Optimization of bypass graft using FSI numerical method

It is well documented in literature that the coronary artery bypass graft is normally fail after a short period of time, due to the development of plaque known as intimal hyperplasia within the graft. Various in vivo and in vitro studies have linked the development of intimal hyperplasia to the abnormal hemodynamics and compliance mismatch. Therefore, it is essential to fully understand the relationship between the hemodynamics inside the coronary artery bypass and its mechanical and geometrical characteristics under the correct physiological conditions. In this work, hemodynamic of the bypass graft is studied numerically. The effect of the host and graft diameters ratio, the angle of anastomosis and the graft configuration on the local flow patterns and the distribution of wall shear stress are examined. The pulsatile waveforms boundary conditions are adopted from in vivo measurement data to study the hemodynamics of composite grafts namely Consequence and Y grafting in terms temporal and spatial distributions of the blood flows. Moreover, various non-Newtonian and Newtonian models of blood have been carried out to examine the numerical simulation of blood flow in stenosis artery. The results are presented and discussed for various operating conditions.

Fracture modelling of DP780 sheets using a hybrid experimental-numerical method and two-dimensional digital image correlation

This paper is concerned with the construction of fracture envelopes of DP780 sheets using two methods: a hybrid experimental-numerical method; two-dimensional digital image correlation (2D-DIC). For the hybrid method, four types of ductile fracture tests were carried out covering a wide range of stress states on specimens: with a central hole; two symmetric circular notches; flat grooved; and diagonally double-notched. Based on the fracture strain and loading paths identified with finite element simulation, a fracture envelope was obtained by employing the three-parameter modified Mohr-Coulomb fracture model. In addition, the fracture surface strain was directly measured using 2D-DIC. Loading histories of each test were extracted from a surface element of a three dimensional finite element model. The comparison of fracture envelopes constructed by the two methods reveals that there is little difference. Thus, it can be concluded that 2D-DIC is applicable to fracture modelling of DP780 sheets despite the assumption of the plane stress condition even after necking

Integral approach and numerical improvement to calculate the carbon concentration profiles in carburising process

In the present work, the carbon diffusion in steel, where the carbon diffusivity varies with the carbon content, was solved with the integral methods under the third boundary condition. The variation of carbon diffusivity in steel with the carbon content was described with two different functions ie. linear dependence and exponential dependence. The integral approximation for both cases was improved with the numerical computation to more accurately predict the carbon profiles. The integral solution is more accurate than the formulation based on the assumption of a constant diffusivity or those based on the assumption of a constant diffusivity and/or constant carbon content at part surface. It is also more easily used in practice than the numerical method to describe the carburising process and predict the carbon content at steel surface and carbon profiles in treated layer.

Integral approach and numerical improvement to calculate carbon concentration profiles in carburising

The carbon diffusion in steel, where the carbon diffusivity varies with the carbon content, was solved with the integral methods under the third boundary condition. The variation of carbon diffusivity in steel with the carbon content was described with two different functions, linear dependence and exponential dependence. The integral approximation for both cases was improved with the numerical computation to more accurately predict the carbon profiles. The integral solution is more accurate than the formulation based on the assumption of a constant diffusivity or those based on the assumption of a constant diffusivity and/or constant carbon content at part surface. It is also more easily used in practice than the numerical method to describe the carburising process and predict the carbon content at steel surface and carbon profiles in treated layer.

Fracture analysis of spherical shells : application to cracking of macadamia nuts

The main consideration in recovering the macadamia kernal is to crack the spherical nutshell without damaging the kernal. Five mechanical cracking tools were tested, and the fracture mechanisms of nutshells, under various cracking loads, were studied. A classical theoretical approach and a numerical method were both used to investigate the influence of crack face closure on the stress intensity factor for a cracked spherical shell subjected to membrane forces and bending moments.

