Superelastic Shape Memory Alloy Fibers Research Articles

The Effect of SMA Fiber Content on the Bending and Self-Recovery Performance of ECC Beams.

The addition of superelastic shape memory alloy fibers (SMAF) into engineering cementitious composites (ECC) can create a new type of SMAF-ECC composite material with good self-recovery and energy dissipation performance, which is very suitable for seismic structures. In this study, 10 groups of beam specimens with different volume contents of SMAF were fabricated, and the bending performance, deflection recovery and energy dissipation ability of these beams were studied through three-point bending cyclic loading tests. The failure mode, peak load, load-deflection curve, crack width and other indicators of the specimens were analyzed, and the relationship expression between fiber content and bending strength was established by fitting analysis. The results show that adding SMA fibers can significantly improve the peak load of ECC beams, with a maximum increase of 48.31%. The knotted SMA fibers can fully exert their superelasticity, providing the beam specimens with crack self-closing and deflection recovery ability. When the volume content of SMA fibers is 0-0.6%, the bending strength, energy dissipation ability and deflection recovery ability of the composite material beams increase with the increase in fiber content. When the volume content of SMA fibers is 0.6-1.0%, the above indicators decrease with the increase in fiber content. The suggested equations can well reflect the relationship between fiber content and beam bending strength. The research results of this paper provide theoretical support for the engineering application of SMAF-ECC composite materials.

Study on the self-recovery performance of SMAF-ECC under cyclic tensile loading

The incorporation of superelastic shape memory alloy (SMA) fibers into engineered cementitious composites (ECC) has led to a novel composite material, SMAF-ECC, which possesses excellent deformation, energy dissipation, and self-recovery capabilities. To understand the self-recovery performance of this new composite material under tensile loading, this study conducted uniaxial cyclic tension tests on SMAF-ECC specimens to investigate stress, strain, crack width, strain recovery rate, and crack recovery rate, while also considering the primary influencing factors such as SMA fiber diameter and content. The results indicate that the addition of SMA fibers improves the ultimate strain and ultimate tensile strength of ECC; increasing fiber content effectively enhances the strain and crack recovery rates of SMAF-ECC specimens, with the maximum strain recovery rate and crack recovery rate of the tested specimens reaching 69% and 77%, respectively. Both too large and too small SMA fiber diameters lead to reduced recovery rates. In this experiment, when the fiber diameter was 0.5 mm, the strain and crack recovery rates of the specimens were the highest, with a crack recovery rate of 53%, approximately 1.7 times that of specimens with 1 mm and 0.2 mm diameter SMA fibers. These findings provide a reliable basis for establishing a theoretical model of the tensile mechanical properties of SMAF-ECC composite materials.

Emerging superelastic SMA core damping elements for seismic application

Structural and non-structural damage reported after recent strong earthquakes promotes a fundamental shift in structural design target from “collapse resistance” to “fast structural function recovery”. A unique class of metal called superelastic shape memory alloy (SMA) has been emerging as a promising solution to enhance the seismic resilience of structures. SMA core damping elements are the basic constituents of many newly-proposed seismic resistant members or devices, and they are directly responsible for the safety, integrity, and economical efficiency of the structures using them. This mini-review paper offers a comprehensive summary of the emerging SMA core damping elements recently developed, covering SMA monofilament wires, fibers, bars, flat plates, U-shaped plates, angles, cables, rings, helical springs, disc springs, and friction springs. These SMA core elements are suitable for various seismic application scenarios, and their efficiency have been verified through either full-scale or proof-of-concept experimental studies. The basic working principles, advantages, and potential shortcomings of these elements are discussed, and future research needs are outlined.

Mechanical Behavior of Shape Memory Alloy Fibers Embedded in Engineered Cementitious Composite Matrix under Cyclic Pullout Loads.

The incorporation of superelastic shape memory alloy (SMA) fibers into engineered cementitious composite (ECC) materials can provide high seismic energy dissipation and deformation self-centering capabilities for ECC materials. Whether the SMA fibers can be sufficiently bonded or anchored in the ECC matrix and whether the mechanical properties of the SMA fibers in the ECC matrix can be effectively utilized are the key scientific issues that urgently need to be studied. In order to study the mechanical behavior of SMA fiber embedded in ECC matrix, four groups of semi-dog-bone pullout specimens were fabricated, and the cyclic pullout tests were conducted in this paper. The pullout stress, displacement, and self-centering capability were analyzed, and different influencing factors were discussed. The results show that the knotted ends can provide sufficient anchorage force for SMA fibers, and the maximum pullout stress of SMA fiber can reach 1100 MPa, thus the superelasticity can be effectively stimulated. The SMA fibers show excellent self-centering capability in the test. The minimum residual deformation in the test is only 0.29 mm, and the maximum self-centering ratio can reach 0.93. Increasing bond length can increase the ultimate strain of SMA fibers with knotted ends, but reduce the maximum pullout stress. Increasing fiber diameter can increase both the ultimate strain and the maximum stress of knotted end SMA fibers. While neither bond length nor fiber diameter has significant effect on the self-centering ratio. This paper provides a theoretical basis for further study of the combination of SMA fibers and ECC materials.

