NiTiNb Shape Memory Alloy Wires Research Articles

The recovery stress of hot drawn and annealed NiTiNb SMA considering effect of temperature and cyclic loads: Experimental and comparative study

Shape memory alloy (SMA) has good memory effect and hyperelasticity, which can be applied in civil engineering for seismic shock absorption and active strengthening. The Nickel-titanium-niobium (NiTiNb) SMA is widely adopted as reinforcement to increase the capacity of bridge piers and/or girders. However, the NiTiNb SMA is inconvenient to apply for two reasons. First, the NiTiNb SMA is generally processed by annealing the row wires at a high temperature (850℃ ± 10℃), meaning that a lot of energy is required. Second, the annealed SMA usually needs to be pre-tensed at a low temperature (about −60℃) to obtain a sufficient recovery stress, which requires a specific device and environment. Comparatively, the NiTiNb SMA treated with hot drawn process doesn’t require high temperature in the fabrication, thus consumes less energy. Meanwhile, the hot drawn SMA can be pre-tensed at room temperature to obtain an adequate recovery stress (around 700 MPa), which overcomes the inconvenience of the annealed SMA. So far, however, the mechanical properties of the hot drawn SMA have not been fully studied, which limits its practical application. In this study, therefore, the mechanical properties of hot drawn NiTiNb SMA wires were investigated through the monotonic tension, thermal excitation and cyclic loading tests. The mechanical properties of the hot drawn SMA were compared with those of the annealed ones. The results showed that, under the monotonic loading, the yield and ultimate strength of the hot drawn SMA is greater than the annealed one by 12.7% and 43.0%. Compared to the annealed SMA, the maximum and remained recovery stress of the hot drawn SMA increases by 70.5% and 18.2%, respectively. Moreover, the hot drawn SMA is better than the annealed SMA in resisting the degradation of effective stress under the incremental cycle amplitude test.

Efficacy of prestressed SMA diagonal loops in seismic retrofitting of non-seismically detailed RC beam-column joints

In this study, a detailed experimental and numerical investigation is carried out to study the efficacy of prestressed SMA diagonal compression loops in seismic retrofitting of non-seismically detailed RC beam-column joints. Prestressed diagonal compression loops exert active confinement to the joint core which is found to perform better than the conventional passive confinement in terms of improving the shear strength, ductility, and energy dissipation capacity. In this study, active confinement of the joint core is achieved by applying in-plane diagonal compression forces using externally fastened prestressed NiTiNb shape memory alloy (SMA) diagonal loops. In the experimental part, two different techniques are investigated to prestress the SMA loops, which are: 1) rapid heat-activated prestressing technique utilizing the shape memory feature of SMAs; and 2) conventional prestressing of SMA loops by mechanical means. In the finite element analysis, a wide range of prestress values (post-tension force) is studied, keeping the basic arrangement same. The efficacy of the retrofitting technique is evaluated in terms of enhancement in strength, ductility, energy dissipation capacity, damage reduction in the specimens, and the ease of application. The results from this study suggest that depending on the applied confinement level, the retrofitting scheme can increase the ultimate strength in the range of 20–30% and the energy dissipation capacity in the range of 60–70% however, no significant enhancement in ductility of the retrofitted specimen may be achieved using this approach. The study also highlights the complexity involved in controlling the material behaviour of NiTiNb SMA wires and the risks associated with the application of SMAs in practical problems.

Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiTiNb shape-memory alloy wires

Concrete girders can suffer from serviceability issues due to excessive cracking and deflection. In response to this problem, a novel heat-induced post-tensioning technique utilizing unbonded near-surface mounted nickel–titanium–niobium (NiTiNb) shape-memory alloy (SMA) wires was proposed and evaluated. SMAs are a class of smart materials that can recover seemingly permanent plastic deformation when heated. The post-tensioning forces, thus, can be generated by restrained heat-induced shape recovery of SMA wires. Material characterization tests showed that 3.92-mm diameter NiTiNb wires with 2.5% prestrain can generate recovery stress of approximately 500 MPa when actuated via Ohmic heating in a restrained condition. The proposed post-tensioning system was experimentally evaluated in reinforced concrete girders measuring 2.3 m in length and 23 × 41 cm in cross-sectional dimensions. The girders were initially cracked to simulate typical girder damage. NiTiNb wires were then installed in the bottom cover of the girders and anchored at both ends. Subsequently, the wires were actuated via Ohmic heating, which triggered shape-recovery and generated post-tensioning stresses in the girder. The post-tensioning technique reduced the crack widths by up to 74% (370 μm) and recovered the residual midspan deflection by up to 49% (1.52 mm) in the cracked girders. Following post-tensioning, flexural loading up to failure showed that the cracked stiffness and ultimate moment capacity of the girders had increased by up to 31% and 45%, respectively, with a relatively small NiTiNb reinforcement ratio of up to 0.17%.

