Pseudoelastic NiTi Shape Memory Alloys Research Articles

Capabilities of self-centering damping device based on pseudoelastic NiTi wires

This experimental study explored a new energy dissipation damper based on pre-stretched pseudoelastic NiTi shape memory alloy (SMA) wires. Due to the pseudoelastic effect and structural parameters, the device combined a self-centering ability and good damping properties. These features were experimentally verified in a prototype composed of four 1.5 mm diameter pseudoelastic NiTi SMA wires. The functional properties of the damping device under various loading frequencies and displacement ranges were studied. The device was pre-loaded by 50 loading cycles before testing to stabilize the functional properties of SMA wires. The dissipation energy grew with the device rod displacement amplitude in the studied frequency range. At all displacement amplitudes, the dissipation energy first rose and falled monotonically with the frequency growth from 0.001 to 0.005 Hz. The results obtained strongly indicate that the proposed pseudoelastic SMA damper can be effectively used in structures undergoing cyclic loading in terms of post-yield strength, energy dissipation, and re-centering capability.

Concentration of transformation-induced plasticity in pseudoelastic NiTi shape memory alloys: Insight from austenite–martensite interface instability

Transformation-induced plasticity (TRIP) in pseudoelastic polycrystalline NiTi shape memory alloys (SMAs) is investigated through tensile tests combined with in-situ infrared and digital image correlation (DIC) observations, and a new TRIP concentration phenomenon is revealed. To this end, an instability evolution model for solid–solid phase transformation is established to explain the multi-scale mechanisms of TRIP in SMAs. It is shown that, during the stress-induced transformation, the local stress at the grain scale evolves discontinuously inside austenite–martensite mixtures, presenting the maximum magnitude at the austenite side of the transformation interfaces. This local stress field triggers TRIP although the macroscopic stress still evolves “smoothly” with a relatively low magnitude. As the temperature increases, a further plasticity in austenite is required to lower the discontinuity at the austenite–martensite interfaces. Due to the accumulation of the heat generated in transformed regions, the temperature at the moving transformation interface continuously increases till reaching the maximum at the end of the transformation. Therefore, TRIP is concentrated at the final transformation band fronts, manifesting as concentration peaks in the residual strain map. The concentration of TRIP also shows a strong rate/frequency dependence: when the loading rate increases, TRIP accumulates to a higher average density throughout the transformation region while the concentration is significantly relieved.

Deformation mechanisms of a NiTi alloy under high cycle fatigue

In this paper, high-cycle fatigue mechanisms of a pseudoelastic NiTi shape memory alloy (SMA) under different mean strains are investigated. The uni-axial cyclic tensile tests with small amplitude of 0.25% are operated at three different states, i.e. austenite/elastic region, austenite and martensite coexisted region and martensite/elastic region with a pre-strain. Mechanical results indicate that the cyclic deformation strain is mainly in the inhomogeneous deformation band when the sample cycling in the B2 + B19′ phase coexist status. Cyclic deformation in the B2 + B19’ phase showed cyclic hardening effect. With increasing of the mean strain, the saturation stress is decreased, and saturation rate is reduced. The mean strain effect on the high-cycle fatigue life of NiTi is attributed to the inhomogeneous transformation of martensite. High resolution electron microscopy (HREM) observations and atomic simulations showed that partial dislocations with the Burgers vectors of 1/2<110> are induced on (001)M twin planes, the interaction of dislocations and edge dislocations in martensite results in cyclic hardening in martensite. While cyclic softening in martensite at the upper plateau of stress–strain (S–S) curve is attributed to the grain reorientation/rotation phenomenon. It suggests that newly generated stress-induced martensite at the interface of martensite and austenite could increase the high-cycle fatigue of NiTi SMA.

