Superelastic NiTi Shape Memory Alloy Research Articles

Punching Shear Capacity of RC Flat Slabs with Steel and Superelastic Ni-Ti Shape Memory Shear Reinforcement under Repeated Loads

Flat slab systems are a popular choice among engineers because of their numerous advantages, such as reduced building height, flexibility in spatial layout by eliminating the need for beams, and cost savings. However, Brittle slab failure under punching shear presents a considerable challenge. The primary objective of this study was to examine how flat slabs respond to PS and conduct an experiment using steel and superelastic Ni-Ti shape memory alloy (SMA) bars as shear reinforcement to assess their effectiveness in improving the PS capacity of flat slab-column connections under vertical Repeated load. Nine half-scale samples were tested, consisting of 1100 × 1100 × 120 mm slabs with a protruding column from one side of 150 × 150 mm and 400 mm height. The factors considered in this research were the type of materials, spacing between stirrups, stabilization method, and inner RFT. After that, finite element analysis by using ABAQUS 6.14 was developed to simulate all the tested specimens. The finite element analysis showed identical results compared to the experimental ones concerning energy absorption, load capacity, absorbed energy, and model stiffness. According to the experimental and finite element analysis findings, the flat slab reinforced with inclined stirrups exhibited a significant increase in punching strength, ranging from 28.8% to 51.3% compared with the control specimen. The stiffness increases by 8.5% when the distance between stirrups is reduced to 0.5d, and inclined-shaped memory alloy stirrups are employed.

Effects of residual stress on the isothermal tensile behavior of nanocrystalline superelastic NiTi shape memory alloy

The residual stress greatly affects the mechanical behavior of a material. In this work, the effect of residual stress on the isothermal tensile behavior of a NiTi shape memory alloy is studied. The focused ion beam and digital image correlation are combined to measure the two-dimensional residual stress in nanocrystalline NiTi plates processed with prestrain laser shock peening. A four-point bending experiment verified the accuracy of this measurement method. The FIB-DIC method is an attractive tool for measuring the two-dimensional residual stress in phase transition nanocrystalline materials. The internal residual stress significantly decreases the phase transition stress, and the mechanism is studied via finite element and theoretical analyses. This work implies that the mechanical behavior of NiTi shape memory alloys can be tailored via residual stress engineering.

Constitutive modeling of functional fatigue with tension–compression asymmetry for superelastic NiTi shape memory alloy

Under cyclic loads, superelastic shape memory alloys (SMAs) exhibit stress–strain responses featured by functional fatigue, i.e., degradation of superelasticity and accumulation of irrecoverable deformation as cycling number increases, together with an asymmetry between tensile and compressive responses. Comprehensive understanding and modeling of these material complexities are crucial for the design and analysis of various superelastic SMA structures in practical applications. This work has developed a novel constitutive model based on irreversible thermodynamics to account for functional fatigue with tension–compression asymmetry. A potential function, defined as a weighted sum of two potentials that are calibrated against the tensile and compressive responses respectively, is employed to generate the asymmetric responses, and functional fatigue is represented by degradation of superelastic properties and growth of plastic strain as martensitic transformation accumulates. The model is adopted in numerical simulations for superelastic SMA tubes under cyclic lateral compression, which is experimentally investigated as a model problem. The agreement between simulations and experiments shows the validity and effectiveness of this constitutive modeling. Through additional finite element simulations incorporating this model, the effects of tension–compression asymmetry under cycling and diameter-to-thickness ratio of the tubular geometry upon mechanical responses of laterally compressed SMA tubes are also unveiled.

