High Recovery Stress Research Articles

Recovery stress formation in a restrained Fe–Mn–Si-based shape memory alloy used for prestressing or mechanical joining

The formation of recovery stresses in a restrained Fe–Mn–Si–Cr–Ni–VC shape memory alloy used for mechanical coupling and prestressing has been investigated with special emphasis on non-ideal restraints that might reduce the recovery stress. The evolution of recovery stresses in the presence of different initial gaps and elastic restraints was investigated experimentally. The results show that the alloy can still produce substantial recovery stresses even under non-ideal restraining conditions. In a second part, the experimental results are analyzed using techniques from constitutive modelling to investigate the evolution of the recovery stress in a more rigorous way. For this purpose the measured strain is decomposed into its elastic, thermal expansion, and transformation parts. With these strain components, the development of the recovery stress during heating and cooling can be described for all different constraint conditions in a unified way. The analysis also shows why optimizing a material for high recovery stress is different from optimizing it for a large recovery strain.

Improved recovery stress in multi-walled-carbon-nanotubes reinforced polyurethane

The shape-recovery performance of multi-walled-carbon-nanotubes (MWCNT) reinforced shape-memory-thermoplastic-polyurethane (SMTPU) was investigated for modified Shape-Memory-Creation-Procedure (SMCP). The modified SMCP replaced the conventional stretching scheme of deformation which is generally employed to attain temporary-shape. A Progressive-Stretch-Relax-Stretch (PSRS) scheme of deformation was introduced to create temporary shape in SMTPU and MWCNT-SMTPU nanocomposites. As compared to the conventional deformation, PSRS deformation exhibited more than 50% increase in the recovery-stress in all the composites. The cast-films of SMTPU having 0, 1, 2, 3 and 5 phr MWCNT were prepared. Significant improvement in recovery-stresses was also observed with the increased content of MWCNT. DSC studies revealed increased glass-transition temperature with MWCNT content. These composites exhibited higher values of shape-fixity with increased content of MWCNT. The samples prepared under PSRS scheme, having 5 phr MWCNT in SMTPU, attained high recovery stress 6.95MPa as compared to 3.68MPa with conventional method at 780% strain of the sample.

Feasibility of Crosslinked Acrylic Shape Memory Polymer for a Thrombectomy Device.

PurposeTo evaluate the feasibility of utilizing a system of SMP acrylates for a thrombectomy device by determining an optimal crosslink density that provides both adequate recovery stress for blood clot removal and sufficient strain capacity to enable catheter delivery.MethodsFour thermoset acrylic copolymers containing benzylmethacrylate (BzMA) and bisphenol A ethoxylate diacrylate (Mn~512, BPA) were designed with differing thermomechanical properties. Finite element analysis (FEA) was performed to ensure that the materials were able to undergo the strains imposed by crimping, and fabricated devices were subjected to force-monitored crimping, constrained recovery, and bench-top thrombectomy.ResultsDevices with 25 and 35 mole% BPA exhibited the highest recovery stress and the highest brittle response as they broke upon constrained recovery. On the contrary, the 15 mole % BPA devices endured all testing and their recovery stress (5 kPa) enabled successful bench-top thrombectomy in 2/3 times, compared to 0/3 for the devices with the lowest BPA content.ConclusionWhile the 15 mole% BPA devices provided the best trade-off between device integrity and performance, other SMP systems that offer recovery stresses above 5 kPa without increasing brittleness to the point of causing device failure would be more suitable for this application.

Recovery stress and shape memory stability in Ni–Ti–Cu thin wires at high temperatures

Abstract The recovery stress evolution during thermal cycling (heating, isothermal holding, cooling) to temperatures significantly above the austenite finish temperature in Ni – Ti – Cu thin wires was evaluated in a dynamic mechanical analyser DMA TQ800. Ni – Ti – Cu wires were annealed at four different temperatures (350 °C, 450 °C, 550 °C and 650 °C). It was observed that the maximum recovery stress significantly decreases with increasing annealing temperature. It was also observed that the recovery stress during four isothermal holdings shows similar behaviour for all annealing temperatures, i. e. stress relaxation occurs after initial stress increase during the first isothermal holding and no stress relaxation is observed during isothermal holdings in subsequent thermal cycles. The maximum recovery stress after each thermal cycle decreases and shows a tendency to saturate with increasing number of thermal cycles. It is found that even when the recoverable strain is significantly decreased during thermal cycling, Ni – Ti – Cu wires are still able to generate very high recovery stresses.

