NiTi Strips Research Articles

Modeling the interaction between instabilities and functional degradation in shape memory alloys

Localization of the stress-induced martensitic phase transformation plays an important role in the fatigue behavior of shape memory alloys (SMAs). The phenomenon of return-point memory that is observed during the subloop deformation of a partially-transformed SMA is a clear manifestation of the interaction between localized phase transformation and degradation of the functional properties. The present study aims to demonstrate this structure–material interaction in the modeling of return-point memory. It seems that this crucial aspect has been overlooked in previous modeling studies. For this purpose, we developed a gradient-enhanced model of pseudoelasticity that incorporates the degradation of functional properties in its constitutive description. The model is employed to reproduce the hierarchical return-point memory in a pseudoelastic NiTi wire under isothermal uniaxial tension with nested subloops. Additionally, a detailed analysis is carried out for NiTi strip with a more complex transformation pattern. Our study highlights the subtle morphological changes of phase transformation under different loading scenarios and the resulting implications for return-point memory.

Rate induced thermomechanical interactions in NiTi tensile tests on strips

The paper uses tensile experiments on NiTi strips at different displacement rates to establish and simulate the thermomechanical interactions caused by the latent heat of the reversible transformation between the austenitic and martensitic phases. The evolution of deformation in the specimen is synchronously monitored with digital image correlation, and the temperature field through infrared imaging, essential for structural modeling. Transformation leads to localized deformation that propagates through the specimen, while the latent heat released/absorbed at the propagating fronts locally heats/cools the specimen. The sensitivity of the transformation stress to temperature results in a complex interaction between the heat transfer conditions and the nucleation and evolution of transformation in the specimen. At low rates of loading, the alternate phase propagates nearly isothermally with a small number of fronts producing relatively flat stress plateaus. Higher rates lead to significant heating/cooling that results in progressive nucleation of multiple fronts and apparent "hardening" responses. The experiments are simulated in a three-dimensional static displacement transient temperature finite element analysis, using a new fully coupled thermomechanical constitutive model. Transformation strain and entropy are its internal variables whose evolution is governed by the motion in the stress-temperature space of a single transformation surface governing both transformations. The prevailing localization is captured by the introduction of softening over the unstable branches of the recorded isothermal material response. The results demonstrate how the important role of the thermal interaction between the specimen and the environment can be addressed. This, together with appropriate calibration of the constitutive and structural model, enable the analysis to reproduce the effect of rate on the recorded response, the evolution of localization patterns, and the associated thermal fields. The results can guide the development of constitutive and structural models of phase transforming materials with strong thermomechanical interactions.

On the underlying material response of pseudoelastic NiTi

In some temperature regimes NiTi can be deformed to strain of 5–7% that recovers fully on unloading. Under tensile loading it traces a closed hysteresis with two stress plateaus associated with the reversible solid-state phase transformation between the austenitic (A) and the martensitic (M) phases, during which the deformation localizes and propagates through the specimen. Hallai and Kyriakides (2013) extracted the underlying softening response associated with A-M transformation from a tensile test on a NiTi strip laminated between two hardening steel face-strips that ensured that it deformed uniformly. The test finishes with the laminate permanently deformed to a strain of 6%. Extraction of the softening response of the M-A transformation requires compressing the thin laminate back to zero strain. The present paper presents a new experimental set-up that enables completion of the load/reverse load test on such a laminate by laterally supporting the specimen during compression to prevent buckling. The extracted response of NiTi exhibits softening branches with up-down-up trajectories for both the A-M and M-A transformations, and Maxwell stresses that match those of the stress plateaus of the tensile hysteresis of a NiTi strip. The extracted response is used to calibrate a custom SMA constitutive model, which when incorporated in a finite element analysis reproduces the measured hysteresis of NiTi nearly perfectly for the first time. Extraction of the complete underlying partially unstable response of SMAs enables modeling of the unstable behavior of SMA structures with the confidence provided by measured data.

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.

