Pseudoelastic Shape Memory Alloys Research Articles

Indentation-induced martensitic transformation in SMAs: Insights from phase-field simulations

Direct experimental characterization of indentation-induced martensitic microstructures in pseudoelastic shape memory alloys (SMAs) is not possible, and thus there is a lack of evidence and understanding regarding the microstructure pattern and related features. To fill this gap, in this work we employ the phase-field method to provide a detailed and systematic analysis of martensitic phase transformation during nanoindentation. A recently-developed finite-element-based computational model is used for this purpose, and a campaign of large-scale 3D simulations is carried out. First, the orientation-dependent indentation response in CuAlNi (a widely studied SMA) is examined. A detailed investigation of the predicted microstructures reveals several interesting features, some of them are consistent with theoretical predictions and some can be (to some extent) justified by experiments other than micro/nanoindentation. The results also highlight the key role of finite-deformation effects and elastic anisotropy of the phases on the model predictions. Next, a detailed study of indentation-induced martensitic transformation in NiTiPd (a potential low-hysteresis SMA) with varying Pd content is carried out. In terms of hysteresis, the results demonstrate the prevailing effect of the transformation volume change over phase compatibility in the conditions imposed by nanoindentation and emphasize on the dominant role of the interfacial energy at small scales. Results of such scope have not been reported so far.

Influence of phase transformation on stress wave propagation in thin-walled tubes

Phase transformation can change the properties of a material and significantly affect the stress wave propagation features. This study presents an analytical model of coupled stress wave propagation in pseudo-elastic shape memory alloy thin-walled tubes subjected to combined longitudinal and torsional impact loading. An incremental constitutive model considering the effect of the hydrostatic pressure and deviatoric stress was employed to describe the mechanical behavior of SMA. The phase transformation ellipses shift along the σ-axis in the σ – τ plane due to the asymmetry of tension and compression induced by the hydrostatic pressure. The generalized characteristic theory was used to obtain the characteristic wave speed,simple wave solution, and the differential equation determining the stress path. The numerical results showed that the wave structure in the thin-walled tube is affected by the initial stress state and the final loading state. For the initially static and unstressed thin-walled tube, there is a special region on the σ–τ plane. When the combined longitudinal and torsion impact load lies in this region, an elastic compression wave, elastic shear wave and coupled phase transformation fast waves appear in the thin-walled tube in turn. The abnormal phenomenon is not observed in the conventional elastoplastic tubes.

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.

The upper bound of low cycle fatigue life of pseudoelastic polycrystalline NiTi shape memory alloys

The coupling between transformation-induced plasticity (TRIP) and low cycle fatigue of pseudoelastic shape memory alloys (SMAs) is investigated and an upper bound of low cycle fatigue life is found. Through combined in-situ infrared and digital image correlation observations on NiTi sheets, we reveal that the low cycle fatigue of SMAs is governed by the transformation band fronts with the highest temperature where TRIP is maximized. The local state at the band front is in fact determined by a coupling effect of the applied loads and thermal conditions, the former driving the transformation with heat release while the latter determining the heat transfer efficiency. This coupling leads the fatigue lives to distribute along a series of isotherms. As the coupling tends to be infinitesimal, local TRIP is limited to a minimum level, thus resulting in an upper bound of low cycle fatigue life. This agrees well with the experiments.

Flexural toughness and compressive stress–strain behaviour of pseudoelastic shape memory alloy fibre reinforced concrete

The results of experimental analyses for the full-range compressive and flexural response of concrete reinforced with pseudoelastic shape memory alloy fibres (PSMAFs) are discussed in this paper. PSMAFs were manufactured with two types of end hooks and were used in four different fibre contents ranging between 0.5% and 1.25%, with an interval of 0.25% by volume. Cracking patterns and full-depth crack width profiles of PSMAFs reinforced concrete (PMSA-FRC) were also generated by the 2D digital image correlation (DIC) technique. The results obtained for PMSA-FRC are compared with those of steel fibre reinforced concrete (SFRC).The flexural responses of PSMA-FRC and SFRC were significantly different at fifferent stages of flexural behaviour. Compared to SFRC, PSMA-FRC showed lower flexural strength but considerably higher flexural toughness and deflection capacity. PSMA-FRC also exhibited slightly lower compressive strength (5-8%) and modulus of elasticity (4–10%) than SFRC. However, no significant difference was noticed between PSMA-FRC and SFRC regarding the overall shape of the compressive stress–strain curve, compressive toughness and failure mode. It is also shown that PSMAFs type 2, the fibre with 45° end hook, is more efficient in distributing stress over a larger cementitious matrix area. DIC analyses displayed better control over cracking, improved multiple-cracking performance, and tighter crack profiles for PSMAFs type 2 at all four fibre contents tested in this study.