Thermo-mechanically coupled deformation of pseudoelastic NiTi SMA helical spring

Based on the cyclic tension-unloading tests with various displacements (40, 60 and 80 mm) and at different loading rates (0.3 mm/s, 1 mm/s and 5 mm/s), the thermo-mechanically coupled deformation and temperature evolution of pseudoelastic NiTi shape memory alloy (SMA) helical spring are investigated. Experimental results reveal that the phase transition between austenite (A) and martensite (M) phases undergoes a two-step process involving an intermediate phase, that is, rhombohedral (R) phase; the force-displacement responses of the spring depend on the loading rate strongly, which originates from Clausius-Clapeyron relation between the critical stresses of phase transitions (including R and M transformations), and the competition between the internal heat generation and heat exchange. Then, a three-dimensional phenomenological constitutive model is developed by addressing three possible phase transition processes (that is, A→R→M, A→mixed A + R→M and A→M) and thermo-mechanical coupling effect. Generalized thermodynamic forces for the phase transitions and the evolution equation of temperature are derived based on the second and first laws of thermodynamics, respectively. To accurately describe the loading amplitude-dependent dissipation and hardening behavior of MT, new dissipation and hardening laws are proposed. Furthermore, a new analytical model of helical spring is developed by simultaneously considering the torsion and bending deformation modes, curvature effect and the coupling effect between the deformation and temperature evolution. Finally, the developed model is validated by comparing the simulated results with the experimental data, and the influences of geometrical parameters and ambient temperature on the thermo-mechanical responses of the spring are also predicted and discussed.

Experimental analysis of corrugated core sandwich panel with smart composite face-sheets under high-velocity impact

The present research work investigated on the behavior of corrugated core composite sandwich panel (CCCS) with composite face-sheets reinforced by pseudo-elastic NiTi shape memory alloy (SMA) wires subjected to high-velocity impact experimentally. The number of SMA wires, their location, and their pre-strain percentage are the parameters studied in this investigation. The sandwich specimens were made by woven glass fibers/epoxy composite face-sheets strengthened by the SMA wires and Aluminum corrugated core and prepared using the wet hand lay-up method. Their behavior is evaluated under high-velocity impact test by a gas gun impactor testing machine. According to the results obtained, a significant change in the amount of energy absorption of the specimens containing SMA wires is observed. By increasing the number of the SMA wires, the energy absorption by the specimen is increased, but by pre-straining the SMA wires, the effect in energy absorption is significant. Also, when the projectile hits the SMA wire in CCCS, better properties in energy absorption can be observed. The results show that the energy absorption increased from 117.247 J for the specimen without the SMA wire to 128.749 J for the six SMA wires specimen. On the other hand, by applying the pre-strain to the SMA wires, the residual stresses in the specimens and hence the energy absorption in the specimen with six SMA wires with 3% pre-strain increased to 142.57 J.

Cyclic Behavior of Masonry Shear Walls Retrofitted with Engineered Cementitious Composite and Pseudoelastic Shape Memory Alloy.

The behavior of masonry shear walls reinforced with pseudoelastic Ni–Ti shape memory alloy (SMA) strips and engineered cementitious composite (ECC) sheets is the main focus of this paper. The walls were subjected to quasi-static cyclic in-plane loads and evaluated by using Abaqus. Eight cases of strengthening of masonry walls were investigated. Three masonry walls were strengthened with different thicknesses of ECC sheets using epoxy as adhesion, three walls were reinforced with different thicknesses of Ni–Ti strips in a cross form bonded to both the surfaces of the wall, and one was utilized as a reference wall without any reinforcing element. The final concept was a hybrid of strengthening methods in which the Ni–Ti strips were embedded in ECC sheets. The effect of mesh density on analytical outcomes is also discussed. A parameterized analysis was conducted to examine the influence of various variables such as the thickness of the Ni–Ti strips and that of ECC sheets. The results show that using the ECC sheet in combination with pseudoelastic Ni–Ti SMA strips enhances the energy absorption capacity and stiffness of masonry walls, demonstrating its efficacy as a reinforcing method.