Cyclic tensile response and crack closure performance of novel superelastic Ni-Ti SMA cable grid-reinforced engineered cementitious composites

To further improve the tensile and self-healing properties of engineered cementitious composites (ECCs) for their special uses such as harsh environments, plastic hinge regions, and old members strengthening, an innovative high-performance superelastic Ni-Ti shape memory alloy (SMA) cable grid-reinforced ECC (SMACG-ECC) was first proposed and tested in this study. The effects of water-binder ratio, cable grid material, and reinforcement ratio on the cyclic tensile properties and crack closure performance of the proposed SMACG-ECC were systematically examined. Test results revealed that the tensile strength, initial crack strength, ultimate tensile strain, dense crack, crack closure, and self-healing properties of the traditional ECC were obviously improved by being reinforced with the SMACG. Compared with the ordinary steel cable grid-reinforced ECC (SCG-ECC), the proposed SMACG-ECC exhibited approximately 50 % lower first crack strength, similar tensile strength and deformation ductility, but a 53.7 %-75.1 % greater energy dissipation, 75.8 %-80.2 % lower residual strain, and 80.4 %-83.4 % smaller maximum residual crack width. The tensile properties, cracking performance, and self-healing ability improved with the incorporation of SMACG as well as the increase in reinforcement ratio. Notably, most specimens showed satisfactory composite operation with no cable ends slipping during the entire cyclic loading process, with a maximum tensile strain amplitude of 8 %.

Recoverable and plastic strains generated by forward and reverse martensitic transformations under external stress in NiTi SMA wires

Although superelastic NiTi shape memory alloy wire displays very high resistance to plastic deformation in austenite and martensite phases, incremental plastic strains are recorded whenever the cubic to monoclinic martensitic transformation (MT) proceeds under external stress leading to functional fatigue degradation. Therefore, special closed-loop thermomechanical loading tests were performed to shed light on the mechanism by which the incremental plastic strain are generated. These tests revealed that both forward and reverse MTs occurring above certain stress thresholds generate plastic strains specific for the [temperature, stress] conditions under which the MTs occurred. While the forward MT upon cooling does not produce plastic strain or permanent lattice defects up to 500 MPa stress, the reverse MT upon heating starts to generate them from 100 MPa. While plastic strain generated by the forward MT merely elongates the wire, plastic strain generated by the reverse MT also reduces the recoverable strain. Since the reverse MT upon heating generates plastic strains at lower external stresses than the forward MT upon cooling, it is largely responsible for cyclic instability of NiTi actuators. The characteristic thresholds and magnitudes of plastic strains generated by the forward and reverse MTs define the functional fatigue limits for specific NiTi wires.

Characterization of the temperature-dependent superelastic and elastocaloric effects of a NiTi tube under compression at 293–330 K

Comprehensive characterizations of the superelastic and elastocaloric effects of NiTi and NiTi-based shape memory alloys (SMA) in the operation temperature region are highly desirable for using them in elastocaloric coolers with a large temperature lift. In this article, we report the superelastic and elastocaloric effects of a commercially available superelastic polycrystalline NiTi SMA tube with an outer diameter of 5 mm and a wall thickness of 1 mm between 293 and 330 K. The NiTi tube sample was subjected to a training of 250 cycles to stabilize its superelastic and elastocaloric effects. We observed that temperature dependencies existed for both superelastic and elastocaloric effects of the NiTi tube, and stress–strain curves differed much between isothermal and adiabatic loading conditions. The largest temperature rise and temperature drop measured at 293 K under an applied strain of 3.66% and a strain rate of 0.1 s−1 during loading and unloading were 21 and 11 K, respectively. The loading conditions (loading function and holding time) also impacted the superelastic effect of the NiTi tube. We identified two major reasons for the irreversibility of the adiabatic temperature change: the hysteresis heat dissipation and the temporary residual strain after unloading, and they affected the cooling performance of the elastocaloric cooler in different ways. We investigated the dependencies of the superelastic and elastocaloric effects on the maximum applied strain and the temperature distribution on the NiTi tube during loading and unloading. The results are beneficial to the modeling of elastocaloric coolers with large temperature lifts.