Active Confinement of Reinforced Concrete Bridge Columns Using Shape Memory Alloys

This paper presents experimental and analytical work conducted to explore the feasibility of using an innovative technique for seismic retrofitting of RC bridge columns using shape memory alloys (SMAs) spirals. The high recovery stress associated with the shape recovery of SMAs is being sought in this study as an easy and reliable method to apply external active confining pressure on RC bridge columns to improve their ductility. Uniaxial compression tests of concrete cylinders confined with SMA spirals show a significant improvement in the concrete strength and ductility even under small confining pressure. The experimental results are used to calibrate the concrete constitutive model used in the analytical study. Analytical models of bridge columns retrofitted with SMA spirals and carbon fiber-reinforced polymer (CFRP) sheets are studied under displacement-controlled cyclic loading and a suite of strong earthquake records. The analytical results proves the superiority of the proposed technique using SMA spirals to CFRP sheets in terms of enhancing the strength and effective stiffness and reducing the concrete damage and residual drifts of retrofitted columns.

A Novel Fe‐Mn‐Si Shape Memory Alloy With Improved Shape Recovery Properties by VC Precipitation

In this work, a nominally new Fe-Mn-Si based shape memory alloy with a small amount of VC was designed. After an optimized thermo-mechanical treatment, a shape recovery of more than 90% after an elongation of 4% could be achieved when the alloys were heated up to 225°C. In addition, high recovery stresses of up to 380 MPa could be obtained after heating to 225°C, whereas 330 MPa were obtained after heating to 160°C.

Structure and Functional Properties of Ti-Ni-Based Shape Memory Alloy after Electroplastic Deformation

Application of electroplastic deformation (EPD) by rolling to bulk long-sized samples of Ti-50.7 at.%Ni alloy allows increasing of the deformation strain without macrofailure by 1.5 to 3 times in comparison to cold rolling without electrical current application. Structure formation and functional properties were studied after various EPD regimes: current density (84 to 168 А/mm2) and impulse duration (80 and 160 ms). When the stage of mixed nanocrystalline and amorphous structure formation is reached as a result of EPD, a post-deformation annealing at 400 °С leads to a nanocrystalline structure formation in austenite and highest recovery stress values generation by the martensite.

Development of a fuel-powered shape memory alloy actuator system: I. Numericalanalysis

This work (Part I) discusses the numerical analysis of a fuel-powered shape memory alloyactuator system (FPSMAAS) that utilizes fuels with high energy densities, such aspropane, as its energy source and thus reduces or eliminates the dependence on electricalpower supplies, such as batteries. The main component of the actuator, a shape memoryalloy (SMA) element, operates as a heat engine and converts the thermal energy of fuelcombustion to mechanical energy. The incorporation of the high energy density fuel andactuator control hardware and software inside the unit makes for a compact actuatorsystem, requiring only low-power, digital actuator control signals as input. Due to therelatively high recovery stress and strain of SMAs, the compact actuator can providesignificant force and stroke. Convection heating and cooling of the SMA also results inrelatively high actuation frequencies. Finally, if the compact actuator is utilized in thecontext of a larger system producing excess parasitic heat, the energy density of theactuator subsystem increases. The numerical analysis of the SMA element/strip wasconducted using commercial software, including FLUENT and ABAQUS. The goal of theanalysis was to estimate actuation frequency and actuation strain. In addition, amulti-channel combustor and a micro-tube heat exchanger were designed and analyzed.These results were compared to experimental tests and measurements (Part II).

Microstructure change and shape memory characteristics in welded Fe–28Mn–6Si–5Cr–0.53Nb–0.06C alloy

In the present study, an Fe–28Mn–6Si–5Cr–0.53Nb–0.06C (mass%) alloy was employed to investigate the influence of laser beam welding (LBW), electron beam welding (EBW) and tungsten-insert gas welding (TIGW) on shape memory properties. This alloy was recently found to exhibit very good shape memory effect with simple thermomechanical treatments without so-called the ‘training’. After homogenizing heating at 1470 K for 10 h, two different treatments were performed: (a) 14% rolled at 870 K followed by aging at 1070 K for 10 min and then welded, (b) first welded and then extended 10% at room temperature, followed by aging at 1070 K for 10 min. The results show that the shape recovery of the alloy was slightly different depending on welding method. The shape recoveries for the samples treated in (a) by LBW, EBW and TIGW were 90%, 87%, 81% and those for the samples treated in (b) were 86%, 84% and 79%, respectively. After welding, the alloy still possesses very high shape recovery stress of about 250 MPa for the samples treated in (a). High strength of the weld zone is due to the dendrite structure with very fine columnar grains. All of these results on welding will make a wide industry application of the Fe–Mn–Si based SMAs in the near future.

