Modeling of NiTiHf Using Finite Difference Method

NiTiHf is a high temperature and high strength shape memory alloy with transformation temperatures above 100oC. A constitutive model based on Gibbs free energy is developed to predict the behavior of this material. Two different irrecoverable strains including transformation induced plastic strain (TRIP) and viscoplastic strain (VP) are considered when using high temperature shape memory alloys (HTSMAs). The first one happens during transformation at high levels of stress and the second one is related to the creep which is rate-dependent. The developed model is implemented for NiTiHf under uniaxial loading. Finite difference method is utilized to solve the proposed equations. The material parameters in the equations are calibrated from experimental data. Simulation results are captured to investigate the superelastic behavior of NiTiHf. The extracted results are compared with experimental tests of isobaric heating and cooling at different levels of stress and also superelastic tests at different levels of temperature. More results are generated to investigate the capability of the proposed model in the prediction of the irrecoverable strain after full transformation in HTSMAs

Pull-in behavior of a bio-mass sensor based on an electrostatically actuated cantilevered CNT with consideration of rippling effect

This paper studies pull-in behavior of a bio-mass sensor with a cantilevered CNT actuated electrostatically considering rippling deformation. Although this phenomenon can change the behavior of CNT remarkably, its effect on the performance of a CNT-based mass sensor is not investigated yet. This investigation is based on modified Euler-Bernoulli beam theory and rippling effect is entered the equations related to cantilevered CNT-bases sensor. Impact of other properties like different masses, mechanical damping and intermolecular force is studied in this paper too. According to the results obtained from this study rippling deformation decrease the pull-in voltage and tip deflection of CNT while it enhances the pull-in time. Results related to the impact of other mentioned properties are presented too. In order to verify the soundness of this study, the obtained results are compared with other pull-in sensor equations in literature and “molecular dynamics”, and an excellent agreement is seen in both