Papers by Keyword: Shape Recovery

Abstract: As new generation of high-temperature shape memory alloys, high-entropy alloys (HEAs) have been attracted for strong solid-solution hardened alloys due to their severe lattice distortion and sluggish diffusion. TiPd is the one potential high-temperature shape memory alloys because of its high martensitic transformation temperature above 500 °C. As constituent elements, Zr expected solid-solution hardening, Pt expected increase of transformation temperature, Au expected keeping transformation temperature, and Co expected not to form harmful phase. By changing the alloy composition slightly, two HEAs and two medium entropy alloys (MEAs) were prepared. Only two MEAs, Ti₄₅Zr₅Pd₂₅Pt₂₀Au₅, and Ti₄₅Zr₅Pd₂₅Pt₂₀Co₅ had the martensitic transformation. The perfect recovery was obtained in Ti₄₅Zr₅Pd₂₅Pt₂₀Co₅ during the repeated thermal cyclic test, training, under 200 MPa. On the other hand, the small irrecoverable strain was remained in Ti₄₅Zr₅Pd₂₅Pt₂₀Au₅ during the training under 150 MPa because of the small solid-solution hardening effect. It indicates that Ti₄₅Zr₅Pd₂₅Pt₂₀Co₅ is the one possible HT-SMA working between 342 and 450 °C.

Abstract: Shape recovery fiber reinforced self-healing plastic which achieves full strength recovery is developed. In this system, microcapsules containing dicyclopentadiene (DCPD) as healing agent are dispersed in epoxy resin matrix. Moreover, thermal shrinkage fibers coated by weak interlayer with Grubbs catalysts are uniaxially oriented. When a crack is introduced to the material, it propagates through the matrix and branches along the interlayer while breaking the microcapsules. Then DCPD flows from microcapsules into the cracks and polymerizes. At the same time, the fibers thermally shrink due to the polymerization heat, and the cracks are completely healed and closed. Furthermore, shape recovery ratio is defined as ratio of "closed crack width" with respect to "initial crack width", and can be rewrote as ratio of loads. Tensile tests were carried out using flat plates with a notch, and they were pulled in the fiber direction. After the crack introduction, the displacement was kept constant. The specimen was heated to about 70°C, and temperature and load change were measured until the stress reduction was done. As a result, the shape recovery ratio was calculated to be 7.89% due to the temperature rose caused by polymerization. Therefore, the full strength recovery can be expected by demonstrating the shape recovery effect of this material.

Abstract: Ti alloys are attractive materials for such applications, they are expensive due to the costly alloying elements such as Nb or Mo. The present authors have adopted Mn as a low-cost alloying element, and melted Ti-7, 7.5 and 8 mass%Mn-1.5 and 3mass%Al alloys using a laboratory-scale arc furnace. All specimens prepared from bottom ingots were heat treated at 1223 K for 3.6 ks and quenched in ice water. In the 7 and 7.5Mn-Al alloys, the β phase and orthorhombic martensite were identified using X-ray diffraction. In the 8Mn-Al alloys, only the β phase was identified. In the 7, 7.5, and 8Mn-Al alloys, the electrical resistivity at room and liquid nitrogen temperature increased with increasing Al content due to dissolution of Al into the β phase, whereas the Vickers hardness decreased with increasing Al content due to decreasing formation of athermal omega by the addition of Al. Heat treatment at 673 K for 60 s almost completely returned deformed Ti-7 and 7.5Mn-3Al specimens to their original shapes, and heat treatment at 773 K for 60 s almost returned deformed Ti-8Mn-Al specimens to their original shapes.

Abstract: The present work deals with the optimization of micro-WEDM process parameters for machining Ti_49.4-Ni_50.6 shape memory alloy (SMA) for orthopedic implant application. Effect of micro-WEDM parameters viz. Gap voltage, capacitance, wire feed and wire tension on the response variables such as material removal rate, surface roughness, kerf width and dimensional deviation is determined. As Ti-Ni SMA has fascinating properties and bio-compatibility, have been considered for present work. Nine experiments have been performed on micro-WEDM based on an orthogonal array of Taguchi method. Subsequently, the grey relational analysis (GRA) method is applied to determine an optimal set of process parameters. It is observed that optimized set of parameters A3B3C3D1 viz. 140 V gap voltage, 0.4 µF capacitance, wire feed 30 µm/sec and 30% of wire tension determined by using GRA offers maximum MRR and minimum SR, KW and DD. From the Analysis of Variance, it is seen that the process parameter capacitance is the most significant parameter for multi-response optimization with a percentage contribution of 77.91%. Young’s modulus also checked for biocompatibility. Also, SEM images are taken to confirm the results offering better surface quality. Heat treatment process like annealing is found to be the most suitable to recover shape memory effect of WEDMed samples.