Design earthquake ground motion prediction for Perth metropolitan area with microtremor measurements for site characterization

Perth is the largest city in Western Australia and home to three-quarters of the state's residents. In recent decades, there have been a lot of earthquake activities just east of Perth in an area known as the South-West Seismic Zone. Previous numerical results of site response analyses based on limited available geology information for PMA indicated that Perth Basin might amplify the bedrock motion by more than 10 times at some frequencies and at some sites. Hence, more detailed studies on site characterization and amplification are necessary. The microtremor method using spatial autocorrelation (SPAC) processing is a useful tool for gaining thickness and shear wave velocity (SWV) of sediments and has been adopted in many previous studies. In this study, the response spectrum of rock site corresponding to the 475-year return period for PMA is defined according to the probabilistic seismic hazard analysis (PSHA) based on the latest ground motion attenuation model of Southwest Western Australia. Site characterization in PMA is performed using two microtremor measurements, namely SPAC technique and H/V method. The clonal selection algorithm (CSA) is introduced to perform direct inversion of SPAC curves to determine the soil profiles of representative PMA sites investigated in this study. Using the simulated bedrock motion as input, the responses of the soil sites are estimated using numerical method based on the shear-wave velocity vs. depth profiles determined from the SPAC technique. The response spectrum of the earthquake ground motion on surface of each site is derived from the numerical results of the site response analysis, and compared with the respective design spectrum defined in the Australian Earthquake Loading Code. The comparison shows that the code spectra are conservative in the short period range, but may slightly underestimate the response spectrum at some long period range.

Slope stability analysis for cohesive materials filled on purely cohesive undrained clay

Stability charts for soil slopes were first produced by Taylor in 1937 and they continue to be used extensively as design tools and draw the attention of many investigators. From a review of literature, it was found that there is no convenient solution has been provided for cohesive materials filled on purely cohesive undrained clay. A recent study revealed that the embankment slope which has two-layered clays failed in an undrained state which shows the importance of this study. In order to obtain the solutions for this type of fill slope. A number of numerical method are employed, namely the finite element upper and lower bound limit analysis methods and limit equilibrium method. The numerical upper and lower bound limit analysis method can bracket true solutions within a small range (6%). The solutions of limit equilibrium analysis are used for comparison purpose.

Slope stability analysis for fill slopes using finite element limit analysis

This paper investigates the stability of fill slopes often found in embankment cases where frictional fill materials are placed on purely cohesive undrained clay with increasing strength. By using finite element upper and lower bound limit analysis for this investigation, the limit load can be truly bounded. It is known that two-dimensional analysis yields a more conservative result due to plain strain condition when compared to three-dimensional analysis. Therefore, this paper will focus on three-dimensional (3D) slope stability analysis and for comparison purposes two-dimensional analysis results will be employed. In fact, the final results are presented in the form of comprehensive chart solutions for the convenience of practicing engineers during preliminary slope design. The failure mechanism will also be discussed in order to further illustrate the situation during failure. It should be highlighted that the failure mechanisms are obtained through the numerical method itself and no prior assumptions are required, therefore, are more realistic and able to provide a better understanding for the slope failure surfaces.

Selecting the individual numerical scale and prioritization method in the analytic hierarchy process : a 2-tuple fuzzy linguistic approach

The validity of the priority vector used in the analytic hierarchy process (AHP) relies on two factors: the selection of a numerical scale and the selection of a prioritization method. The traditional AHP selects only one numerical scale (e.g., the Saaty scale) and one prioritization method (e.g., the eigenvector method) for each particular problem. For this traditional selection approach, there is disagreement on which numerical scale and prioritization method is better in deriving a priority vector. In fact, the best numerical scale and the best prioritization method both rely on the content of the pairwise comparison data provided by the AHP decision makers. By defining a set of concepts regarding the scale function and the linguistic pairwise comparison matrices (LPCMs) of the priority vector and by using LPCMs to unify the format of the input and output of AHP, this paper extends the AHP prioritization process under the 2-tuple fuzzy linguistic model. Based on the extended AHP prioritization process, we present two performance measure criteria to evaluate the effect of the numerical scales and prioritization methods. We also use the performance measure criteria to develop a 2-tuple fuzzy linguistic multicriteria approach to select the best numerical scales and the best prioritization methods for different LPCMs. In this paper, we call this type of selection the individual selection of the numerical scale and prioritization method. We also compare this individual selection with traditional selection by using both random and real data and show better results with individual selection.