Production and characterization of functionally graded NiTi shape memory alloys by Joule effect

Abstract Localized heat treatments via Joule effect were performed on cold-drawn NiTi strips to produce a functionally graded material (FGM). Three zones where locally heat treated at 300, 350, and 400 °C for 10 min followed by air cooling. Multiscale and multiphenomena characterization of the obtained FGM was performed through infrared temperature testing, four-point probe and eddy current testing, mechanical testing and synchrotron X-ray diffraction. The effect of these localized heat treatments is clearly observed by different techniques. The use of these short and localized heat treatments avoids the need of highly expensive manufacturing routes typically used to obtain the same effect on NiTi shape memory alloys, thus opening new possibilities for processing these advanced engineering alloys.

Nonlocal modeling and analysis of spatiotemporal patterns in non-isothermal phase transformation of NiTi strips

The spatiotemporal evolution of macro-domain patterns in a NiTi thin strip during stress-induced non-isothermal phase transformation (PT) are investigated through 2D FEM simulations using nonconvex nonlocal continuum model and 1D theoretical analysis. The FEM simulations shows that, with the increase of applied stretching rate (nominal strain rate), the way of PT in a thin strip (30 mm × 2.5 mm × 0.5 mm) changes from the sequential nucleation and growth mode in the low rates range (≤0.03/s) to the emergence of periodic patterns in the medium rates range (0.03/s~3.0/s) and eventually to stable homogeneous deformation mode in the high rates range (≥3.0/s). The domain spacing is governed by a power-law scaling to the strain rate, where the exponent changes from −1/2 (low rates range) to −1/6 (medium rates range). The 1D theoretical analysis further demonstrates that the rate-dependent patterns are essentially governed by a nondimensional internal material parameter A-0 and a nondimensional external driving parameter k-. A-0 represents the ratio of adiabatic hardening to isothermal softening in PT of the material, while k- controls the time scale competition of heat release and heat conduction, through which the thermodynamic condition of PT changes from near isothermal (k-→∞) to near adiabatic (k-→0) with the increase of the strain rate. It is shown that, as long as the material has the properties of isothermal softening and adiabatic hardening (i.e., A-0>1), the conventional nucleation-growth paradigm of PT will eventually break down for small k- (or large stain rate). This is due to the fact that the increase of the energy barrier caused by the thermal and the interfacial effects can convexify the energy landscape of the system near the adiabatic condition and suppress the nucleation events at the macroscopic level.

An experimentally-driven approach to model bending in a thermally activated SMA-based beam

In this paper, the bending behavior of an active SMA-based beam, made up of a NiTi alloy strip externally joined to a PA66-GF30 polymeric lamina, is examined. Firstly, experimental investigations were conducted in a purpose-built test bench where a forced airflow promoted the reverse and forward phase transformations of the NiTi strip via heating and cooling ramps. The macroscopic shape changes associated with the shape memory effect deforms the structure in the cantilevered condition. The evolution of the shape memory behavior was investigated via digital image analysis, performed at both the end of heating and the end of the thermal cycle on cooling. Next, a theoretical prediction combining the classical beam model for bending with the assumption of incomplete recovery upon cooling was developed. The theoretical results indicate good agreement with experimental data showing how the proposed approach can be effectively used to predict the behavior in bending of SMA-based actuator working in controlled recovering condition.

Nonuniform transformation behaviour of NiTi in a discrete geometrical gradient design

Shape memory alloys exhibit unique thermomechanical properties, e.g., the shape memory effect and the pseudoelasticity. By proper geometrical gradient design, these alloys can be made to exhibit different and more intricate thermomechanical behaviour to enable innovative applications. This paper reports the design of geometrically gradient NiTi and characterisation of its complex and nonuniform transformation and deformation fields. The study investigated two designs, with one using multiple pseudoelastic NiTi strips of different lengths in parallel arrangement to create a discrete geometrical gradient in the direction perpendicular to the loading axis and the other a tapered plate to give a continuous length gradient in the lateral direction for comparison with the former. The geometrically gradient structures exhibited partial stress gradient during stress-induced transformation. A maximum stress window of 520 MPa was achieved, giving an expanded stress interval for shape memory actuation control. Finite element modelling was applied to characterise the deformation behaviour of such structures under tensile loading and to reveal the complex propagation of the stress-induced transformation in such structures.