Experimental Study on Self-Centering Performance of the SMA Fiber Reinforced ECC Composite Beam.

The combination of superelastic shape memory alloy fibers and ECC materials can form a new SMA fiber reinforced ECC composite material (SMAF-ECC) with good self-centering performance. In order to study the self-centering performance of the new composite material, 6 groups of pre-notch beam specimens were made for three-point bending cyclic loading tests, and the failure phenomenon, hysteresis curve, self-centering effect and influencing factors of the specimens were analyzed. The research results show that when the SMA fibers are effectively anchored in the ECC matrix, the SMA fibers can exert the superelastic properties to provide the ECC beams with recoverying force, and realize the crack self-closure and deflection self-recovery function for the beams, with the minimum residual crack width and deflection is only 0.9 mm and 1.3 mm respectively. Increasing fiber content can cause a small increase in the self-centering ability of the beams. However, only when the fiber diameter is appropriate, better self-centering effect can be achieved, but the difference caused by fiber diameter in the test was only 5%. SMA Fiber end forms have significant influence on self-centering performance. The knotted end beam can get a more than 70% self-centering ratio, while the straight end beams and bended end beams have no self-centering ability. The research results provide important reference for the research and application of this new self-centering materials and their structures.

Effect of 3D, 4D, and 5D hooked-end type and loading rate on the pull-out performance of shape memory alloy fibres embedded in cementitious composites

This paper investigates the single fibre pull-out performance of superelastic shape memory alloy fibres (SMAFs) embedded in a highly flowable cementitious matrix suitable for producing ultra-high performance fibre reinforced cementitious composites. The research emphasis is placed on the influence of hooked-ends and loading rate on the pull-out behaviour of SMAFs. Full-range pull-out load-slip responses are captured for straight and 3D, 4D, and 5D hooked-end SMAFs under three loading rates ranging from 0.025 mm/s (representing quasi-static loading) to 25 mm/s (representing seismic loading). Pull-out test is also carried out on steel fibres (SFs) of similar geometries as SMAFs for comparison reasons. Pull-out responses are analysed in terms of maximum tensile stress in the fibre, material usage factor, pull-out energy, average and equivalent bond strength. The results obtained show lower pull-out resistance for SMAFs comparing to their steel counterparts. Although the use of more bends at hooked-end of SFs (5D versus 3D) significantly enhances the average and equivalent bond strength of SFs at a given loading rate, its efficiency was much lower for SMAFs. The deformation process that fibres undergo when they travel through the matrix tunnel can explain such a difference between SMAFs and SFs. Experimental results show loading rate dependency for pull-out resistance of hooked-end SMAFs in terms of both average and equivalent bond strength. In contrast, loading rate dependency is observed only for equivalent bond strength in the case of straight SMAFs.

The effect of shape memory alloy, steel, and carbon fibres on fresh, mechanical, and electrical properties of self-compacting cementitious composites

This paper presents the initial results of a two-phase research project for developing re-centring self-sensing cementitious composites. The focus here is on fresh, mechanical, and electrical properties of the developed cementitious composites. Nickel-titanium (NiTi) superelastic shape memory alloy (SMA) fibres and carbon fibres were used in this study. The performance of composites was studied based on the slump test, four-point bending test, flexural toughness, compression test, direct tensile test, and electrical conductivity test. The performance of SMA fibre-reinforced self-compacting cementitious composites was also compared with their steel fibre-reinforced counterparts. The addition of SMA and steel fibres slightly decreased the relative slump while carbon fibres reduced the flowability of mixture significantly. The increase in SMA and steel fibre content enhanced the flexural and tensile post-peak performance of composite, especially for specimens containing 1%–1.5% fibres by volume. Compressive strength was observed to decrease slightly by using SMA and steel fibres up to 1%, after which a reverse trend was detected. Adding SMA and steel fibres up to 1.5% did not affect the conductivity of composite considerably. On the contrast, carbon fibres even at low content (i.e., 0.1%) increased the conductivity of the composite substantially. The percolation transition zone detected for carbon fibre-reinforced cementitious composites showed an optimum carbon fibre dosage of 0.3% in terms of electrical conductivity.

