A finite element method for thermo-mechanical coupling analysis of shape memory alloys

Due to strong thermo-mechanical coupling in shape memory alloys (SMAs), heat generation/absorption during forward/reverse phase transformation can cause temperature variations in the material; influencing its mechanical behaviour. It is usually assumed that this coupling is only affected by the loading rate. But, recently studies have shown that the size of the structure and the boundary conditions are also important. Therefore, only the definition of quasistatic or slow loading rate can not guarantee an isothermal process and so further considerations need to be made. Based on the powerful model, proposed by Lagoudas et al. [1] and later improved for computer programming using the return mapping algorithm by Qidwai and Lagoudas [2], this contribution presents a three-dimensional thermo-mechanically coupled extension which describes two important typical phenomena of material model of SMAs: superelasticity and superplasticity (shape memory effect). An algorithm is then proposed to implement the coupled model into a finite element code. Performed simulations with different boundary conditions demonstrate that the both loading rate and the size dependency can be captured within the proposed framework. The results are in good agreement with available data in the literature

Design of Finite-Length Irregular Protograph Codes with Low Error Floors over the Binary-Input AWGN Channel Using Cyclic Liftings

We propose a technique to design finite-length irregular low-density parity-check (LDPC) codes over the binary-input additive white Gaussian noise (AWGN) channel with good performance in both the waterfall and the error floor region. The design process starts from a protograph which embodies a desirable degree distribution. This protograph is then lifted cyclically to a certain block length of interest. The lift is designed carefully to satisfy a certain approximate cycle extrinsic message degree (ACE) spectrum. The target ACE spectrum is one with extremal properties, implying a good error floor performance for the designed code. The proposed construction results in quasi-cyclic codes which are attractive in practice due to simple encoder and decoder implementation. Simulation results are provided to demonstrate the effectiveness of the proposed construction in comparison with similar existing constructions.Comment: Submitted to IEEE Trans. Communication

Lowering the Error Floor of LDPC Codes Using Cyclic Liftings

Cyclic liftings are proposed to lower the error floor of low-density parity-check (LDPC) codes. The liftings are designed to eliminate dominant trapping sets of the base code by removing the short cycles which form the trapping sets. We derive a necessary and sufficient condition for the cyclic permutations assigned to the edges of a cycle $c$ of length $\ell(c)$ in the base graph such that the inverse image of $c$ in the lifted graph consists of only cycles of length strictly larger than $\ell(c)$. The proposed method is universal in the sense that it can be applied to any LDPC code over any channel and for any iterative decoding algorithm. It also preserves important properties of the base code such as degree distributions, encoder and decoder structure, and in some cases, the code rate. The proposed method is applied to both structured and random codes over the binary symmetric channel (BSC). The error floor improves consistently by increasing the lifting degree, and the results show significant improvements in the error floor compared to the base code, a random code of the same degree distribution and block length, and a random lifting of the same degree. Similar improvements are also observed when the codes designed for the BSC are applied to the additive white Gaussian noise (AWGN) channel

Improved Energy Detector for Wideband Spectrum Sensing in Cognitive Radio Networks

In this paper, an improved energy detector for a wideband spectrum sensing is proposed. For a better detection of the spectrum holes the overall band is divided into equal non-overlapping sub-bands. The main objective is to determine the detection thresholds for each of these subbands jointly. By defining the problem as an optimization problem, we aim to find the maximum aggregated opportunistic throughput of cognitive radio networks. Introducing practical constraints to this optimization problem will change the problem into a convex and solvable one. The results of this paper show that the proposed improved energy detector will increase the aggregated throughput considerably

Prediction of the cyclic durability as a function of cycle duration and temperature of an air plasma sprayed coating using inelastic strain

A detailed study of the failure mechanisms in an APS TBC was carried out involving over 1000 micrographs. As a result the kinetics of oxidation and rumpling were characterized. In addition it was found that the failure was always within the ceramic caused by progressive cracking. At approximately half the spallation life, crack linking became the dominant growth mode for cracks. This pattern of damage remained constants over the temperature range of 1066 °C to 1149 °C and for cycle durations of 0.5 hours to 50 hours. The change in temperature resulted in a variation in spallation life of a factor of 5 and the variation in hold time resulted in a variation in life of a factor of 7 and for the entire data set the ratio of the longest failure life to shortest was 17. Through the use of a finite element analysis (FEA) that used an experimentally validated viscoplastic model driven by imposed shape changes derived from measured oxidation and rumpling behavior as well as thermal expansion mismatch, the hot and cold inelastic strains were predicted and used to predict failure. It was possible to predict the entire data set using data from only two cyclic life tests at two different test conditions. The life as a function of temperature can be predicted from two experiments run with two different hot times at a single temperature. The life as a function of hold time can be predicted from tests run at two temperatures at a single hold time. This suggests that the hold time dependence and temperature dependence are closely tied to the factors controlling inelastic strain. The inelastic strains can be determined form a combination of measured and computed behavior. It was also found that the finite element results can be captured without running the FEA using simple expressions that are calibrated using a large set of FEA run