Mechanical behaviour of NiTiNb shape memory alloy wires– strain localisation and effect of strain rate

Over the past few years, shape memory alloys (SMAs) have received increased attention in civil engineering research due to their unique features such as superelasticity, shape memory effect (SME), high ductility and good corrosion resistance. SMAs, however, have complex material behaviour which depends on many parameters such as chemical composition, thermo-mechanical treatment and ambient temperature. SMAs exhibit high strain-rate sensitivity and strong strain localisation during the stress-induced martensitic (SIM) phase transformation. Both have serious implication in civil engineering applications. Strain rate effects and the strain localisation in SMAs have been commonly investigated separately in the literature which has been mostly focused on NiTi SMA. This paper investigates the strain localisation phenomenon, the effect of strain rate and their interaction in NiTiNb SMA. The digital image correlation (DIC) technique was used to investigate the strain localisation phenomenon. The effect of strain rates is investigated within the quasi-static range and varies from 3.3 × 10−5/s to 3.3 × 10−2/s in strain-controlled tests and 2 × 10−5/s to 2 × 10−1/s in displacement-controlled tests. Significant non-uniform strain distribution is observed during the SIM phase transformation, which progresses through the nucleation and broadening of the transformation bands (TBs). The nucleation of the TBs and the unloading strain recovery within the TBs show high strain-rate sensitivity. At high strain rates, the nucleation of a TB is accompanied by a significant stress-drop.

Heat-activated prestressing of NiTiNb shape memory alloy wires

Pre-strained shape memory alloys (SMAs) can develop significant level of recovery stress (prestress) when heated above their transformation temperature in the constrained conditions. In this study, thermo-mechanical tests were conducted to investigate heat-activated prestressing (HAP) of NiTiNb SMA wires. The tests were conducted to investigate: (1) the effect of pre-strain on reverse transformation temperatures (As and Af) of the NiTiNb SMA; (2) the effect of strain rate and the pre-strain level on the recovery stress; (3) post-HAP behaviour under monotonic and cyclic loading; and (4) reusability of the NiTiNb SMA wires for HAP applications. The digital image correlation technique was used to gain deeper insight into micro-mechanical behaviour of the SMA. The study highlights the complex material behaviour of the NiTiNb SMAs. The results from this study suggest that NiTiNb SMAs can be effectively used for HAP. Recommendations on optimal strain rate for pre-straining NiTiNb SMAs and the optimal pre-strain level for achieving maximum recovery stress in NiTiNb SMAs are given in this paper.

Shape memory alloy-carbon fiber reinforced polymer system for strengthening fatigue-sensitive metallic structures

This paper provides an overview of the basic elements, course of development, experimental evaluation, and numerical simulation of a thermally activated shape memory alloy (SMA) and carbon fiber reinforced polymer (CFRP) composite system for fatigue repair or retrofit of metallic structures. Nickel titanium niobium (NiTiNb) SMA wires, which are able to generate 400 MPa recovery stress upon thermal loading and maintain that stress level at a wide range of temperatures, was adopted to apply compressive stresses near the crack. Monotonic bond behavior of single and multiple SMA wires to CFRP was investigated; the debonding onset load and maximum capacity were quantified. The fatigue behavior of patches consisting of multiple wires bonded to CFRP was studied. Results indicated that the system could maintain 80% of the recovery stress, after up to 2 million load cycles, so long as the maximum applied stress was below the debonding onset level. A fatigue strengthening system, using such multiple SMA wire system as underlay and CFRP patch as overlay, was applied to fatigue sensitive steel plates for fatigue life improvement evaluation. The average fatigue life of the patched steel plates was over 26 times longer than that of the unpatched plates tested at the same load range. Finally, a numerical framework was developed to simulate the fatigue crack growth in steel plates patched with such strengthening system and was validated by the experimental data. The findings suggest that the proposed system could be a promising alternative to traditional techniques for fatigue crack repair.

Development of a self-stressing NiTiNb shape memory alloy (SMA)/fiber reinforced polymer (FRP) patch

The objective of this research is to develop a self-stressing patch using a combination of shape memory alloys (SMAs) and fiber reinforced polymer (FRP) composites. Prestressed carbon FRP patches are emerging as a promising alternative to traditional methods to repair cracked steel structures and civil infrastructure. However, prestressing these patches typically requires heavy and complex fixtures, which is impractical in many applications. This paper presents a new approach in which the prestressing force is applied by restraining the shape memory effect of NiTiNb SMA wires. The wires are subsequently embedded in an FRP overlay patch. This method overcomes the practical challenges associated with conventional prestressing. This paper presents the conceptual development of the self-stressing patch with the support of experimental observations. The bond between the SMA wires and the FRP is evaluated using pull-out tests. The paper concludes with an experimental study that evaluates the patch response during activation subsequent monotonic tensile loading. The results demonstrate that the self-stressing patch with NiTiNb SMA is capable of generating a significant prestressing force with minimal tool and labor requirements.