Experimental investigation of the pseudoelastic behavior on zirconium modified Cu–Al–Be shape memory alloys for seismic applications

This paper examines the effect of 0.05–0.3 wt.% of zirconium-doping on microstructure, transformation temperatures, tensile properties, and pseudoelastic behavior of the parent -phase Cu87.93–Al11.5–Be0.57 shape memory alloys (SMAs). Results reveal that alloying zirconium in the evaluated SMA samples exhibits an excellent grain refinement up to 0.15 wt.%. Further, higher additions of Zr ⩾ 0.2 wt.% lowers the grain refinement efficiency due to precipitates agglomeration. Larger the size and volume fraction of Al3Zr precipitates led to higher transformation temperatures. Tensile properties were improved with Zr-doping, resulting enhancements in the maximum tensile strength and ductility with the addition of 0.15 wt.% Zr. The alloy with 0.05 wt.% of Zr-dope showed a good pseudoelastic strain recovery of deformation strain and then lowered by retaining large residual strain, indicating deterioration in the pseudoelasticity of SMAs.

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.

Effect of Ultrasound on the Pseudoelasticity of Shape Memory Alloys

Effect of Ultrasound on the Pseudoelasticity of Shape Memory Alloys

Study of cyclic response of SMA based damping device

In this paper, there is study the main properties of damping device, that is based on shape memory alloy pre-stretched wires made of pseudoelastic NiTi alloy and compressed springs which provide the wires stretching. Due to the structural parameters and pseudoelastic effect the device provides the 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. There was studied a damping properties of device under cyclic loading in the wide range of frequencies from 0.001 to 2 Hz and displacement amplitude in the range of 3–9 mm. To stabilize the functional properties of SMA wires the device had been loaded for 50 cycles before testing.

Methodology and some results of studying the influence of frequency on functional properties of pseudoelastic SMA

The mechanical loading frequency affects the functional properties of shape memory alloys (SMA). Thus, it is necessary to study the effect of frequency in order to use successfully these materials in real structures. Based on the pseudoelastic cyclic behavior, the experimental methodology that allows testing of NiTi wires in stress controlled mode is proposed. Cyclic tensile tests are carried out using universal testing machine STM-100 at room temperature with loading frequencies of 0.1 Hz and 10 Hz. The functional dependencies are determined based on the experimentally obtained hysteresis loops. These functional dependencies comprise dissipated energy and damping factor. It iis found that the increase of loading frequency results in the worsening of functional properties, namely, to the decrease of dissipated energy and damping factor. This is caused by the fact that the regions of austenitic and martensitic transformation under the high loading frequency are absent. That is, the transformation of austenite into martensite does not occur, that stands for the pseudoelasticity effect. Nevertheless, it should be noted that the increase of loading frequency in 100 times augments the lifetime of pseudoelastic wire made of NiTi alloy roughly by 30%. It is determined that the increase of loading frequency results in the decrease of maximum strain in two times in the first loading cycle, and practically in 5 times after 200 cycles of loading.