Fatigue resistance of branching phase-transformation fronts in pseudoelastic NiTi polycrystalline strips

Inhomogeneous martensitic phase transformation induced by tensile stress can trigger Lüders-like band formation in pseudoelastic NiTi polycrystalline which has influence on its fatigue behaviors. It is often intuitively conjectured that the phase-transformation front (band front) has lower fatigue resistance than the regions inside or outside the band (i.e., the Martensite or Austenite domains), but no experiment in literature can confirm or disprove this conjecture. In this paper, a systematic fatigue study is performed on NiTi strips with nucleated bands under cyclic “elastic” loading, where the nominal strain amplitude is set as a small value (0.25%) to keep the band front immobile. Meanwhile, a close relation between the front morphology and the strip's fatigue failure is revealed with the help of a “global camera” to achieve the full-field strain distribution (by Digital Image Correlation) and a “local camera” to observe the local front morphology. It is found that the band front branches into many small-scale narrow bands in thin strips while the front has no branching and just becomes diffusive in thick strips, and the fatigue life of the thin strip is shorter than the thick strip. Moreover, for the strips cut along the rolling direction, the fatigue failures of the strips with different thicknesses always occur at the phase-transformation front. The reason is attributed to the stress concentration in the front due to the branching front morphology and geometry necking, which depend on the structure geometry (strip thickness here), and the dissipative microscopic interface motion of Austenite-Martensite phase mixtures.

Functionally graded NiTi shape memory alloys

In this study, in order to obtain a functionally graded material, NiTi strips were annealed at 350°C, 450°C and 550°C in a furnace using an assembly that allowed a temperature gradient along them, and their transformation temperatures were studied by Differential Scanning Calorimetry (DSC). Furthermore, the strips were bent at both ends and dipped into a water bath at room temperature which was then heated to 61°C in order to observe the influence of the gradient annealing on their strain recovery. It was found that the strips’ coolest regions presented the greatest strain recovery, particularly the strips annealed at 350°C and 450°C, although any strip exhibited a full strain recovery, due to plastic deformation during bending. These results, together with the DSC analysis at both regions (coolest and hottest), allow us to conclude that the graded annealing was successful for the intended functional gradient, as a gradient of transformation temperatures along the strips has been obtained, despite the primitive assembly, thus presenting an interesting result for a first approach. Further tests will be performed with a new experimental procedure especially designed for this purpose.

Development of Ni-Ti Shape Memory Alloys through Novel Powder Metallurgy Route and Effect of Rolling on their properties

The present paper deals with the development of Ni-Ti shape memory alloys through a novel powder metallurgy process. The Ni-Ti shape memory alloy are widely used in automobile, aerospace, sensor, vibration, actuators, biomedical and other applications. The Ni-Ti shape memory alloys mixed in the weight percent of their atomic number. The powders were then cold compacted in a hydraulic press at pressure of 300 MPa and subsequently sintered at 1150°C for 30 minutes at 25 MPa pressure in vacuum (10-5Torr). The microstructure and effect of rolling is to done in sintered and hot pressed sample. Further the rolling of hot pressed sample was done to improve the mechanical properties and testing of shape memory effect on Ni-Ti strip was investigated. The rolled thin strip was further used for testing of mechanical properties and shape memory behaviour. The hot-pressed sample rolling in thin strip. The effect of the rolling on the hot-pressed sample was marginal on the mechanical properties and transformation temperature of Ni-Ti shape memory alloys.

Lüders-like band front motion and fatigue life of pseudoelastic polycrystalline NiTi shape memory alloy

We conducted tensile fatigue tests on polycrystalline NiTi strips with in-situ optical observation on the evolution of the localized deformation (Lüders-like band) in order to trace the deformation history of the fatigue-failure material point in the strips under various loadings. It is found that the band front motion triggering localized cyclic phase transformation leads to a short fatigue life while the “elastic” deformation (without the band front motion) results in a much longer life whose failure crack nucleates away from the immobile band front. The results provide hints to derive a material fatigue criterion from the structure fatigue tests.

