrusion

Co–Ni–Ga alloys represent a new class of promising high-temperature shape memory alloys allowing realization of functional components for applications at elevated temperatures. Single crystals show a fully reversible pseudoelastic response at temperatures up to 500 °C. However, for most industrial applications, the application of polycrystalline material is needed. Polycrystalline Co–Ni–Ga alloys suffer from the anisotropic properties inherent to shape memory alloys, i.e., a strong orientation dependence of transformation strains, and therefore, are prone to intergranular fracture. This drawback can be curtailed by using appropriately textured material with a favorable grain-boundary orientation distribution. The current study discusses the impact of a hot-extrusion process on microstructural evolution and functional properties of polycrystalline Co–Ni–Ga high-temperature shape memory alloys paving the way to their robust application.

On the microstructural and functional stability of Fe-Mn-Al-Ni at ambient and elevated temperatures

In the present study the effect of multiple aging treatments ranging from room temperature to 150 °C on the pseudoelastic behavior of previously aged bamboo-structured as well as single crystalline specimens of a Fe-34.8Mn-13.5Al-7.4Ni (at.%) shape memory alloy was investigated. No additional room temperature aging effects occurred in the initially aged bamboo-structured tensile specimen stored for two years, whereas small changes in the pseudoelastic response in single crystalline specimens stored for 96 h at elevated temperatures were seen. It appears that an average precipitate sizes of about 13 nm is sufficient to impede undesired aging effects at least at room temperature.