Effect of Equal Channel Angular Pressing and Repeated Rolling on Structure, Phase Transformations and Properties of TiNi Shape Memory Alloys

The nanostructured TiNi-based shape-memory alloys were synthesized by multi-step SPD deformations – ECAP plus cold rolling or drawing. It is found that the SPD processing changed the morphology of the martensite and temperature of martensite transformation. Also, we found that the mechanical and shape memory properties can be enhanced by forming nanostructures in these alloys. SPD processing renders higher strength, higher yield dislocation strength and in results - higher recovery stress and maximum reverse strain of shape memory.

Composition design principles for Fe–Mn–Si–Cr–Ni based alloys with better shape memory effect and higher recovery stress

To investigate the factors affecting the recovery stress in Fe–Mn–Si–Cr–Ni shape memory alloy, the transformation behavior under constraints was studied through in situ electrical resistivity analysis. The recovery stress equations during constrained heating and cooling have been established. Composition design principles for Fe–Mn–Si–Cr–Ni shape memory alloys with better shape memory effect and higher recovery stress at room temperature have been proposed and they are different from ones which produce only the best shape memory effect. To test the principles, Fe–14.8Mn–4.78Si–7.95Cr–4.16Ni–0.18C alloy was designed according to the principles. The result shows that the recovery stress of the designed alloy is 360 MPa, 135 MPa higher than that of the alloy designed according to the principles for only obtaining the best shape memory effect under no constraints. Decreasing the amount of the plastic deformation and the ɛ martensite transformation can remarkably increase the recovery stress.

Effects of silicon nitride interlayer on phase transformation and adhesion of TiNi films

TiNi films with different Ti/Ni ratios were deposited on Si substrates with and without silicon nitride interlayer. Near-equiatomic TiNi films were found to have the lowest residual stress and the highest recovery stress regardless of the existence of silicon nitride interlayer. The addition of silicon nitride interlayer between film and Si substrate did not cause much change in phase transformation behavior as well as adhesion properties. X-ray photoelectron spectroscopy (XPS) analysis revealed that there is significant interdiffusion of elements and formation of Ti–N and Si–Si bonds at TiNi film/silicon nitride interface. Scratch test results showed that adhesion between the TiNi film and substrate was slightly improved with the increase of Ti content in TiNi films.

Robust position regulation of a shape memory alloy wire actuator

This paper presents design and experiment results of active position control of a shape memory alloy (SMA) wire actuator using a sliding mode based robust approach. In this research, an SMA wire was chosen as an actuating element for position control owing to its high recovery stress (gt;500MPa) and tolerance to high strain (up to 5 per cent). To compensate for the inherent non-linearity associated with the SMA, a sliding mode based robust controller was designed and implemented actively to control the position of the SMA wire actuator. Experiments demonstrated the effectiveness of the robust control. For a 12 in long SMA wire actuator, the position can be controlled within 30m.

Robust position regulation of a shape memory alloy wire actuator

This paper presents design and experiment results of active position control of a shape memory alloy (SMA) wire actuator using a sliding mode based robust approach. In this research, an SMA wire was chosen as an actuating element for position control owing to its high recovery stress (gt;500MPa) and tolerance to high strain (up to 5 per cent). To compensate for the inherent non-linearity associated with the SMA, a sliding mode based robust controller was designed and implemented actively to control the position of the SMA wire actuator. Experiments demonstrated the effectiveness of the robust control. For a 12 in long SMA wire actuator, the position can be controlled within 30m.

Effect of initial strengthening on recovery stress generation and isothermal relaxation processes in TiNi alloys

Recovery stress (RS) generation and forced isothermal relaxation processes in Ti-50.5 at. % Ni and Ti-50.7 at. % Ni shape memory alloys were investigated using in situ X-ray structure analysis, X-ray texture analysis, transmission electron microscopy and mechanical tests. The alloys were subjected to thermomechanical treatments (TMT) including cold rolling followed by post-deformation heating in the range from 400 to 700°C for obtaining partially recovered, completely polygonized or recrystallized B2-austenite structures. Specimens containing partially recovered, polygonized or recrystallized structures obtained by thermomechanical treatment (TMT) were deformed in tension by 3 - 12 % strain inducing shape memory. When the austenite get low hardening by TMT or by deformation inducing SME, the RS generation is accompanied by elastic and plastic deformations. When the austenite get high hardening, the RS generation is accompanied by elastic deformation only. The isothermal RS relaxation is accompanied by elastic deformation of the austenite excluding the case of very high RS (> 1000 MPa) and recovery strain during relaxation (8 - 9%) when plastic deformation also takes place. During RS generation the reverse martensite transformation by the orientation variant « directly backwards» is restricted by external counteraction and other orientation variants can partially realize.