The synergistic effects of shape memory alloy, steel, and carbon fibres with polyvinyl alcohol fibres in hybrid strain-hardening cementitious composites

The current paper discusses the first results of a comprehensive research project, aiming at developing a strain-hardening cementitious composite (SHCC) with self-sensing and re-centring capabilities. Superelastic shape memory alloy (SMA) can be used to provide the composite with re-centring behaviour as SMA is capable of recovering large strains. Carbon fibres are proved to be very useful for the self-sensing performance of cementitious composites. Therefore, an experimental programme was designed in the first stage of the project to investigate the effect of fibre hybridisation on the fresh, mechanical, and electrical properties of SHCC with polyvinyl alcohol (PVA) fibres. Steel fibres (SFs) were also involved in the experiments for comparison reasons. It was found that such hybridisation using SMA fibres (SMAFs) and SFs enhanced the flexural performance of composite significantly, but with a decrease in compressive characteristics of composite for some of the studied fibre contents. The electrical resistivity of SHCC with PVA fibre remained almost unchanged by the addition of SMAFs and SFs within the studied range. Furthermore, considering the type of CFs used in this study, 0.2% CFs by volume is the optimum CF content for developing a conductive SHCC without any adverse effect of CFs on the flexural performance of the composite.

Investigating Self-Centering Capacity of Superelastic Shape Memory Alloy Fibers with Different Anchorages Through Pullout Tests.

This study investigated the pull-out resistance of superelastic shape memory alloy (SMA) short fibers in mortar with consideration of various end-anchorages that provide different anchoring actions. For the purpose, four types of SMA fibers were prepared using NiTi SMA wires with a diameter of 1.0 mm and the following four end shapes: straight (ST), L-shaped (LS), N-shaped (NS), and spearhead-shaped (SH). The straight-ended fiber was a reference with no working on the end, and the fiber with the spearhead-shaped end was crimped to make the end part flat. The fibers with L- and N-shaped ends were bent with single or double bending. The results showed that only the spearhead-shaped fibers showed self-centering behavior because of the superelasticity of the SMA after slip occurred. This paper discusses the reasons that the ST, LS, and NS fibers do not show self-centering behavior and proposes a concept to induce superelastic behavior in SMA fibers in mortar or concrete.

Tensile and fatigue behavior of polymer composites reinforced with superelastic SMA strands

This study explores the use of superelastic shape memory alloy (SMA) strands, which consist of seven individual small-diameter wires, in an epoxy matrix and characterizes the tensile and fatigue responses of the developed SMA/epoxy composites. Using a vacuum assisted hand lay-up technique, twelve SMA fiber reinforced polymer (FRP) specimens were fabricated. The developed SMA-FRP composites had a fiber volume ratio of 50%. Tensile response of SMA-FRP specimens were characterized under both monotonic loading and increasing amplitude loading and unloading cycles. The degradation in superelastic properties of the developed SMA-FRP composites during fatigue loading at different strain amplitudes was investigated. The effect of loading rate on the fatigue response of SMA-FRP composites was also explored. In addition, fractured specimens were examined using the scanning electron microscopy (SEM) technique to study the failure mechanisms of the tested specimens. A good interfacial bonding between the SMA strands and epoxy matrix was observed. The developed SMA-FRP composites exhibited good superelastic behavior at different strain amplitudes up to at least 800 cycle after which significant degradation occurred.

Pullout behavior of superelastic SMA fibers with various end-shapes embedded in cement mortar

This study investigated the effects of superelastic shape memory alloy (SMA) fibers end-shape on pullout resistance through hysteretic pullout testing. Superelastic NiTi SMA wire of 1.0 mm diameter was employed in manufacturing short fibers. SMAs were fabricated with four end-shapes: 1) prismatic and straight end, 2) L-shaped end, 3) N-shaped end, and 4) crimped end with a spearhead. The embedded length of a fiber into the mortar matrix having compressive strength of 50.0 MPa was 18.0 mm, except for the N-shaped end fiber for which the embedded length was 21.0 mm. The pullout test was conducted with displacement control to obtain hysteretic pullout behavior by four cycling loadings. The results showed that crimped-end fibers considerably magnified the pullout strength, and the deformation was recovered by their superelastic behavior. As N-shaped end fibers could not show flag-shaped behavior, another test was performed through which fibers were annealed to induce superelasticity, so, a perfect strain recovery was obtained. Results showed that additional annealing of the N-shaped fibers after the manufacturing improved superelastic behavior at the bended part.