Design and Experimental Evaluation of Passive Ankle-Foot Orthoses Hinges Using Superelastic NiTi Shape Memory Alloy

Design and Experimental Evaluation of Passive Ankle-Foot Orthoses Hinges Using Superelastic NiTi Shape Memory Alloy

Experimental investigation of mechanical properties of NiTi superelastic shape memory alloy cables

As a self-centering element in a recoverable functional structure, shape memory alloys (SMAs) usually require a large diameter to provide sufficient bearing capacity, and pre-strain is applied to obtain good self-centering performance and initial stiffness. This study aims to explore the mechanical properties of large-diameter NiTi SMA cables and their potential applications in recoverable functional structures. Through the uniaxial tensile test of 11 SMA cables with 7 × 7 × 1.0 mm, the mechanical properties of SMA cables under different strain amplitudes, cyclic loading effects, loading rates, and different pre-strain modes were evaluated. The results indicate that the energy dissipation capacity of SMA cable increases with the increase of strain amplitude, but at the same time, it will reduce its self-centering capabilities. The equivalent viscous damping ratio decreases after the SMA cable enters the martensitic hardening stage. The residual strain of SMA cable depends on the combined effect of strain amplitude and number of cycles. The larger the strain amplitude and the number of cycles, the more significant the degradation of self-centering performance. After 20 times of constant amplitude loading and unloading, the mechanical properties of SMA cable are relatively stable. The reverse phase transformation stress of the pre-strained SMA cable gradually decreases with the increase of the tensile strain amplitude. The pre-strained SMA cable exhibits a stable restoring force strength in the sub-cycle, and the residual strain is zero. The test results support experimental data for applying large-diameter SMA cables in recoverable functional structures.

Direct-Current-Assisted Healing for Functional Degradation of Nanocrystalline Superelastic NiTi Shape Memory Alloys

Direct-Current-Assisted Healing for Functional Degradation of Nanocrystalline Superelastic NiTi Shape Memory Alloys

Development and preliminarily tests of superelastic Ni-Ti SMA ribbed bars for reinforced concrete applications

The research on superelastic shape memory alloy (SMA) bars reinforced concrete members and structures has received a great deal of attention recent years. In this study, novel superelastic Ni-Ti SMA ribbed bars were developed and investigated to improve the bonding performance and to avoid the problem that the threaded or mechanical anchorage parts of the SMA plain round bars (used in many previous studies) were easy to break in advance. A comprehensive experimental program was conducted to characterize the cold-rolled superelastic Ni-Ti SMA ribbed bars, including scanning electron microscopy to observe the micromorphology, differential scanning calorimeter tests to analyze the phase transformation temperatures, and cyclic tensile tests to examine their mechanical properties. Moreover, five reduced-scale concrete beams were prepared and tested to verify the advantages of the superelastic SMA ribbed bars as bottom longitudinal reinforcements. Results shown that the cyclic tensile response of the cold-rolled SMA ribbed bar subjected to some degree of annealing can be similar with that of the annealed SMA plain round bar. Compared with the beam specimen incorporating ECC and superelastic SMA plain round bars and the beam specimen incorporating normal concrete and superelastic SMA ribbed bars, the specimen incorporating ECC and superelastic SMA ribbed bars exhibited a significantly improved deformation ductility, ultimate bearing capacity, energy dissipation ability, self-centering capability, and cracking performance under the cyclic flexural loads. Besides, no reinforcement slippage or fracture were observed during the entire loading process in all beams, suggesting satisfactory bonding effect and uniformity.

Development of a crankshaft driven single long NiTi tube compressive elastocaloric cooler

Elastocaloric cooling has no environmental effects during operation, and achieving a compact structure especially the driver is significant for its commercialization. In this article, a compact, standalone crankshaft driven single long NiTi tube compressive elastocaloric cooler is developed. A crankshaft driver was designed and fabricated to drive a compressive elastocaloric regenerator utilizing a single long polycrystalline superelastic NiTi shape memory alloy tube (outer diameter 5 mm, wall thickness 1 mm, and initial length 305 mm). A novel design of ceramic heat insulation plate was applied to the cooler to reduce the conduction heat loss from the NiTi tube to the stainless-steel loading heads. The cooling performance of the cooler was characterized using synchronized thermocouples and infrared thermography, and the specific cooling(heating) power, temperature span, and coefficient of performance of up to 65(125) W·kg−1, 9.1 K, and 5.0, respectively were measured. The progressions of the temperature span, specific cooling(heating) power, and coefficient of performance with the operation cycle and temperature lift were analyzed. An energy analysis revealed that the heat transfer fluid carried out only 14% of the latent heat generated by the NiTi tube, which demonstrated a potential to enhance the cooling performance by the improvement in the regenerator structure.

