Masood Taheri Andani

Microplane modeling of shape memory alloy tubes under tension, torsion, and proportional tension-torsion loading

In this study, a three-dimensional thermomechanical constitutive model based on the microplane theory is proposed to simulate the behavior of shape memory alloy tubes. The three-dimensional model is implemented in ABAQUS by employing a user material subroutine. In order to validate the model, the numerical results of this approach are compared with new experimental findings for a NiTi superelastic torque tube under tension, pure torsion, and proportional tension–torsion performed in stress- and strain-controlled manners. The numerical and experimental results are in agreement indicating the capability of the proposed microplane model in capturing the behavior of shape memory alloy tubes. This model is capable of predicting both superelasticity and shape memory effect by providing closed-form relationships for calculating the strain components in terms of the stress components.

A rate dependent tension–torsion constitutive model for superelastic nitinol under non-proportional loading; a departure from von Mises equivalency

In this work, a modified 3D model is presented to capture the multi-axial behavior of superelastic shape memory alloys (SMAs) under quasi-static isothermal or dynamic loading conditions. General experimental based equivalent stress and strain terms are introduced and improved flow rule and transformation surfaces are presented. The 3D constitutive equations are found for both isothermal and dynamic loading states. An extended experimental study is conducted on NiTi thin walled tubes to investigate the performance of the model. The proposed approach is shown to be able to capture the SMA response better than the original model in tension‐torsion loading conditions.

Modifying the torque–angle behavior of rotary shape memory alloy actuators through axial loading: A semi-analytical study of combined tension–torsion behavior

In this article, a semi-analytical modeling approach is presented to study the pseudoelastic response of shape memory alloy rods and tubes subjected to combined tension–torsion loading states. A three-dimensional phenomenological shape memory alloy constitutive model is used to obtain the corresponding two-dimensional constitutive relations. The rod is partitioned into a finite number of narrow annular regions, and the equilibrium equations are found in each region for both loading and unloading paths. The derived equations are then solved based on an iterative algorithm. A set of experiments is conducted on a shape memory alloy thin-walled tube, and the results are then used to evaluate the performance of the modeling approach. Several numerical examples along with a finite element analysis are finally presented to demonstrate the capabilities of the proposed method. Using this approach, it is possible to design active biomedical devices with shape memory alloy actuators under combined loading conditions.

Coupled rate-dependent superelastic behavior of shape memory alloy bars induced by combined axial-torsional loading: a semi-analytic modeling

In this article, the coupled thermomechanical response of superelastic shape memory alloy bars and tubes in combined tension and torsion is studied both analytically and experimentally. Using the Gibbs free energy as the thermodynamic potential and choosing appropriate internal state variables, a three-dimensional phenomenological macroscopic constitutive model for shape memory alloys is derived. Taking into account the effect of latent heat during the forward and reverse martensitic phase transformation, the appropriate form of the energy balance relation is obtained. The three-dimensional coupled relations for the energy balance in the presence of the internal heat flux and the constitutive equations are reduced to a two-dimensional form for the tension-torsion case. An explicit finite difference approach is utilized to discretize the governing boundary conditions of bars. An empirical expression for the free heat convection from the surface of a shape memory alloy bar was proposed and experimentally validated. Several sets of experiments were then carried out to evaluate the mechanical and thermal responses of the model for a shape memory alloy tube subjected to uniaxial, pure torsion and non-proportional tension and torsion loading–unloading conditions. The approach could be used in the design of shape memory alloy devices undergoing combined loads with high strain rates or in the fatigue design of shape memory alloy devices subjected to cyclic loading.

An SMA Passive Ankle Foot Orthosis: Design, Modeling, and Experimental Evaluation

Shape memory alloys (SMAs) provide compact and effective actuation for a nvariety of mechanical systems. In this work, the distinguished superelastic behavior of these materials is utilized to develop a passive ankle foot orthosis to address the drop foot disability. Design, modeling, and experimental evaluation of an SMA orthosis employed in an ankle foot orthosis (AFO) are presented in this paper. To evaluate the improvements achieved with this new device, a prototype is fabricated and motion analysis is performed on a drop foot patient. Results are presented to demonstrate the performance of the proposed orthosis.