A Shape Memory Alloy-based Morphing Axial Fan Blade: Functional Characterization and Perspectives

In a traditional automotive cooling system, energy optimization could be achieved by controlling the engine temperature by means of several sensors placed inside the cooling circuit. Nevertheless, in some cases the increasing use of a great number of sensor devices makes the control system too bulky, expensive and not sufficiently robust for the intended application. This paper presents the development of a heavy-duty automotive cooling axial fan with morphing blades activated by Shape Memory Alloy (SMA) strips that work as actuator elements in the polymeric blade structure. The blade was designed to achieve the activation of the strips (thermo-mechanically treated on purpose) by means of an air stream flow. With the aimof studying the morphing capability of the adaptive structure together with the recovery behavior of the NiTi strips, four different polymeric compounds have been compared in a specifically-designed wind tunnel. As the airstream flow increases its temperature, the strips recover the memorized bent shape, leading to a camber variation. To study the possibility of employing SMA strips as actuator elements, a comparison with common viscous clutchbehavior is proposed. The time range actuator response indicates that the SMA strips provide a lower frequency control that fits well with the engine coolant thermal requirement. The experimental results demonstrate the capability of SMA materials to accommodate the lower power actuators in the automotive field. This innovative passive control system results from the selection of (i) the memorized shape of the SMA strips and (ii) the polymeric compound used for the blade structure.

Morphing blades with embedded SMA strips: An experimental investigation

Abstract This paper focuses on the development and experimental testing of a composite blade for an automotive axial fan. A novel concept of morphing blade is proposed with the aim of replacing the conventional actuator systems. The structure is made up of a polymeric matrix equipped with NiTi shape memory alloy strips as active elements. The morphing blade changes its shape as the phase transformations of the strips are thermally activated by airflow. To study the morphing capability of the blade, together with the recovery behavior of the NiTi strips, four different polymeric compounds have been compared. Digital image analysis techniques have been performed to quantitatively analyze the blade deflections and to evaluate the most suitable polymeric matrix for the intended application. Finally, the blade shape modifications, which occur along the blade span during the activation cycles, have been reconstructed by three-dimensional non-contact surface detection. Results from the comparison between these two reverse engineering methods could provide a powerful support for the assessment of the aerodynamic performance of the morphing blade.

TWSME of a NiTi strip in free bending conditions: experimental and theoretical approach

This paper deals with the two-way shape memory effect (TWSME) induced on a strip of a nearequiatomic NiTi alloy by means of the shape memory cycling training method. This procedure is based on the deformation in martensite state to reach the desired cold shape followed by cycling the temperature from above Af to below Mf. To this end, the sample was thermally treated to memorise a bent shape, thermomechanical trained as described and thermally cycled in unloaded conditions in order to study the stability of the induced TWSME. Heating to Af was reached by a hot air stream flow whereas cooling to Mf was achieved through natural convection. The evolution of the curvature with the increasing number of cycles was evaluated. The thermomechanical behaviour of the strip undergoing uniform bending was simulated using a one-dimensional phenomenological model based on stress and the temperature as external control variables. Both martensite and austenite volume fractions were chosen as internal parameters and kinetic laws were used in order to describe their evolution during phase transformations. The experimental findings are compared with the model simulation and a numerical prediction based on the approach proposed in [25].

Stress-induced martensitic transformation in nanometric NiTi shape memory alloy strips: An in situ TEM study of the thickness/size effect

Abstract Ultrathin NiTi miniature strips of 40–83 nm in thickness were fabricated by means of focused ion beam milling from a polycrystalline NiTi shape memory alloy. The NiTi strips were subjected to tensile deformation inside a transmission electron microscope using a self-designed tension apparatus for in situ examination of the effect of thickness on the stress induced martensitic transformation behavior in the strips. The study revealed that the transformation was completely suppressed in a strip of 40 nm in thickness whereas it was possible in thicker strips. In these strips, the stress induced martensitic transformation was found to commence sequentially in thicker strips first and then in thinner strips at higher strain (stress) levels, demonstrating the size effect. This size effect is attributed to the effect of damaged surfaces, including a Ga + -impregnated amorphous layer on one side of the strip caused by sample fabrication using FIB and oxidation affected layers on both sides. This means that the observed “size effect” is not an intrinsic behavior of the martensitic transformation in NiTi but of extrinsic influences.