Tension-compression asymmetric functional degeneration of super-elastic NiTi shape memory alloy: Experimental observation and multiscale constitutive model

The functional degeneration of super-elastic NiTi shape memory alloy (SMA) rods is investigated in the cyclic tension-unloading and compression-unloading experiments. The results show that the functional degeneration exhibits a strong tension–compression asymmetry, which originates from the initial 〈111〉 and 〈110〉 mixed textures along the axial direction of the rods, the polarity of martensite transformation, and the complex interaction between martensite transformation and dislocation slipping. Moreover, the dependence of functional degeneration on the applied stress level is discussed. To understand and predict the effect of initial texture on the cyclic deformation of polycrystalline NiTi SMA, a multiscale constitutive model is constructed at finite deformation. At the mesoscopic scale, two major inelastic deformation mechanisms, i.e., the transformation between austenite and martensite phases and the transformation-induced plasticity caused by the strain mismatch at the austenite–martensite interface, are considered. The total deformation gradient is multiplicatively decomposed into the martensite transformation, transformation-induced plasticity and elastic parts. The deformation gradient parts at the mesoscopic scale are obtained by upscaling these physical quantities at a smaller spatial scale (i.e., the microscopic scale). The proposed model is formulated within the framework of irreversible thermodynamics. The driving forces for martensite transformation and transformation-induced plasticity are derived according to the established Helmholtz free energy and Clausius-Duhem inequality. The complex interaction between martensite transformation and dislocation slipping, and the forward/reverse inheritance of dislocations caused by the moving austenite–martensite interfaces are considered. To calculate the overall cyclic responses of polycrystalline NiTi SMA at the macroscopic scale, the developed constitutive model is implemented into the finite element software ABAQUS/Explicit using a user-defined material subroutine. A polycrystalline representative volume element is constructed by introducing the initial 〈111〉 and 〈110〉 mixed textures observed in the experiment. By comparing the simulated results with the experimental data, the capability of the proposed model to predict the texture-induced tension–compression asymmetric functional degeneration is verified, and the influence of texture type on the cyclic deformation is discussed.

Superelastic Nickel-Titanium (NiTi)-Based Smart Alloys for Enhancing the Performance of Concrete Structures.

Recent advances in materials science have led to the development of smart materials that can continuously adapt to different loading conditions and changing environment to meet the growing demand for smart structural systems. The unique characteristics of superelastic NiTi shape memory alloys (SMAs) have attracted the attention of structural engineers worldwide. SMAs are metallic materials that can retrieve their original shape upon exposure to various temperatures or loading/unloading conditions with minimal residual deformation. SMAs have found increasing applications in the building industry because of their high strength, high actuation and damping capacities, good durability, and superior fatigue resistance. Despite the research conducted on the structural applications of SMAs during the previous decades, the existing literature lacks reviews on their recent uses in building industry such as prestressing concrete beams, seismic strengthening of footing-column connections, and fiber-reinforced concrete. Furthermore, scarce research exists on their performance under corrosive environments, elevated temperatures, and intensive fires. Moreover, the high manufacturing cost of SMA and the lack of knowledge transfer from research to practice are the main obstacles behind their limited use in concrete structures. This paper sheds light on the latest progress made in the applications of SMA in reinforced concrete structures during the last two decades. In addition, the paper concludes with the recommendations and future opportunities associated with expanding the use of SMA in civil infrastructures.

Intrinsic response of nanocrystalline superelastic NiTi shape memory alloy

Intrinsic response of nanocrystalline superelastic NiTi shape memory alloy

A multi-scale diffusional-mechanically coupled model for super-elastic NiTi shape memory alloy wires in hydrogen-rich environment