Behavior of mortar beams with randomly distributed superelastic shape memory alloy fibers

The addition of fibers to cementitious composites can provide improved ductility, energy dissipation, and resistance to cracking. However, it is also important to minimize residual deformations and...

Sustainability of civil infrastructure using shape memory technology

The last few decades have clearly demonstrated the vulnerability of our civil infrastructure systems to problems like aging, natural, and man-made hazards including earthquakes, hurricanes, blasts, etc. The conventional materials such as steel and concrete have proven to be limited in terms of their ability to withstand the extreme demands imposed on them by modern societies. The limitations in currently used construction materials combined with the consistently growing population worldwide present new challenges and demands for researchers in the field of structural engineering. Hence, there is an urgent need for new materials that are capable of extending the service life of structures with minimal or no need for maintenance or repairs against natural and man-made hazards. Shape memory alloy (SMA) is a class of “Smart Materials” that have recently emerged as potential construction material with unique thermomechanical properties, namely shape memory effect and superelasticity. Two applications of SMAs in civil structures are discussed in this paper. The first application involves the use of SMA in performing seismic rehabilitation of RC bridge columns that lack flexural ductility. In this application, SMA is used in the form of thermally prestressed spirals that can apply large active confinement pressure to the columns at their plastic hinge regions to improve their flexural ductility. The experimental results of large scale shake table tests performed on two RC columns, one of which is retrofitted with SMA are discussed. The results demonstrate the great ability of SMA spirals in mitigating the damage even under strong levels of ground shaking. The second application focuses on utilizing superelastic SMA fibers as reinforcement for polymeric composite bars. The newly developed composite material is named SMA–Fiber Reinforced Polymer (SMA–FRP), and is studied as seismic reinforcing bars for moment resisting concrete frames. The results of nonlinear time history analysis prove that using SMA–FRP bars at the plastic hinge regions of the frames helps significantly in limiting the residual drifts and enhancing the energy dissipation of the frames.

Acoustic emission analysis of cyclically loaded superelastic shape memory alloy fiber reinforced mortar beams

Superelastic shape memory alloys (SMAs) are metallic alloys that can recover their nonlinear deformations upon unloading. The use of SMAs in cementitious composites as fibers can enable crack recovery and re-centering capabilities. In this study, the crack recovery characteristics of SMA fiber reinforced mortar specimens subjected to flexural cyclic loading are studied through acoustic emission analysis. SMA fiber reinforced mortar specimens with fiber volume ratios of 0.3%, 0.5% and 1.0% are tested under cyclic flexural loads. Acoustic emissions (AE) signals are captured and analyzed to characterize the crack formation and crack width development at each cycle of loading. In addition, the crack width propagation of the specimens is monitored by Digital Image Correlation measurement system and is used as a basis in AE analyses. Results show that the SMA fiber reinforced mortars exhibit considerable re-centering and crack recovery characteristics at large deformation amplitudes.

Monotonic and hysteretic pullout behavior of superelastic SMA fibers with different anchorages

Abstract The aim of this study is to assess the pullout resistance of superelastic shape memory alloy (SMA) short fibers with different end shapes, which provide different anchoring actions, through monotonic and hysteretic pullout tests. For this study, NiTi superelastic SMA wire with a diameter of 1.0 mm was prepared and cut by a length of 40 mm to make SMA short fibers. Four types of SMA fibers with different end shapes were manufactured namely: 1) straight end shape, 2) crimped end, 3) bended end with L-shape, and 4) spearhead end. The straight end-shaped fiber was one without any anchoring action on the end part. The crimped fiber had grooves on the two sides manufactured by crimping an end part of 5 mm. The end-bended fiber had an L-shaped end with a 30° bending angle. The fiber with a spearhead was manufactured by pressing the end part of 5 mm. The pullout tests of the fibers from the mortar matrix were first monotonically conducted with displacement control, and the hysteretic pullout behavior was obtained by 4 cyclic loadings. The L-shaped fibers increased the pullout resistance significantly compared with the straight and crimped fibers. However, they did not reach the upper plateau stress of the SMA fiber to induce state transformation. Only the fibers with a spearhead end exceeded the stress for the state transformation because they provided sufficient anchoring resistance due to the spearhead. The maximum pullout resistance of the spearhead fiber was 3.74 times that of the L-shaped fiber. Moreover, they showed flag-shaped behavior during the hysteretic tests.