Shape Memory Alloys: A Material and a Technology

Shape memory alloys offer attractive potentials such as: superelastic behaviour, reversible strains of several percent during heating or cooling over a limited temperature range, generation of high recovery stresses, and a work output with a high power/weight ratio. This paper describes the origin of those properties and gives an overview of the shape memory functions and shape memory applications.

Adaptive polymer composites with embedded shape memory elements

Adaptive materials integrate actuating and sensing technologies into a structural material. The approach discussed here is the integration of shape memory alloy (SMA) wires as actuating elements in fibre reinforced polymer composites. SMA's offer attractive potentials such as reversible strains of several percent, generation of high recovery stresses, and high power/weight ratios. T he development of polymer composites with embedded SMA's can open new perspectives with respect to the development of engineering structures with adaptive shape, stiffness, damping and other properties. The development of these advanced composites with embedded SMA-wires is still in an early stage. An overview is given of the current state-of-the-art and of the many fundamental issues which require further research before these adaptive composites can be used in industrial applications.

Adaptive composites incorporating shape memory alloy wires. Part 2: development of internal recovery stresses as a function of activation temperature

Shape memory alloy (SMA) wires have been integrated in composite laminates consisting of an epoxy resin reinforced by aramid fibres. In the first part of this series, a new methodology was proposed for the simultaneous measurement of stress and temperature in the reinforcing fibres during shape memory wire activation. Those tests were conducted at three activation temperatures (40, 60 and 100°C) and the internal compressive stress distribution was extracted for low wire volume fraction composites. It was observed that for higher wire volume fractions the high compressive recovery stresses generated during electrical resistive heating of the wires led to the geometric failure of the composite coupons. In this work, measurements are conducted on hybrid laminate coupons of dimensions that prevent geometric (global) specimen failure. The internal stress distribution is measured at relatively high activation temperatures of 80 and 100°C and a filtering technique based on fast Fourier transform (FFT) is employed to simulate the stress and temperature variability and to filter the associated experimental noise. The results show that the higher stresses appear in the middle of the mid-wire distance and that the values at 100°C activation are not significantly different than those at 80°C indicating the presence of an upper limit in the transmission of wire recovery stresses in these systems.

Active position control of a shape memory alloy wire actuated compositebeam

This paper presents the design and the experimental result of theactive position control of a shape memory alloy (SMA) wire actuated compositebeam. The composite beam has a honeycomb structure with SMA wires embedded inone of its face sheets for the active actuation. The potential applications ofthis experiment include thermo-distortion compensation for precision spacestructure, stern shape control for submarines, and flap shape control foraeronautical applications. SMA wires are chosen as the actuating elements dueto their high recovery stress ({>}500 MPa) and tolerance to highstrain (up to 6%). However, SMA wires are inherently nonlinear and pose achallenge for control design. A robust controller is designed and implementedto actively control the tip position of the composite beam. The experimentset-up consists of the composite beam with embedded SMA wires, a programmablecurrent/voltage amplifier to actuate the SMA wires, an infrared laser rangesensor to detect the beam tip displacement, and a real-time data acquisitionand control system. The experimental result demonstrates the effectiveness ofthe robust control.

Application of Shape Memory Alloy Wire Actuator for Precision Position Control of a Composite Beam

In this paper are presented the design and experimental results of using a shape memory alloy (SMA) wire as an actuator for position control of a composite beam. The composite beam is honeycomb structured, having wires of SMA embedded in one of its face sheets for the purposes of active actuation. Nickel-titanium SMA wires were chosen as actuating elements due to their high recovery stress (>700 MPa) and tolerance to high strain (up to 8%). A simple proportional and derivative controller plus a feed-forward current is designed and implemented for controlling the tip position of the composite beam. Experiments have demonstrated the effectiveness of the SMA wire as an actuator for active position control of a composite beam.