The functional devices made by NiTi shape memory alloys (SMAs) are often serviced in hydrogen (H)-rich environment. Recently experimental results showed that the martensite transformation (MT) of NiTi SMA changed strongly after charging H, which originates from the H diffusion in the material and an additional transformation resistance caused by the absorbed H atoms. In this work, a multi-scale diffusional-mechanically coupled constitutive model is constructed to describe the super-elastic deformation of NiTi SMA wires in H-rich environment. In the grain scale, a crystal plasticity-based constitutive model is developed within the framework of irreversible thermodynamics. Four deformation mechanisms, i.e., elasticity, MT, transformation-induced plasticity (TRIP) and H expansion are taken into account. Since the H atoms can be trapped by the lattice defects, the total H concentration is further decomposed into two parts, i.e., the lattice hydrogen concentration and the trapped one. The generalized thermodynamic forces of MT and TRIP are derived from the Gibbs free energy and Clausius-Duhem inequality. The evolution of H concentration field is derived by combining the chemical potential, diffusion balance equation and the Fick's law. In the polycrystalline aggregate scale, in order to measure the interaction among the grains and obtain the overall response of the polycrystalline aggregate, a diffusional-mechanically coupled self-consistent homogenization method is developed. Meanwhile, the finite volume method is employed to calculate the H concentration in each grain. An assumption of uniform stress field in the macroscopic scale is adopted to achieve the scale transition from the polycrystalline aggregate to the whole wire. To validate the prediction capability of the proposed model, the predictions are compared with the experiments. Moreover, the influences of loading rate on the deformation of the wires in the process of in-situ charging H are predicted and discussed.

Thermo-mechanically coupled functional fatigue of NiTi shape memory alloys under multiaxial cyclic loadings

In this work, the rate-dependent and loading path-dependent cyclic deformation behaviors of super-elastic NiTi shape memory alloy (SMA) are investigated under multiaxial cyclic loadings with various loading frequencies. The evolutions of stress–strain response, temperature and dissipation energy with increasing the number of cycles are investigated systematically. It is found that the functional fatigue occurs in the process of cyclic deformation, and such a phenomenon is more significant under non-proportionally multiaxial loadings and with higher frequency. The temperature variation induced by the internal heat generation becomes more obvious with increasing the loading frequency. Moreover, it is revealed that the path-dependent functional fatigue has a positive correlation to a newly defined indicator for different loading paths, i.e., the degree of non-proportionality, and the rate-dependent functional fatigue can be attributed to the thermo-mechanically coupled nature of martensite transformation. The experimental results performed in this work and the revealing deformation mechanisms are helpful to establish a three-dimensional thermo-mechanically coupled cyclic constitutive model of NiTi SMAs.

Highly efficient grooved NiTi tube refrigerants for compressive elastocaloric cooling

Elastocaloric (eC) cooling without environmental issues has been a promising substitute for the conventional vapor-compression refrigeration. Commercial superelastic NiTi shape memory alloy tubes show large elastocaloric cooling capacity and ultrahigh fatigue life under cyclic compression but suffer from the limited specific heat transfer area (Aht) and insufficient cooling performance. Up to now, compressive NiTi tube eC refrigerant with Aht larger than 1 m2 Kg−1 has not been available due to the easy buckling problem. In this work, the buckling-free vertically and spirally grooved NiTi tubes (VGT and SGT) with Aht of 1.04 m2 Kg−1 and 1.25 m2 Kg−1 respectively are developed as the compressive eC refrigerants. The VGT and SGT are fabricated by the slow-feeding wire electric discharge machining based on the bared NiTi tubes. Then a compressive eC refrigeration prototype (ERP) is built to experimentally characterize the specific cooling power (SCP) and system temperature span (ΔTspan) of the grooved tubes. Under the regenerative flow configuration, the VGT and SGT achieve large zero-ΔTspanSCP of 2.18 W g−1 and 2.61 W g−1 at the operation frequency (fERP) = 0.5 Hz. Besides, the VGT and SGT produce maximum no-cooling-load ΔTspan of 25.2 K and 27.7 K respectively at fERP = 0.417 Hz. The experimental SCP and ΔTspan exceed most of the reported eC regenerators and there are still rooms for enhancing the cooling results by improving the ERP in future. Overall, the developed VGT and SGT with large Aht records and excellent cooling performance show big potentials of heat transfer structure design in developing highly efficient eC devices.