Underlying material response for Lüders-like instabilities

The initial yielding of some low carbon steels exhibits a material instability known as Lüders banding. This is a dislocation driven phenomenon that macroscopically manifests as inhomogeneous deformation. For example, in a displacement controlled uniaxial test Lüders banding leads to coexistence of two deformation regimes while the stress remains relatively unchanged. Shape memory alloys whose behavior is governed by solid–solid phase transformations, in some temperature regimes exhibit similar localizations albeit reversible. Such localizations have undesirable consequences in structural applications such as stretch marks and structural instabilities. Modeling of this behavior in order to assess its consequences in structures is hampered because the true material response over the stress plateau is unknown. This work used an experimental technique, outlined in Shioya and Shiroiri (1976) that extracts the underlying material response of such materials from a tensile test. Laminates consisting of face-strips of a hardening material and an unstable core, if properly designed, can suppress the inhomogeneous deformation of the core resulting in a monotonically increasing response. This method revealed that both steel and NiTi strips have up-down-up responses. The extracted responses incorporated in finite element models are shown to reproduce both the laminate experiments and the behavior of strips made of unstable materials alone.

Functionally graded NiTi strips prepared by laser surface anneal

In this study, two-dimensional functionally graded NiTi thin plates were created by laser surface scanning anneal. Owing to the natural degradation of heat penetrating into the material, a temperature gradient within the thickness was created by laser surface interaction and heat conduction. A microstructural gradient was created due to partial annealing within the temperature field. The microstructural variation through the thickness was characterized by hardness measurement and layered differential scanning calorimetry analysis. The microstructural gradient led to a unique shape memory effect, involving shape change in two opposite directions upon one heating, analogous to stingray motion. Such behaviour also experiences enlarged temperature windows for both the forward and reverse martensitic transformations, rendering high controllability of actuation.

CHARACTERIZATION OF CONSTRAINED AGED NiTi STRIPS FOR USING IN ARTIFICIAL MUSCLE ACTUATORS

Marvelous bending/straightening effects of two-way shape memory alloy (TWSMA) help their employment in design and manufacturing of new medical appliances. Constrained ageing with bending load scheme can induce two-way shape memory effect (TWSME). Scanning electron microscope (SEM) analysis, electrical resistivity measurement (ERM) and differential scanning calorimetry (DSC) are employed to determine the property change due to flat strip constrained aging. Results show that flat-annealing prior to the aging shifts NiTi transformations temperatures higher. Superelastic behavior of the as-received/flat-annealed/aged samples with more adequate transition temperatures due to biological tissue replacement is studied by three-point flexural tests. Results show that curing changes the transition points of the NiTi strips. These changes affect the shape memory behavior of the NiTi strips embedded within the biocompatible flexible composite segments.

Experimental study on rate dependence of macroscopic domain and stress hysteresis in NiTi shape memory alloy strips

NiTi polycrystalline shape memory alloys, when stretched, can deform through the formation and growth of localized macroscopic martensite domains. In this paper, we study the effects of stretching rate on the stress-induced domains and stress hysteresis in NiTi strips. Synchronized measurements of the nominal stress–strain curve, macroscopic domain pattern and the associated temperature field were conducted in the strain rate range of 10 −4–10 −1/s. It was found that the nominal stress–strain curve changed from the near-isothermal plateau-type with distinct stress drops at the very low strain rate to the near-adiabatic smooth hardening-type in the high strain-rate region. The corresponding deformation mode changed from the nucleation propagation mode with a few parallelepiped martensite domains to the near-homogeneous multiple-nucleation mode with many fine alternating austenite–martensite stripes. The number of the domains (domain spacing) increased (decreased) monotonically with the strain rate and followed a power law scaling, while the stress hysteresis (or material damping capacity) changed non-monotonically with the strain rate, reaching a peak at strain rate of 2.0×10 −3/s. We show that, though the rate dependence of both pattern and hysteresis originates from the transfer of the released/absorbed heat and the thermo-mechanical coupling, the domain spacing in the test of static air is mainly controlled by heat conduction and the hysteresis change is mainly controlled by the heat convection with the ambient.