Papers by Keyword: Superelastic

Abstract: Over the past decades, Shape Memory Alloys (SMAs), have revolutionized the field of seismic and structural engineering, offering their unprecedented unique properties such as superelasticity, energy dissipation, and the ability to undergo remarkable deformations and reverting to their original shape. The origins of SMA date back to the 1930s when Swedish scientist Arne Ölander initiated revolutionary research on iron alloys, exploring the distinctive characteristics of Iron-Manganese (Fe-Mn) alloy. Ever since, researchers have extensively investigated the mechanical properties of SMAs, leading to increasingly utilizing them in a wide variety of applications, including self-centering braces, structural elements, and systems frequently exposed to harsh working conditions, such as in regions susceptible to earthquakes and dynamic loading. However, a critical limitation has emerged, particularly those made of Nitinol (Nickel­­­­­­–Titanium), which possesses a smooth surface that makes it hard to implement in most structural elements, therefore anchorage systems are often required. Consequently, this smooth surface increases the possibility for slippage, therefore conventional methods to anchor steel reinforcement bars may not be applicable. A few recent studies have investigated the anchorage of SMA rebars, but there is still a big research gap. To fill this research gap, this paper presents an experimental test to evaluate the possible anchorage systems for smooth-surfaced Nitinol-SMA rebars. A total of 6 specimens were tested under uniaxial tensile loading reaching a maximum strain level up to 6%, utilizing the two different anchorage systems. The tests were conducted at a constant loading rate of 0.5 mm/min to evaluate the effectiveness of these anchorage systems. The findings show that both proposed anchorage systems are appropriate for high-deformation seismic zones since it preserved the nitinol bar to sustain up to 6% strain without showing any signs of slippage. These results provide vital insights for creating structural parts with SMA integration that are more dependable. This paper's key findings include ultimate tensile strength, force/displacement relationship, and stress/strain relationship under different constant strain. This paper highlights the need for a more thorough investigation of innovative anchorage systems suitable with SMA bars to pave the way for researchers to enable their wider application in more structural elements.

Abstract: The force transferred from the NiTi arch wire to the tooth during orthodontic treatment is strongly dependent on the geometry of the wire and brackets used by the orthodontist. This study investigated the effect of orthodontic bracket geometry on the bending behaviour of superelastic NiTi arch wire. A three-dimensional finite element models of wire bending in three-brackets configuration was developed by employing superelastic user material subroutine and contact interaction. The finite element model was used to anticipate the bending forces and stresses of NiTi wires as they were engaged in various slot width dental brackets. Four different bracket widths were considered, ranging from 1.5 mm to 4.5 mm. Throughout the bending course, the superelastic NiTi wire bent in the 4.5 mm width bracket continuously demonstrated the highest bending force in comparison to the other bracket widths. As the bracket width was raised from 1.5 mm to 4.5 mm, the wire unloading force measured at 2.5 mm deflection increased gradually from 0.40 N to 1.7 N. The bending stress of the wire deformed in the 1.5 mm width bracket was 543 MPa, band it increased to 1051 MPa when the 4.5 mm width bracket was used. The proportion of complete martensite structure at the wire curvature gradually increased as the width of the bracket slot was increased.

Abstract: The material assumptions made to facilitate Thermoelastic Stress Analysis (TSA) are linear elasticity, material homogeneity and isotropy, and mechanical properties that are independent of temperature. The unusual shape memory and superelastic properties of near equiatomic NiTi alloys complicate the application of any experimental stress analysis technique, and in the case of TSA, make these assumptions invalid. This paper describes a detailed analysis conducted to characterise the material properties of NiTi shape memory alloys and to identify loading conditions suitable for quantitative stress analysis using TSA. The mechanical behaviour of the material in three distinct regions is considered and the suitability of each region for TSA is discussed. It is shown that the thermoelastic response is dependent on the mean stress when tested at room temperature in the pre-martensitic phase, due the presence of an intermediate R-phase. Theoretical calculations are used to confirm that this effect is related to the high temperature dependence of the material’s Young’s modulus.

Abstract: The strain-resistivity behavior of Ti-45.0Ni-%.0Cu wires has been measured during superelastic cycling. The results are in good agreement with previous results, showing that the main changes of resistivity during superelastic cycling are due to Austenite ↔ Martensite phase changes and anisotropic variant orientation. The true plastic strain (dislocations) does not seem to play an important role for this alloy.