Properties and response of composite material with spheroidal superelastic shape memory alloy inclusions subject to three-dimensional stress state

This communication illustrates the method of evaluation of the stiffness tensor and the stress–strain relationship of a particulate composite material with randomly distributed spheroidal superelastic shape memory alloy (SMA) inclusions that is subject to three-dimensional stresses. The Mori–Tanaka homogenization technique in conjunction with the Tanaka phenomenological SMA theory is applied to develop relations between the properties of the composite material and the applied stresses. In addition, the composite stress–strain loop reflecting hysteresis within the SMA material is generated. It is shown that the energy dissipation within a metal matrix composite with spherical SMA inclusions is comparable to that observed in composites with SMA fibres. The solution is exact within the framework of the assumptions of the Mori–Tanaka and phenomenological SMA theories.

Damping, tensile, and impact properties of superelastic shape memory alloy (SMA) fiber-reinforced polymer composites

The potential of superelastic shape memory alloy (SMA) fibers to enhance the damping capacity and toughness of a thermoset polymer matrix was evaluated. A single-fiber winder was designed and built to manufacture a pre-form consisting of 102 μm diameter SMA fibers aligned parallel to each other. This pre-form was loaded to varying amounts of pre-strain and impregnated with vinyl ester to manufacture SMA fiber composites with 20% fiber volume fraction. The composites were tested using a Differential Scanning Calorimeter (DSC) and a Dynamic Mechanical Analysis (DMA), to evaluate the improvement in damping capacity of the polymer matrix due to the SMA fibers. Tensile and instrumented impact testing were carried out to evaluate improvements in mechanical properties and toughness of the composites. Appreciable improvement was observed in damping, tensile, and impact properties of the polymer matrix due to reinforcement with superelastic SMA fibers, highlighting the advantages of their use in polymer composites.

Embedding of Superelastic SMA Wires into Composite Structures: Evaluation of Impact Properties

Shape memory alloy (SMA) represents the most versatile way to realize smart materials with sensing, controlling, and actuating functions. Due to their unique mechanical and thermodynamic properties and to the possibility to obtain SMA wires with very small diameters, they are used as smart components embedded into the conventional resins or composites, obtaining active abilities, tunable properties, self-healing properties, and damping capacity. Moreover, superelastic SMAs are used to increase the impact resistance properties of composite materials. In this study, the influence of the integration of thin superelastic wires to suppress propagating damage of composite structures has been investigated. Superelastic SMAs have very high strain to failure and recoverable elastic strain, due to a stress-induced martensitic phase transition creating a plateau region in the stress-strain curve. NiTi superelastic wires (A f = −15 °C fully annealed) of 0.10 mm in diameter have been produced and characterized by SAES Getters. The straight annealed wire shows the typical flag stress-strain behavior. The measured loading plateau is about 450 MPa at ambient temperature with a recoverable elastic strain of more than 6%. For these reasons superelastic SMA fibers can absorb much more strain energy than other fibers before their failure, partly with a constant stress level. In this paper, the improvement of composite laminates impact properties by embedding SMA wires is evaluated and indications for design and manufacturing of SMA composites with high-impact properties are also given.

Ballistic Impact Resistance of SMA and Spectra Hybrid Graphite Composites

The effect of adding small amounts of high strain hybrid components on the impact resistance of graphite epoxy composites subjected to projectiles traveling at ballistic velocities (greater than 900 ft/sec) has been studied. The hybrid components tested include superelastic shape memory alloy (SMA) and a high performance extended chain polyethylene (ECPE) known as Spectram. In all cases, the embedded SMA fibers were pulled through the graphite without straining to their full potential. It is believed that this is due to high strain rate effects coupled with a strain mismatch between the tough SMA and the brittle epoxy resin. However, a significant increase in energy absorption was found by adding ECPE and ECPE/SMA layers to the backface of the composite.

The Response of SMA Hybrid Composite Materials to Low Velocity Impact

Composite material impact damage tolerance and resistance are often limiting crite ria when composites are considered for critical load bearing applications. Often undetectable to visual inspection, foreign object impact causes varying degrees of damage to composite structures which can initiate fatal material failures under various operating loads. This paper examines a new concept of composite material toughening by embedding small amounts of fibrous superelastic shape memory alloy (SMA) materials into brittle composite materials. In this preliminary study, a 2.8 per cent volume fraction of activated (i.e., superelastic) shape memory alloy fibers were embedded within G40-600/5245C graphite/bismaleimide to create a hybridized composite material. Impact test results show that with the SMA fibers, impact resistance of the hybrid composite materials was increased. At impact energies that perforate the standard graphite/bismaleimide, the SMA fibers stopped perforation of the hybridized composite. Preliminary results show that composite-ply delamination was reduced as much as 25 percent because of the embedded SMA fibers.