# Browsing by Subject "shape memory alloys"

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Item Discrete Preisach Model for the Superelastic Response of Shape Memory Alloys(2012-02-14) Doraiswamy, SrikrishnaShow more The aim of this work is to present a model for the superelastic response of Shape Memory Alloys (SMAs) by developing a Preisach Model with thermodynamics basis. The special features of SMA superelastic response is useful in a variety of applications (eg. seismic dampers and arterial stents). For example, under seismic loads the SMA dampers undergo rapid loading{unloading cycles, thus going through a number of internal hysteresis loops, which are responsible for dissipating the vibration energy. Therefore the design for such applications requires the ability to predict the response, particularly internal loops. It is thus intended to develop a model for the superelastic response which is simple, computationally fast and can predict internal loops. The key idea here is to separate the elastic response of SMAs from the dissipative response and apply a Preisach Model to the dissipative response as opposed to the popular notion of applying the Preisach Model to the stress{strain response directly. Such a separation allows for the better prediction of internal hysteresis, avoids issues due to at/negative slopes in the stress{strain plot, and shows good match with experimental data, even when minimal input is given to the model. The model is developed from a Gibbs Potential, which allows us to compute a driving force for the underlying phase transformation in the superelastic response. The hysteresis between the driving force for transformation and the extent of transformation (volume fraction of martensite) is then used with a Preisach model. The Preisach model parameters are identi ed using a least squares approach. ASTM Standards for the testing of NiTi wires (F2516-07^sigma 2), are used for the identi cation of the parameters in the Gibbs Potential. The simulations are run using MATLAB R . Results under di erent input conditions are discussed. It is shown that the predicted response shows good agreement with the experimental data. A couple of attempts at extending the model to bending and more complex response of SMAs is also discussed.Show more Item Modeling of Shape Memory Alloys Considering Rate-independent and Rate-dependent Irrecoverable Strains(2011-02-22) Hartl, Darren J.Show more This dissertation addresses new developments in the constitutive modeling and structural analysis pertaining to rate-independent and rate-dependent irrecoverable inelasticity in Shape Memory Alloys (SMAs). A new model for fully recoverable SMA response is derived that accounts for material behaviors not previously addressed. Rate-independent and rate-dependent irrecoverable deformations (plasticity and viscoplasticity) are then considered. The three phenomenological models are based on continuum thermodynamics where the free energy potentials, evolution equations, and hardening functions are properly chosen. The simultaneous transformation-plastic model considers rate-independent irrecoverable strain generation and uses isotropic and kinematic plastic hardening to capture the interactions between irrecoverable plastic strain and recoverable transformation strain. The combination of theory and implementation is unique in its ability to capture the simultaneous evolution of recoverable transformation strains and irrecoverable plastic strains. The simultaneous transformation-viscoplastic model considers rate-dependent irrecoverable strain generation where the theoretical framework is modfii ed such that the evolution of the viscoplastic strain components are given explicitly. The numerical integration of the constitutive equations is formulated such that objectivity is maintained for SMA structures undergoing moderate strains and large displacements. Experimentally validated analysis results are provided for the fully recoverable model, the simultaneous transformation-plastic yield model, and the transformation-viscoplastic creep model.Show more Item Neuro-fuzzy model of superelastic shape memory alloys with application to seismic engineering(2009-05-15) Ozbulut, Osman EserShow more Shape memory alloys (SMAs) have recently attracted much attention as a smart material that can be used in passive protection systems such as energy dissipating devices and base isolation systems. For the purpose of investigating the potential use of SMAs in seismic engineering applications a soft computing approach, namely a neurofuzzy technique is used to model dynamic behavior of CuAlBe shape memory alloy wires. Experimental data are collected from two test programs that have been performed at the University of Chile. First, in order to evaluate the effect of temperature changes on the behavior of superelastic SMA wires, a large number of cyclic, sinusoidal, tensile tests are conducted at various temperatures. Second, to assess dynamic effects of the material, a series of laboratory experiments are conducted on a scale model of a three story model of a building that is stiffened with SMA wires and given excitation by a shake table. Two fuzzy inference systems (FISes) that can predict hysteretic behavior of CuAlBe wire have been created using these experimental data. Both fuzzy models employ a total of three input variables (strain, strain-rate, and temperature or prestress) and one output variable (predicted stress). Values of the initially assigned membership functions for each input are adjusted using a neural-fuzzy procedure to accurately predict the correct stress level in the wires. Results of the trained FISes are validated using test results from experimental records that had not been previously used in the training procedure. Finally, numerical simulations are conducted to illustrate practical use of these wires in a civil engineering application. In particular, dynamic analysis of a single story frame and a three story benchmark building that are equipped with SMA damping elements are conducted. Then, an isolated bridge that utilizes a linear rubber bearing together with SMA elements is analyzed. Next, in order to show recentering ability of SMAs, nonlinear time history analysis of a chevron like braced frame is implemented. The results reveal the applicability for structural vibration control of CuAlBe wire whose highly nonlinear behavior is modeled by a simple, accurate, and computational efficient FIS.Show more Item Structural Thermomechanical Models for Shape Memory Alloy Components(2014-04-18) Rao, AshwinShow more Thermally responsive shape memory alloys (SMA) demonstrate interesting properties like shape memory effect (SME) and superelasticity (SE). SMA components in the form of wires, springs and beams typically exhibit complex, nonlinear hysteretic responses and are subjected to tension, torsion or bending loading conditions. Traditionally, simple strength of materials based models/tools have driven engineering designs for centuries, even as more sophisticated models existed for design with conventional materials. In light of this, an effort to develop strength of materials type modeling approach that can capture complex hysteretic SMA responses under different loading conditions is undertaken. The key idea here is of separating the thermoelastic and the dissipative part of the hysteretic response by using a Gibbs potential and thermodynamic principles. The dissipative part of the response is later accounted for by a discrete Preisach model. The models are constructed using experimentally measurable quantities (like torque?twist, bending moment?curvature etc.), since the SMA components subjected to torsion and bending experience an in-homogeneous non-linear stress distribution across the specimen cross-section. Such an approach enables simulation of complex temperature dependent superelastic responses including those with multiple internal loops. The second aspect of this work deals with the durability of the material which is of critical importance with increasing use of SMA components in different engineering applications. Conventional S-N curves, Goodman diagrams etc. that capture only the mechanical loading aspects are not adequate to capture complex thermomechanical coupling seen in SMAs. Hence, a novel concept of driving force amplitude v/s number of cycles equivalent to thermodynamical driving force for onset of phase transformations is proposed which simultaneously captures both mechanical and thermal loading in a single framework. Recognizing the paucity of experimental data on functional degradation of SMAs (especially SMA springs), a custom designed thermomechanical fatigue test rig is used to perform user defined repeated thermomechanical tests on SMA springs. The data from these tests serve both to calibrate the model and establish thermodynamic driving force and extent of phase transformation relationships for SMA springs. A drop in driving force amplitude would suggest material losing its ability to undergo phase transformations which directly corresponds to a loss in the functionality/smartness of SMA component. This would allow designers to set appropriate driving force thresholds as a guideline for analyzing functional life of SMA components.Show more