Adjustment of the scan track spacing and linear input energy to fabricate dense, pseudoelastic Nitinol shape memory alloy parts by selective laser melting

Nitinol is a well-known shape memory alloy (SMA) which is widely used due to its unique properties such as shape memory effect and pseudoelasticity. However, challenges fabricating Nitinol parts have limited the use of this alloy. Nowadays, additive manufacturing methods, specifically selective laser melting (SLM), are being used as an alternative to conventional methods for fabricating Nitinol specimens. Achieving a dense structure and controlling the transformation temperatures in such products have been among the most important challenges for several research groups. In the present study, fabrication of dense Nitinol parts by SLM together with control of their transformation temperatures is investigated with the main purpose of achieving pseudoelastic products at room temperature. For this purpose, the effect of process parameters on density, transformation temperatures, microstructure, hardness, and shape memory response are studied. The influence of process parameters on transformation temperatures varies depending on the amount of power so that the effect of scan tracks spacing for high powers is more pronounced than that for low powers. The hardness and compressive strength of the parts are also affected by the process parameters. Accordingly, optimal parameters are found to fabricate dense pseudoelastic parts with the ability of strain recovery at ambient temperature.

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.

A cyclic integrated microstructural-mechanical model for a shape memory alloy

Shape memory alloys are able to recover the initial shape even after a large deformation. The memory property is due to the ability to change the microstructure under a variation of the stress state or the temperature. The stress–strain curve is characterized by the presence of a stage where the slope of the curve sharply decreases and the initial microstructure transforms into a new one. In this work, an integrated microstructural-mechanical model is proposed for the equiatomic NiTi shape memory alloy, that is suitable to predict the cyclic behaviour of a pseudoelastic shape memory alloy starting from the microstructure composition.

Crack recovery and re-centring performance of cementitious composites with pseudoelastic shape memory alloy fibres

Newly developed hooked-end pseudoelastic shape memory alloy fibres (PSMAF) provide re-centring and crack-closing behaviour in cementitious composites. Relatively large PSMAF-reinforced specimens are tested under cyclic flexural loading at small and large deflection amplitudes to examine these capabilities realistically. Crack propagation, maximum crack width at each cycle, crack-closing ratio, and full-depth crack width are quantified through 2D digital image correlation (DIC) technique for composites with 0.5% to 1.25% PSMAF. The re-centring ratios are established for the developed composites, and their flexural performances are evaluated based on various parameters, including cyclic modulus, toughness, and ductility. Furthermore, X-ray micro-computed tomography and statistical analysis of spatial point patterns are employed to examine fibre dispersion in the cracked sections. Results indicate that PSMAF-reinforced composites with 0.75% and 1% fibre content exhibit considerable re-centring and crack-closing capabilities with minimal fibre clumping.

Application and potential of shape memory alloys for dowel-type connections in timber structures

ABSTRACT The present study aims at identifying and analyzing challenges and prospects related to the use of pseudoelastic shape memory alloys for well-established double shear connections in timber engineering. The unique behavior of shape memory alloys will allow to bear large reversible deformations without loss of cohesion. Thereby, the main objective of the present work is to study the behavior of a connection that is able to stay completely reversible even upon very large deformations. In this context, the stiffness of the connection as well as functional and structural integrity and damping behavior have been improved by stepwise adaption of the detailing. Proof of concept is provided, and current limitations are discussed, based on the application of bolts made from a commercially available shape memory alloy.

Quantification of Shape Memory Alloy Damping Capabilities Through the Prediction of Inherent Behavioral Aspects

In this work the time response of pseudoelastic Shape Memory Alloy (SMA) wires is numerically simulated. In particular, the effect of their operation under partial phase transformation is investigated and quantified. Additionally, the effect of the thermomechanical coupling under cyclic operation is evaluated both under adiabatic and natural convection conditions. To this end, proper finite element models are generated considering a low-frequency harmonic sinusoidal excitation. The effect of the partial transformation and thermomechanical coupling on the operation of the SMA is highlighted by comparison with respective results acquired by finite element models which neglect the modified hardening function that accounts for the partial loops. The results suggest that the latent heat produced during forward transformation highly affects the energy dissipation potential of SMAs. The hardening behavior also affects the transformation evolution and therefore impacts the amount of heat generation/absorption. Although both phenomena, when accounted for, result in the prediction of an altered hysteresis area and consequently different dissipation capabilities, the scope of the paper is to highlight their importance on the calculated values of dissipated energy and loss factor. These quantities are of particular interest since they constitute crucial design parameters for the development of smart dampers employing SMA materials.