A multi-physics, multi-scale and finite strain crystal plasticity-based model for pseudoelastic NiTi shape memory alloy

A crystal plasticity-based constitutive model is developed to describe the thermomechanical behavior of pseudoelastic NiTi single crystal. The model includes, for the first time in the literature, all inelastic mechanisms influencing the fatigue behavior of NiTi SMAs in a finite strain framework: martensite transformation, deformation slip in austenite at high-temperature, deformation twinning in martensite at large strain, transformation-induced plasticity (TRIP) as well as thermomechanical coupling. Furthermore, new internal variables and evolution laws are introduced in the monocycle model (referred as basic model in the remainder of the paper) to reproduce the main features of anisotropic cyclic deformation of pseudoelastic NiTi single crystal. The numerical implementation of the constitutive model is performed in the CAST3M (2019) finite element software through a user-defined UMAT subroutine. A series of simulations were performed to verify the basic and generalized cyclic models under various conditions. Moreover, the robustness of the model is attested by comparing the simulation results with the reported data of the pseudoelastic NiTi single crystal. The effect of crystallographic orientation and anisotropic cyclic deformation behavior are revealed and shown to be quantitatively in a good agreement with experimental results. Finally, the evolution of dislocation density and stored energy is discussed from the perspective of fatigue analysis of SMAs.

Novel insight into the strain-life fatigue properties of pseudoelastic NiTi shape memory alloys

The whole low-to-high cycle fatigue behavior of a pseudoelastic NiTi SMA was analyzed. Global and local strains measurements were carried out by an extensometer and digital image correlation. Significant differences were observed at the two scales, due to the localized nature of stress-induced transformations. Local mechanisms play a crucial role on fatigue damage and have a direct effect on the Z-shaped strain-life curves observed in previous works. The effective nature of this unusual behavior was revealed by systematic analysis of local strain. Finally, strain decomposition was made and data were analyzed by a recent modified Coffin–Manson approach.

Effects of Temperature on Fatigue Crack Propagation in Pseudoelastic NiTi Shape Memory Alloys

The effects of temperature on fatigue crack propagation in a pseudoelastic NiTi shape memory alloy (SMA) were analyzed. Single edge crack specimens were used and near crack tip displacements were captured by in situ digital image correlation (DIC). The effective stress intensity range was estimated from displacement data by a fitting procedure involving the William’s solution. Stress intensity range was also estimated using a recent analytical model that accounts for the complex thermo-mechanical response of SMAs. In addition, comparisons with the linear elastic fracture mechanics (LEFM) solution were made. Results revealed an important role of temperature on crack propagation rate, that is, the higher the temperature the longer the fatigue life. However, it was demonstrated that these effects are attributed to the marked influence of temperature on near crack tip fields and, consequently, on the effective stress intensity range. This trend is correctly captured by the DIC method as well as by the analytical model. On the contrary, LEFM does not consider the effects of temperature and, consequently an apparent change in the material properties is observed. Therefore, a novel approach is proposed to analyze crack propagation in SMAs, where both stress and temperature are considered as significant loading parameters.

Analyses of orientation dependent nanoindentation response of pseudoelastic NiTi alloy using a crystal plasticity model

In the present study, a crystal plasticity finite element model has been employed to mechanically characterize small scale deformation in pseudoelastic NiTi shape memory alloy (SMA) single crystal during nanoindentation. The model includes various inelastic mechanisms of deformation such as austenite to martensite transformation, dislocation glide in the austenite phase and twinning in the martensite phase. Simulations were conducted for nanoindentation of single crystals of NiTi along various crystallographic orientations and temperatures. The simulation results have been validated by comparing against experimental observations reported in the literature. The load-depth curves for various crystallographic orientations are consistent with the experimental results. The total martensite volume fraction is predicted and compared among the three orientations. Furthermore, in order to study the effect of temperature on deformation characteristics during nanoindentation of NiTi SMA, simulations are carried at two different temperatures. The results show the depth recovery ratio and the slope of the load-depth curve increase with the increase of temperature. The simulation results are satisfactorily consistent with the experimental results. The martensite volume fraction evolution and surface profile of the indentation surface are also analyzed.