Optimal Rate for Irregular LDPC Codes in Binary Erasure Channel

In this paper, we introduce a new practical and general method for solving the main problem of designing the capacity approaching, optimal rate, irregular low-density parity-check (LDPC) code ensemble over binary erasure channel (BEC). Compared to some new researches, which are based on application of asymptotic analysis tools out of optimization process, the proposed method is much simpler, faster, accurate and practical. Because of not using any relaxation or any approximate solution like previous works, the found answer with this method is optimal. We can construct optimal variable node degree distribution for any given binary erasure rate, {\epsilon}, and any check node degree distribution. The presented method is implemented and works well in practice. The time complexity of this method is of polynomial order. As a result, we obtain some degree distribution which their rates are close to the capacity.Comment: 5 pages, to be presented at the 2011 IEEE Information Theory Workshop (ITW 2011), Paraty, Brazil, October, 201

Acoustic phonon scattering in a low density, high mobility AlGaN/GaN field effect transistor

We report on the temperature dependence of the mobility, $\mu$, of the two-dimensional electron gas in a variable density AlGaN/GaN field effect transistor, with carrier densities ranging from 0.4$\times10^{12}$ cm$^{-2}$ to 3.0$\times10^{12}$ cm$^{-2}$ and a peak mobility of 80,000 cm$^{2}$/Vs. Between 20 K and 50 K we observe a linear dependence $\mu_{ac}^{-1} = \alpha$T indicating that acoustic phonon scattering dominates the temperature dependence of the mobility, with $\alpha$ being a monotonically increasing function of decreasing 2D electron density. This behavior is contrary to predictions of scattering in a degenerate electron gas, but consistent with calculations which account for thermal broadening and the temperature dependence of the electron screening. Our data imply a deformation potential D = 12-15 eV.Comment: 3 pages, 2 figures, RevTeX. Submitted to Appl Phys Let

A comparative analysis of particle tracking in a mixer by discrete element method and positron emission particle tracking

Characterisation of particle flow using Positron Emission Particle Tracking (PEPT) is based on tracking the position of a single particle in a dynamic system. Recent developments in PEPT have facilitated tracking multiple particles aiming at improvements in data representation. Nevertheless for systems with a wide residence time distribution and/or dead zone, the conditions for getting representative data which could reflect the bulk behaviour of the powders need to be analysed and specified. In the present work, an attempt is made to simulate PEPT experiments for a paddle mixer using Discrete Element Method (DEM), with a view to investigate the effect of increasing the number of tracers on their time-averaged velocity distribution and whether it can represent the data on whole population of particles. The time averaged velocity distribution of the individual tracer particles (resembling simulated PEPT) is obtained and compared with the time averaged data on entire particle population. The DEM results indicate that for the investigated paddle mixer, it takes 251. s for one tracer to travel adequately in all the active space of the system. The instantaneous tracer velocity fluctuates around the average value obtained for all the particles, suggesting that the average tracer velocity is adequately representative of the average particle velocity in the system. The data of the PEPT experiment with one tracer with those of DEM with one tracer are in good agreement; however, DEM simulation suggests that increasing the number of tracers in the paddle mixer system does not influence the average velocity distribution. Furthermore, the velocity for all particles in the DEM shows a smooth distribution with a peak frequency of the velocity distribution that is lower than PEPT and DEM tracer. When tracking a single tracer in DEM or PEPT, it may not be detected to have zero velocity at any instant of time, whilst the data for all particles show that about 0.3% of particles are stagnant

A kernel least mean square algorithm for fuzzy differential equations and its application in earth's energy balance model and climate

Abstract This paper concentrates on solving fuzzy dynamical differential equations (FDDEs) by use of unsupervised kernel least mean square (UKLMS). UKLMS is a nonlinear adaptive filter which works by applying kernel trick to LMS adaptive filter. UKLMS estimates multivariate function which is embedded to estimate the solution of FDDE. Adaptation mechanism of UKLMS helps for finding solution of FDDE in a recursive scenario. Without any desired response, UKLMS finds nonlinear functions. For this purpose, an approximate solution of FDDE is constructed based on adaptable parameters of UKLMS. An optimization algorithm, optimizes the values of adaptable parameters of UKLMS. The proposed algorithm is applied for solving Earth energy balance model (EBM) which is considered as a fuzzy differential equation for the first time. The method in comparison with the other existing approaches (such as numerical methods) has some advantages such as more accurate solution and also that the obtained solution has a functional form, thus the solution can be obtained at each time in training interval. Low error and applicability of developed algorithm are examined by applying it for solving several problems. After comparing the numerical results, with relative previous works, the superiority of the proposed method will be illustrated