Adaptive ankle–foot orthoses based on superelasticity of shape memory alloys

This article presents two innovative adaptive solutions for the ankle–foot orthosis based on mechanical and structural stiffness control of shape memory alloys. These concepts address gait abnormality in drop foot patients for various walking conditions such as different walking speeds. In the first design, a superelastic rod provides variable torsional stiffness that is adjusted by a controlled axial load. In the second design, the active length of superelastic hinge is adjusted in order to control the bending stiffness of the element. By adjusting the stiffness, variable level of compliance is achieved at the ankle. In both concepts, during powered plantarflexion in the stance phase of the gait, energy is stored in the shape memory alloy element. Release of this energy through superelasticity enables the ankle–foot orthosis to provide the desired controlled dorsiflexion motion in the sagittal plane and to raise the foot during the swing phase of the gait. The ultimate goal is to assist the patients in achieving a more natural gait and to prevent muscle atrophy. For the presented designs, numerical simulations are carried out to evaluate the stiffness properties of the active component under different gait speeds. To this end, experimental data of human gait are used to calculate the variation in ankle stiffness. The superelastic elements mimic the experimental ankle stiffness profiles.

Design, modeling and experimental evaluation of a minimally invasive cage for spinal fusion surgery utilizing superelastic Nitinol hinges

Spinal fusion surgery is performed to alleviate low back pain, and a cage implant is a spacer that sits in between two vertebrae to allow for bone growth and fusion, all while relieving compression of the spinal cord. This paper presents the design, modeling, and experimental evaluation of a minimally invasive cage which utilizes superelastic Nitinol elliptical shaped hinges. The actuation mechanism is presented and the employed additive manufacturing technique is discussed. The modeling approach is also introduced and experimental results are presented to demonstrate the performance of the model.

A Modified Microplane Model Using Transformation Surfaces to Consider Loading History on Phase Transition in Shape Memory Alloys

In most of the existing SMA constitutive models, it is assumed that transformation starts when a thermodynamic driving force reaches a specified amount regardless of loading history. In this work, a phenomenological approach is used to develop an enhanced one-dimensional constitutive model in which loading history is directly considered as one of the main parameters affecting the transformation start conditions. To generalize the model to three-dimensional cases, a microplane formulation based on volumetric-deviatoric is employed. A free energy potential is defined at the microplane level, integrated over all orientations at a material point to provide the macroscopic free energy. Experiments are carried out on Nitinol superelastic tubes to validate the newly proposed constitutive model. In these experiments, interruptions are applied during transformations to show the effects of loading history on transformation start conditions. Numerical results are compared with the experimental data to demonstrate the accuracy of the enhanced model.© 2014 ASME

Railway track irregularity and curvature estimation using doppler LIDAR fiber optics

The application of Doppler-based LIght Detection and Ranging (LIDAR) technology for determining track curvature and lateral irregularities, including alignment and gage variation, are investigated. The proposed method uses track measurements by two low-elevation, slightly tilted LIDAR sensors nominally pointed at the rail gage face on each track. The Doppler LIDAR lenses are installed with a slight forward angle to measure track speed in both longitudinal and lateral directions. The lateral speed measurements are processed for assessing the track gage and alignment variations, using a method that is based on the frequency bandwidth dissimilarities between the vehicle speed and track geometry irregularity. Using the results from an extensive series of tests with a body-mounted Doppler LIDAR system on-board a track geometry measurement railcar, the study indicates a close match between the LIDAR measurements and those made with existing sensors on-board the railcar. The field testing conducted during this study indicates that LIDAR sensors could provide a reliable, non-contact track monitoring instrument for field use in various weather and track conditions, potentially in a semi-autonomous or autonomous manner.

A Compliant Ankle-Foot Orthoses (AFO) Based on Multi-Axial Loading of Superelastic Shape Memory Alloys

This paper presents a novel actuation solution to address the drop foot disorder. The proposed actuator consists of a superelastic Nitinol rod with a variable torsional stiffness that is adjusted by the controlled application of an axial load. The superelastic SMA element enables the AFO to provide sufficient torque during dorsiflexion to raise the foot. The provided torque at the ankle joint assists the patient in walking more naturally and subsequently prevents further issues such as muscle atrophy.By appraising experimental data of the human gait, ankle stiffness is assessed in order to compare ankle behavior for various walking speeds during the swing phase. The adjustable compliance concept for the AFO is then explained, followed by a description of the actuation mechanism and complex loading configuration. Numerical modeling is also presented for the superelastic element of the AFO under specified multiaxial torsion-tension loading. Simulations are performed in MATLAB and variable stiffness results are compared with experimental data for verification.Copyright