Abstract: TiPd was investigated as a candidate of high-temperature shape memory alloys. To improve shape recovery, solid-solution hardening by addition of alloying element has been performed. The effect of alloying on martensite transformation temperature, shape memory effect, and yield strength of martensite and austenite phases were investigated. Zr and Hf were found to be effective element to improve shape memory effect. The most important factor to improve shape memory effect of TiPd is temperature to form Ti₂Pd₃ precipitates rather than strengthening.

Abstract: In this study, multilayered composite plates with shape memory properties were produced: carbon fibers prepreg are alternated with layers of shape memory epoxy powder obtaining composite with different number of layers. The differences in the load exerted during shape recovery, and percentage and time of recovery of the composites as a function of layers number have been evaluated. In particular, the actuation load and the shape recovery percentage were measured after a V-shape memorizing step of the composites. The experimental results are very promising, showing that such multilayers can successfully recover the original shape without noticeable damages and an increasing of actuation load per layer has been found at the increase of the layers number.

Abstract: Shape memory effects, the course and characteristic temperatures of phase transitions and mechanical properties and surface properties of NiTi alloys strongly depend on the chemical composition, the production process used and the plastic working and thermomechanical treatment as well as surface treatment. The test alloy was obtained by vacuum metallurgy by melting the components in a graphite crucible and casting into a graphite ingot mould. In order to obtain the rods in the process of hot plastic working, hot forging was applied using a smith hammer and rotary hot forging on a swaging machine. The resulting rods were subjected to an appropriate heat treatment and thermo mechanical treatment to obtain, at room temperature, a parent phase structure B2.The paper presents the results of the research of NiTi rods after hot rotary forging. The phase composition of the samples of the tested alloy after different heat treatments were determined by X-ray powder diffraction technique. It was found that at room temperature, depending upon the processing the samples had a structure parent phase with a small amount of martensite. The courses of phase transitions and the changes of the temperature characteristic were determined on the basis of the recorded DSC curves. It was found that the test rate after aging in the temperature range of 400-500 °C transitions take place involving the rhombohedral R phase. The temperature ranges of shape recovery of samples after various heat treatments were determined by recording the recovery of the shape during heating, in tests performed according to the standard ASTM 2082-06.

Abstract: A shape memory composite plate has been produced by adding a shape memory interlayer between two carbon-fibre epoxy-matrix prepregs. A Small plate has been obtained which was used for testing under bending at different rates up to the final failure. The first test was performed at low rate and final damage was recovered by heating. A second bending test was performed on the self-repaired plate at high rate. As a result, a good shape recovery after damaging has been observed and a fairly good residual stiffness was measured. The residual stiffness after shape recovery was about 80% and the composite plate was able to withstand a second test at higher rate (with a residual strength about 40%). Results show that it is possible to combine shape recovery of shape memory polymers with structural properties of composite materials: the goal is producing composite structural

Abstract: Shape memory epoxy foams are a new class of materials for aerospace applications as light actuators, structural parts with reduced size during transport, and expandable/deployable structures. They were tested in an experiment onboard of the International Space Station in May 2011 (Shuttle Mission STS 134) and in April 2013, on board the BION-M1 capsule through the Soyuz-2 launch vehicle, with the aim to study the behavior in microgravity for future applications. The experiments were performed by an autonomous device which was in turn composed of control and heating system, battery pack and data acquisition system. Micro-gravity does not affect the ability of the foams to recover their shape but it poses limits for the heating system design because of the difference in heat transfer on earth and on orbit. This could be very significant for the behaviour of complex multi-functional structures in which shape memory epoxy foams are integrated. In this work, the main results of the experiments in microgravity are discussed and some results of tests on ground are shown in order to evaluate new possible developments in the field.

Abstract: Shape memory characteristics of woven glass and carbon fiber fabric reinforced epoxy resin-based composites were assessed in bending mode using a dynamic mechanical analyzer. The reinforcement strongly improved the recovery stress but impaired the bending deformability. Composites with asymmetric fabric lay-up showed better performance when the reinforced section experienced local tension than compression during flexural loading.