1,721,099 research outputs found
Thermomechanical Characterization and Modeling of Shape Memory Polymers
Get PDFThis work focuses on the thermomechanical characterization and constitutive
model calibration of shape memory polymers (SMPs). These polymers have the ability to recover seemingly permanent large deformations under the appropriate thermomechanical load path. In this work, a contribution is made to both existing experimental and modeling efforts. First, an experimental investigation is conducted
which subjects SMPs to a thermomechanical load path that includes varying the value
of applied deformations and temperature rates. Specifically, SMPs are deformed to
tensile extensions of 10% to 100% at temperature rates varying from 1 degree C /min to
5 degree C/min, and the complete shape recovery profile is captured. The results from this
experimental investigation show that the SMP in question can recover approximately
95% of the value of the applied deformation, independent of the temperature rate
during the test.
The data obtained in the experimental investigation are then used to calibrate,
in one-dimension, two constitutive models which have been developed to describe
and predict the material response of SMPs. The models include a model in terms of
general deformation gradients, thus making it capable of handling large deformations.
In addition, the data are used to calibrate a linearized version of the constitutive
model for small deformations. The material properties required for calibrating the
constitutive models are derived from portions of the experimental results, and the
model is then used to predict the shape memory effect for an SMP undergoing various levels of deformation. The model predictions are shown to match well with the
experimental data
A cohesive model of fatigue of ferroelectric materials under electro-mechanical cyclic loading
Get PDFA cohesive fatigue-crack nucleation and growth model for ferroelectric materials under electro-mechanical loading is presented. The central feature of the model is a hysteretic cohesive law which couples the mechanical and electrical fields. This law can be used in conjunction with general constitutive relations of bulk behavior, possibly including domain switching, in order to predict fatigue crack growth under arbitrary loading conditions. Another appealing feature of the model is its ability to predict fatigue-crack nucleation. Despite the scarcity and uncertainty of the experimental data, comparisons with PZT fatigue-life data are encouraging
Modeling Shape Memory Polymer fill Honeycomb as a Composite Skin for a Morphing Wing
Get PDFDue to its complex phase transformation behavior, a Shape Memory Polymer filled honeycomb composite has been proposed as an efficient material for skin on a morphing wing. This work develops a finite element model of the honeycomb composite that captures the material behavior while morphing through all geometric phases. To model the shape memory polymer filling, the simulation implements an experimentally calibrated user defined material subroutine in Abaqus, a commercially available finite element software. In order to validate the model, the modeled behavior is compared to experimentally determined behavior of shape memory polymers. The geometry and deformations of representative unit cells are then discussed
Analysis of the Size Effects on the Pseudoelastic Behavior of Shape Memory Alloy Micro-pillars
Get PDFSize dependent properties of Shape Memory Alloys (SMAs) in micro and nano scales have gained an increasing attention due to the existing and potential applications of SMAs in microelectromechanical systems (MEMS) and small scale biomedical devices. Such applications exploit the pseudoelastic and shape memory properties the SMAs. In order to enhance the applicability of SMA micro and nano structures, the size dependency of the thermo-elastic behavior of SMAs should be understood. In this study, the dependency of the pseudoelastic behavior of Nickel-Titanium (NiTi) micro-pillars on their diameter was analyzed. Isothermal compression experiments from literature of bulk and micro-pillars were analyzed to determine the critical transformation stresses for different pillar diameters. The analysis of experimental data shows that the critical transformation stresses increase as the micro-pillar average diameter decreases. The relations between the critical transformation stresses and the average pillar diameter were represented using power functions. It was assumed that the elastic modulus and Poisson���s ratios of the austenite and martensite phases, the transformation strain parameters, and the stress influence coefficients were unaffected by the micro-pillar size. Parametric studies were performed using the finite element analysis to find the effects of the taper angle and the aspect ratio on the micro-pillars behavior. Comparisons of the results found from finite element simulations and experiments show that the model accurately predicts the pseudoelastic response of bulk and micro-pillars. The results of the parametric studies show that the hysteresis of the compression response decreases as the taper angle increases. The effect of the micro-pillar diameter on the compression response is less significant for micro-pillars of higher aspect ratios and higher taper angles
Interaction of oxygen vacancies with domain walls and its impact on fatigue in ferroelectric thin films
Get PDFThe role of oxygen vacancies in fatigue and dielectric breakdown has been a topic of intense research in ferroelectric perovskites like BaTiO_3. This paper presents a comprehensive model that treats the ferroelectrics as polarizable wide band-gap semiconductors where the oxygen vacancies act as donors. First, a fully coupled nonlinear model is developed with space charges, polarization, electric potential and elastic displacements as variables without making any a priori assumptions on the space charge distribution and the polarization. Second, a Pt/BaTiO_3/Pt structure is considered. Full-field coupled numerical simulations are used to investigate the structure of 180° and 90° domain walls in both perfect and defected crystals. The interactions of oxygen vacancies with domain walls are explored. Numerical results show that there is pronounced charge trapping near 90° domain walls, giving rise to possible domain wall pinning and dielectric breakdown. Third, a simple analytical solution of the potential profile for a metal/ferroelectric semiconductor interface is obtained and the depletion layer width is estimated. These analytical estimates agree with our numerical results and provide a useful tool to discuss the implications of our results
Modeling large strain electrostriction of ferroelectrics under combined electromechanical loads
Get PDFA computational model for investigating domain switching and macroscopic electromechanical properties of ferroelectric materials is developed. Various aspects of domain nucleation and growth, and their effects on macroscopic hysteresis are examined. The model is validated against recent experimental observations. It is thus validated as a design tool to investigate various aspects of a novel thin film ferroelectric microactuator in future work
Interactions Of Electromagnetic Fields With Defects In Deformable Bodies (gauge Theory(ies), Dislocation(s), Disclination(s), Elastic Continuum(a), Exterior Calculus).
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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