In this paper, the grain microstructure and strain partitioning in a polycrystalline NiTi wire subjected to tensile loading was reconstructed from an experimental 3D-XRD dataset. The reconstruction of a volume containing more than 8000 stressed grains involved optimization with respect to both the geometrical features and material elastic properties. The geometrical features of the microstructure were reconstructed using Laguerre tessellations based on the experimental 3D-XRD dataset. Two different algorithms fitting Laguerre tessellations were applied in order to assess the sensitivity of the reconstruction to the choice of the algorithm. The material properties in terms of elastic anisotropy were refined from an initial published value to minimize the mismatch between experiment and simulation using an optimization algorithm based on linear elasticity simulations. As a result of this, we constructed a numerical microstructure model that statistically matches the experimentally probed material in terms of positions and sizes of grains as well as partitioning of elastic strain and stress in the microstructure (average elastic properties and standard deviations of piecewise constant components of elastic strain and stress tensors in grains).
In spite of the fact that the US Food and Drug Administration (FDA) has approved Intracoil and s.m.a.r.t Nitinol stents for superficial femoral artery (SFA), some alternative designs of Nitinol stents are being implanted today, representing the off-label use of the devices. Among the currently stents used for the SFA, s.m.a.r.t and Intracoil stents show the most desirable long-term results but it is not understood why. In the present work, delivery of the s.m.a.r.t and Intracoil Nitinol stents and their release inside a stenotic point of the artery was simulated by FE implemented SMA model using a creative manual controlled method. The influence of the stent design on the stent–vessel interactions and stress state within the stent material after completion of the deployment was revealed. It was found that the Intracoil stent shall be more successful in eliminating the stenosis and less prone to fatigue failure, even though it had less thickness and less mean coil diameter.
Numerous attachment systems exist for implant-supported overdentures, with each having specific limitations in terms of retention, cost, wear, maintenance and cleanability. A retrospective analysis of patients restored with implant-supported overdentures using bars, telescopic crowns and Locator-type attachments was performed and the patients were interviewed. An in vitro strain gauge study compared telescopic crowns, Locator-type attachments and a novel flexible attachment system employing a shape memory alloy (NiTi) with respect to peri-implant strain development during insertion, loading and removal of an overdenture. A significantly lower number of attachment-related complications was observed in bars as compared to telescopic crowns (p = 0.00007) and Locator-type attachments (p = 0.00000), respectively. Greater overall patient satisfaction was noted in bar-retained restorations while Locator-type attachments led to lower levels of satisfaction regarding prosthesis retention. In vitro, telescopic crowns caused maximum strain development during prosthesis insertion and loading, while during removal this was observed in Locators with white retentive inserts. NiTi attachments caused significantly lower strain development during insertion as compared to telescopic crowns (p = 0.027). During loading, NiTi attachments caused significantly lower strain development than Locators with blue retentive inserts (p = 0.039). During removal, NiTi attachments caused significantly less strain development as compared to Locators with white retentive inserts (p = 0.027). Positional discrepancies between male and female attachment parts affected the retention and reaction force between both components, which may be minimized by using the novel NiTi attachment system. This may be beneficial in terms of component wear and implant loading.
The shape memory mechanism associated with R-phase transformation was investigated using electromechanical tests on Ni-Ti shape memory alloy thin wires. To provide a more precise insight into the effects of R-phase on phase transformation of shape memory alloys, wires were prepared in three different initial states prior to conducting the experiments: pure martensite, mixture of R-phase and martensite, and pure R-phase. The electromechanical tests were done in two different loading states (under constant and variable stresses) using specially designed and manufactured apparatuses. Tests under different constant stresses were carried out for three cases of pure martensite, a mixture of martensite and rhombohedral, and pure rhombohedral phase, while those under variable stresses were done on pure martensite as well as mixed martensite and rhombohedral. For all the expressed cases, strain–time responses were investigated during electric heating–cooling cycles to interpret possible microstructural changes in the samples. According to the results, a phase diagram containing R-phase transition strip was proposed for Ni-Ti shape memory alloys.
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