Modeling of finite deformation of pseudoelastic NiTi shape memory alloy considering various inelasticity mechanisms

A crystal plasticity finite element (CPFE) model is developed for pseudoelastic NiTi shape memory alloy (SMA). The model includes various inelastic mechanisms of deformation such as martensite transformation, dislocation glide in austenite phase and twinning in the martensite phase. This model is based on the finite deformation theory and predicts phase transformation, stress field and residual strain due to slip and twinning. Following the work of Anand and Gurtin (2003), the Helmholtz free energy is derived and further the thermodynamic driving forces for slip, twin and phase transformation are obtained by using Clausius-Duhem inequality. The developed constitutive model has been implemented within a user material subroutine interface VUMAT in ABAQUS™/Explicit. The activation of different inelasticity mechanisms at different temperatures and strains are first verified by performing various simulations of uniaxial tensile deformation. Then the model is calibrated and validated against the experimental stress-strain response of NiTi single crystal as reported in the literature. The single crystal constitutive model is extended for modeling the response of a polycrystal with both initial random and textured crystallographic orientations using Taylor scale transition technique. A series of simulations are performed on polycrystalline representative volume element (RVE) at various temperatures and strains. The effect of temperature and imposed strain on phase transformation and residual strain is investigated systematically. The residual strain increases due to slip in austenite phase and twinning in the martensite phases that are activated as temperature and imposed strain increase respectively. Further the uniaxial loading conditions are extended to multiaxial loading and the results are compared against the experimental data.

High fatigue life and cooling efficiency of NiTi shape memory alloy under cyclic compression

We investigate the cyclic compressive fatigue behavior of pseudoelastic polycrystalline NiTi shape memory alloy (SMA) under maximum applied stresses of 800, 950 and 1200 MPa. A fatigue life of over 20 million cycles under 800 MPa is recorded with an average coefficient of performance (COP) of 17.6. Rock-like splitting and chipping through nucleation and slow stable growth of compression-parallel cracks driven by very small stress intensity factor are the major fatigue failure modes of the material. The high fatigue life and cooling efficiency of NiTi SMA under cyclic compressive phase transformation provide a solid mechanical base for its application in solid-state cooling technology.

Virtual Extensometer Analysis of Martensite Band Nucleation, Growth, and Strain Softening in Pseudoelastic NiTi Subjected to Different Load Cases.

Pseudoelastic NiTi shape memory alloys exhibit different stress–strain curves and modes of deformation in tension vs. compression. We have recently shown that under a combination of compression and shear, heterogeneous deformation can occur. In the present study, we use digital image correlation to systematically analyze how characteristic features of the nominally uniaxial engineering stress–strain curves (particularly the martensite nucleation peak and the plateau length) are affected by extensometer parameters in tension, compression, and the novel load case of shear-compression. By post-experimental analysis of full surface strain field data, the effect of the placement of various virtual extensometers at different locations (with respect to the nucleation site of martensite bands or inhomogeneously deforming regions) and with different gauge lengths is documented. By positioning an extensometer directly on the region corresponding to the nucleating martensite band, we, for the first time, directly record the strain-softening nature of the material—a specific softening behavior that is, for instance, important for the modeling community. Our results show that the stress–strain curves, which are often used as a basis for constitutive modeling, are affected considerably by the choice of extensometer, particularly under tensile loading, that leads to a distinct mode of localized deformation/transformation. Under compression-shear loading, inhomogeneous deformation (without lateral growth of martensite bands) is observed. The effects of extensometer gauge length are thus less pronounced than in tension, yet systematic—they are rationalized by considering the relative impact of differently deforming regions.

Mechanical Behaviour and Phase Transition Mechanisms of a Shape Memory Alloy by Means of a Novel Analytical Model

Abstract The aim of the present paper is to examine both the fatigue behaviour and the phase transition mechanisms of an equiatomic pseudo-elastic NiTi Shape Memory Alloy through cyclic tests (up to 100 loading cycles). More precisely, miniaturised dog-bone specimens are tested by using a customised testing machine and the contents of both austenite and martensite phase are experimentally measured by means of X-Ray diffraction (XRD) analyses. On the basis of such experimental results in terms of martensite content, an analytical model is here formulated to correlate the stress-strain relationship to the phase transition mechanisms. Finally, a validation of the present model by means of experimental data pertaining the stress-strain relationship is performed.