A new method for deriving priority weights by extracting consistent numerical-valued matrices from interval-valued fuzzy judgement matrix

It is important to derive priority weights from interval-valued fuzzy preferences when a pairwise comparative mechanism is used. By focusing on the significance of consistency in the pairwise comparison matrix, two numerical-valued consistent comparison matrices are extracted from an interval fuzzy judgement matrix. Both consistent matrices are derived by solving the linear or nonlinear programming models with the aid of assessments from Decision Makers (DMs). An interval priority weight vector from the extracted consistent matrices is generated. In order to retain more information hidden in the intervals, a new probability-based method for comparison of the interval priority weights is introduced. An algorithm for deriving the final priority interval weights for both consistent and inconsistent interval matrices is proposed. The algorithm is also generalized to handle the pairwise comparison matrix with fuzzy numbers. The comparative results from the five examples reveal that the proposed method, as compared with eight existing methods, exhibits a smaller degree of uncertainty pertaining to the priority weights, and is also more reliable based on the similarity measure. © 2014 Elsevier Inc. All rights reserved.

Consensus building in a local context for the AHP-GDM with the individual numerical scale and prioritization method

The methodology for selecting the individual numerical scale and prioritization method has recently been presented and justified in the analytic hierarchy process (AHP). In this study, we further propose a novel AHP-group decision making (GDM) model in a local context (a unique criterion), based on the individual selection of the numerical scale and prioritization method. The resolution framework of the AHP-GDM with the individual numerical scale and prioritization method is first proposed. Then, based on linguistic Euclidean distance (LED) and linguistic minimum violations (LMV), the novel consensus measure is defined so that the consensus degree among decision makers who use different numerical scales and prioritization methods can be analyzed. Next, a consensus reaching model is proposed to help decision makers improve the consensus degree. In this consensus reaching model, the LED-based and LMV-based consensus rules are proposed and used. Finally, a new individual consistency index and its properties are proposed for the use of the individual numerical scale and prioritization method in the AHP-GDM. Simulation experiments and numerical examples are presented to demonstrate the validity of the proposed model.

Fast algorithm for the cutting angle method of global optimization

The cutting angle method for global optimization was proposed in 1999 by Andramonov et al. (Appl. Math. Lett. 12 (1999) 95). Computer implementation of the resulting algorithm indicates that running time could be improved with appropriate modifications to the underlying mathematical description. In this article, we describe the initial algorithm and introduce a new one which we prove is significantly faster at each stage. Results of numerical experiments performed on a Pentium III 750 Mhz processor are presented.

Cutting angle method - a tool for constrained global optimization

Cutting angle method (CAM) is a deterministic global optimization technique applicable to Lipschitz functions f: Rn → R. The method builds a sequence of piecewise linear lower approximations to the objective function f. The sequence of solutions to these relaxed problems converges to the global minimum of f. This article adapts CAM to the case of linear constraints on the feasible domain. We show how the relaxed problems are modified, and how the numerical efficiency of solving these problems can be preserved. A number of numerical experiments confirms the improved numerical efficiency.

Numerical modeling of the dynamic tensile behavior of irregular fibers

Most fibers are irregular, and they are often subjected to rapid straining during mechanical processing and end-use applications. In this paper, the effect of fiber dimensional irregularities on the dynamic tensile behavior of irregular fibers is examined, using the finite element method (FEM). Fiber dimensional irregularities are simulated with sine waves of different magnitude (10%, 30% and 50% level of diameter variation). The tensile behavior of irregular fibers is examined at different strain rates (333%/sec, 3,333%/sec and 30,000%/sec). The breaking load and breaking extension of irregular fibers at different strain rates are then calculated from the finite element model. The results indicate that strain rate has a significant effect on the dynamic tensile behavior of an irregular fiber, and that the position of the thinnest segment along the fiber affects the simulation results markedly. Under dynamic conditions, an irregular fiber does not necessarily break at the thinnest segment, which is different from the quasi-static results.

Numerical study of the mode I delamination toughness of Z-pinned laminates

A finite element (FE) model is developed to investigate mode I delamination toughness of z-pin reinforced composite laminates. The z-pin pullout process is simulated by the deformation of a set of non-linear springs. A critical crack opening displacement (COD) criterion is used to simulate crack growth in a double-cantilever-beam (DCB) made of z-pinned laminates. The toughness of the structure is quantified by the energy release rate, which is calculated using the contour integral method. The FE model is verified for both unpinned and z-pinned laminates. Predicted loading forces from FE analysis are compared to available test data. Good agreement is achieved. Our numerical results indicate that z-pins can greatly increase the mode I delamination toughness of the composite laminates. The influence of design parameters on the toughness enhancement of z-pinned laminates is also investigated, which provides important information to optimise and improve the z-pinning technique.