A two-scale thermo-mechanically coupled constitutive model for grain size- and rate-dependent deformation of nano-crystalline NiTi shape memory alloy

In this paper, a two-scale thermo-mechanically coupled constitutive model is established to predict the grain size (GS)- and rate-dependent deformation of nano-crystalline super-elastic NiTi shape memory alloy (SMA). At the mesoscopic scale, the polycrystalline representative volume element (RVE) is considered as an aggregation of individual grains. Owing to the relatively large volume fraction (VF) of grain boundary in nano-crystalline NiTi SMA, an individual grain is further considered as a composite containing the grain interior (GI) and grain boundary (GB) phases. For the GI phase, a crystal plasticity-based model accounting for the internal heat generation is developed through the fundamental laws of irreversible thermodynamics. The constraint of GB phase on the martensite transformation in the GI phase is considered by introducing a scaling law between the transformation hardening modulus (THM) and GS. Owing to its un-transformable nature and relatively high yielding stress, the GB phase is assumed to behavior constitutively linear thermo-elasticity. To evaluate the thermo-mechanical interactions among the grains, GI phase and GB phase, and obtain the overall response of the polycrystalline RVE, a double inclusion self-consistent homogenization scheme involving the material nonlinearity and thermo-mechanical coupling effect is developed in an incremental form. At the macroscopic scale, the overall thermo-mechanical responses for the specimen of nano-crystalline NiTi SMA are obtained by solving the equations of force balance, deformation compatibility, and thermodynamic equilibrium through an approximation method. Finally, considering the specific texture measured in experiments, the proposed model is validated by predicting the GS- and rate-dependent deformation of nano-crystalline NiTi SMA sheet. Furthermore, the anisotropic thermo-mechanical responses of the sheet caused by the texture are predicted and discussed.

Pre-tensioned SMA cable tests and its application in a novel self-centering viscoelastic damper

The elastic modulus of superelastic NiTi shape memory alloy (SMA) is between normal structural steel and viscoelastic materials. To enhance the initial stiffness and self-centering capability of viscoelastic (VE) dampers, an innovative self-centering high-damping rubber damper (SC-HRD), which is parallel composed of an SMA-cable-controlled self-centering system and a traditional high-damping rubber damper, was proposed and investigated in this study. The influence of initial strain, loading amplitude, and strain rate on the behavior of SMA cables after training and pre-tensioning was also tested for the SC-HRD application. Configuration and working principles of the SC-HRD were subsequently illustrated, followed by numerical studies on the behavior of the novel damper. Parametric analyses were then carried out on several design parameters of the SC-HRD to optimize its mechanical performance. Results show that the initial stiffness and residual self-centering strength of the SMA cables, which are essential for the SC-HRD application, can be enhanced with the increasing initial strain and loading strain rate. Compared with traditional VE dampers, the SC-HRD has better initial stiffness and self-centering capacities. Initial strain and relative area ratio of the pre-tensioned SMA cables in the proposed SC-HRD are suggested to be designed as about 2% and 0.123%, respectively, to achieve appropriate initial stiffness, energy dissipation, and residual deformation of the proposed SC-HRD.

Enhanced Fatigue Resistance of Nanocrystalline Ni50.8Ti49.2 Wires by Mechanical Training

In this paper, the fatigue resistance of superelastic NiTi shape memory alloy (SMA) wires was improved by combining mechanical training and nanocrystallization. Fatigue tests were performed after mechanical training with a peak stress of 600 MPa for 60 cycles of nanocrystalline (NC) NiTi wires, and the associated microscopic mechanism was investigated by using transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD). The results showed that stress-controlled training effectively improved the functional stability (the accumulated residual strain decreased by 83.8% in the first 5000 cycles) of NC NiTi SMA wires, as well as increased the average structural fatigue life by 187.4% (from 4538 cycles to 13,040 cycles). TEM observations and TKD results revealed that training-induced dislocations resulted in lattice rotation and preferential grain orientation. The finite element method (FEM) simulation results indicated that the training-induced preferential grain orientation tended to decrease the local stress concentration and strain energy density. Combined with fractography analysis, the uniform deformation caused by mechanical training changed the crack growth mode from multi-regional propagation to single-regional propagation, improving the structural fatigue life.