Fatigue analysis of a near-equiatomic pseudo-elastic NiTi SMA

In the present paper, the fatigue behaviour of a near-equiatomic pseudo-elastic NiTi SMA (Shape Memory Alloy) is experimentally examined by means of tensile cyclic loading tests. In situ X-ray Diffraction (XRD) analyses are also performed in order to measure the contents of both austenite and martensite phase during testing. The experimental campaign highlights a stress–strain diagram defined through five different stress–strain functions. Such a diagram is here proposed to be analytically described by a model in two steps: (i) firstly, the domain for each of the above functions is determined by exploiting the experimental measurements of both austenite and martensite phase content; (ii) then, the equations defining each function are deduced by assuming that the fatigued material behaves as a single-spring system or a two-spring system (working in parallel or in series).

On the competition between the stress-induced formation of martensite and dislocation plasticity during crack propagation in pseudoelastic NiTi shape memory alloys

Abstract

List of Abstracts

. Production efficiency as well as the reduction of energy consumption and waste are the main drivers for the improvement of large-scale production in chemical industry. In addition, the demand for new processes – these days especially related to the chemical energy conversion as basis for the Energiewende – are additional triggers for innovation. Catalysis, well recognized by the public e.g. in form of the 3-way automobile catalyst, plays an important role for about 90% of all processes in chemical industry. The development of new and improved catalysts is a challenge for chemists – as is the search for new structural materials for mechanical engineers. Due to the often-necessary noble and thus expensive metals (e.g. palladium) in many catalysts, structural and chemical function have been developed separately in the two communities. Recent progress in catalytically active materials allows replacing expensive noble metals in selected catalytic processes by significant cheaper elements [1]. This offers the new perspective to combine structural and chemical functionality by innovative materials. Within this area, intermetallic compounds with their peculiar combination of crystal and electronic structure [2], represent an interesting class of materials to address this vision as will be shown in the contribution.

Experimental investigation of the pseudoelastic behaviour of NiTi wires under strain- and stress-controlled cyclic tensile loadings

Pseudoelastic NiTi based shape memory alloys (SMA) wires have a good potential in civil engineering applications and often used as dampers in anti-seismic or cable structures when a rate varied cyclic loadings is applied. In accordance with these applications, an experimental characterization of NiTi wires under strain- and stress-controlled cyclic loadings at various strain rates is presented. Based on these loadings, it is shown that the stress–strain response of the material evolves up to a complete stabilization with the increase of the number of cycles. This evolution is accompanied with the degeneration of pseudoelasticity and transformation ratcheting in the case of strain- and stress-controlled mode, respectively. Also, it was found that the cyclic deformation of pseudoelastic NiTi SMA is significantly rate dependent.

Three Dimensional Crystal Plasticity Finite Element Simulation of Notched Tensile Samples of Pseudoelastic NiTi Shape Memory Alloys

We present a crystal plasticity finite element (CPFE) model for pseudoelastic NiTi shape memory alloy (SMA). Simulations of uniaxial tensile deformation of notched single crystal specimens of NiTi SMA at isothermal condition have been carried out. Predicted martensitic transformation is compared qualitatively against the one reported in literature. Constitutive model has been implemented within a user material subroutine VUMAT in ABAQUS/Explicit. Two different crystallographic orientation samples with tensile axes along the [100] and [111] are studied. Martensitic transformation in the surrounding region of crack tip in notched samples are determined in simulation and the results are compared with the existing experimental data from literature. For [100] notch sample, martensitic transformation starts above and below the notch tip and for [111] sample, the transformation starts ahead of notch tip. The simulation results show orientation dependence of martensitic transformation and are qualitatively in good agreement with the experimental results.

Experimental validation of a one-dimensional model for monolithic shape memory alloys with multiple pseudoelastic plateaus

A one-dimensional approach to simulate the mechanical properties of multifunctional pseudoelastic NiTi shape memory alloy has been developed. A novel laser processing technique was applied to a NiTi wire to locally alter the pseudoelastic properties, giving the wire different regions with unique material properties. An innovative analytical expression was developed and validated experimentally. The simulated material responses closely predicted the experimentally measured laser-processed shape memory wires.