Hysteretic behavior of NiTi and NiTiNb SMA wires under recovery or pre-stressing stress

Abstract Shape memory effect of shape memory alloy (SMA) wires generates recovery stress and provides external confining pressure on concrete. Previous studies showed the effectiveness of SMA wire jackets to increase peak strength and failure strain of concrete through experimental tests. However, the behavior of SMA wires under recovery stress was not investigated seriously. Thus, this study plans a series of tensile tests for NiTi and NiTiNb martensitic SMA wires under recovery stress with varying pre-strain. This study also assesses the behavior of SMA wires under pre-stressing. Remaining residual stress is reduced with increasing additional loading and unloading strain subsequently. In general, initial and effective stiffness of the hysteretic curves are stable with increasing strain, however, unloading stiffness decreases with increasing strain. Damping ratio also decrease generally with increasing strain. When the SMA wires are heated up to the transformation temperature under pre-stress, the stress of the wires increases due to transformation of the state. Also, the stress decreases with decreasing temperature of the wires down to room temperature. The stress of the NiTiNb wires with heating is higher than the pre-stress though that of the NiTi SMA wires is less than the pre-stress. Thus, the developed stress under pre-stress seems to depend on the composition and temperature windows of SMAs.

Inducing recovery stress of NiTiNb SMA wires using heat of hydration for confining concrete

This study suggests the utilization of heat of hydration of concrete to activate the shape memory effect (SME) of shape memory alloy (SMA) wires embedded in concrete and produce recovery and residual stress on the wires. This method is more convenient than the previous electronic resistance heating. For the purpose, this study prepares NiTiNb SMA wires that show appropriate temperature window for the use of heat of hydration. Axial compressive tests of concrete cylinders confined by the NiTiNb SMA wire jackets are used to prove that the utilization of heat of hydration is valid to generate recovery and residual stress in the SMA wires. The confined cylinders show increased peak strengths and much larger failure strains than those of the plain concrete. The general behavior of the SMA wire-confined specimens in this study is similar to that of specimens heated by electronic heating jacket. Also, this study explains two examples for the utilization of heat of hydration for the SME in reinforced concrete beams and columns.

Comparing the cyclic behavior of concrete cylinders confined by shape memory alloy wireor steel jackets

Shape memory alloy (SMA) wire jackets for concrete are distinct from conventional jacketsof steel or fiber reinforced polymer (FRP) since they provide active confinement which canbe easily achieved due to the shape memory effect of SMAs. This study uses NiTiNb SMAwires of 1.0 mm diameter to confine concrete cylinders with the dimensions of300 mm × 150 mm(L × D). The NiTiNb SMAs have a relatively wider temperature hysteresis than NiTi SMAs; thus,they are more suitable for the severe temperature-variation environments to which civilstructures are exposed. Steel jackets of passive confinement are also prepared inorder to compare the cyclic behavior of actively and passively confined concretecylinders. For this purpose, monotonic and cyclic compressive loading tests areconducted to obtain axial and circumferential strain. Both strains are used toestimate the volumetric strains of concrete cylinders. Plastic strains from cyclicbehavior are also estimated. For the cylinders jacketed by NiTiNb SMA wires, themonotonic axial behavior differs from the envelope of cyclic behavior. The plasticstrains of the actively confined concrete show a similar trend to those of passiveconfinement. This study proposed plastic strain models for concrete confinedby SMA wire or steel jackets. For the volumetric strain, the active jackets ofNiTiNb SMA wires provide more energy dissipation than the passive jacket of steel.

Confining jackets for concrete cylinders using NiTiNb and NiTi shape memory alloy wires

This study used prestrained NiTiNb and NiTi shape memory alloy (SMA) wires to confine concrete cylinders. The recovery stress of the wires was measured with respect to the maximal prestrain of the wires. SMA wires were preelongated during the manufacturing process and then wrapped around concrete cylinders of 150 mm×300 mm (ϕ×L). Unconfined concrete cylinders were tested for compressive strength and the results were compared to those of cylinders confined by SMA wires. NiTiNb SMA wires increased the compressive strength and ductility of the cylinders due to the confining effect. NiTiNb wires were found to be more effective in increasing the peak strength of the cylinders and dissipating energy than NiTi wires. This study showed the potential of the proposed method to retrofit reinforced concrete columns using SMA wires to protect them from earthquakes.

Bond strength of concrete confined by SMA wire jackets

This study pursuits a goal to assess bond strength of concrete confined by shape memory alloy (SMA) wire jackets that can provide active confinement due to shape memory effect. This study uses NiTiNb SMA wires which show more wider temperature hysteresis than normal NiTi SMA wire. Thus, NiTiNb SMA wires are more applicable for the use of shape memory effect for civil structures which are exposed severe environmental conditions such as large temperature variation. Concrete cylinders of 200 mm × 100 mm (LxD) are used for this study. The SMA wire jackets increases the bond strength of concrete and changes the failure mode from split to pull-out failure. A particular thing in the tests is a split crack developing even with the SMA wire jackets. In general, the crack is not developed in the pull-out mode failure. The SMA wire jackets increase the bonding strength even with the crack developing since it seems that they provide active confinement as well as external passive confinement.