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Simulation of Stress Concentration Problems by Hexahedral Hybrid-Trefftz Finite Element Models
Get PDFHybrid-Trefftz stress finite elements have been applied with success to the analysis of linear and non-linear problems in structural mechanics. Two independent fields are approximated: stresses within the elements and displacements on their boundary. The stress field satisfies the Trefftz constraint a priori, i.e., it is extracted from the Navier equation solution. This type of element has provided remarkable improvement in stress predictions compared to the standard displacement-based finite elements. In this work, solution of stress concentration problems is carried out by hexahedral hybrid-Trefftz stress element models. Stress concentration factors and stress intensity factors are then identified and compared with available results. The hierarchicalp-refinement strategy is exploited in the numerical tests
Local Defect Correction for Boundary Integral Equation Methods
Get PDFThe aim in this paper is to develop a new local defect correction approach to gridding for problems with localised regions of high activity in the boundary element method. The technique of local defect correction has been studied for other methods as finite difference methods and finite volume methods. The initial attempts to developing such a technique by the authors for the boundary element method was based on block decomposition and manipulation of the coefficient matrix and right hand side of the system of equations in three dimension. It ignored the inherent global nature of the boundary integral equation, that is, each node of the grid contributes to all the others in the grid through integration. In this paper we present a better approach that takes this into account. We use a new integral approach to defect correction and develop the technique for the boundary element method. The technique offers an iterative way for obtaining the solution on an equivalent composite grid. It uses two grids: a global uniform coarse grid covering the whole boundary and a local fine grid covering the local active boundary. The solution of the local problem on the local fine grid is used to estimate the defect on the fine grid. The effect of the defect onto the right hand side is then considered. We demonstrate the technique’s strength using an example and show that it offers a cheaper alternative to either solving on a global uniform grid or directly on a composite grid
Simulation of a Single Red Blood Cell Flowing Through a Microvessel Stenosis Using Dissipative Particle Dynamics
Get PDFThe motion and deformation of a single red blood cell flowing through a microvessel stenosis was investigated employing dissipative particle dynamics (DPD) method. The numerical model considers plasma, cytoplasm, the RBC membrane and the microvessel walls, in which a three dimensional coarse-grained spring network model of RBC’s membrane was used to simulate the deformation of the RBC. The suspending plasma was modelled as an incompressible Newtonian fluid and the vessel walls were regarded as rigid body. The body force exerted on the free DPD particles was used to drive the flow. A modified bounce-back boundary condition was enforced on the membrane to guarantee the impenetrability. Adhesion of the cell to the stenosis vessel surface was mediated by the interactions between receptors and ligands. Firstly, the motion of a single RBC in a microfluidic channel was simulated and the results were found in agreement with the experimental data cited by [1]. Then the mechanical behavior of the RBC in the microvessel stenosis was studied. The effects of the bending rigidity of membrane, the size of the stenosis and the driven body force on the deformation and motion of red blood cell were discussed
Fluid-Structure Interaction Analysis of Pulsatile Flow within a Layered and Stenotic Aorta
Get PDFIn this paper, the hemodynamic characteristics of blood flow and stress distribution in a layered and stenotic aorta are investigated. By introducing symmetrical and unsymmetrical stenosis, the influence of stenosis morphology and stenotic ratio on the coupled dynamic responses of aorta is clarified. In the analysis, the in-vivo pulsatile waveforms and fully fluid–structure interaction (FSI) between the layered elastic aorta and the blood are considered. The results show that the fluid domain is abnormal in the stenotic aorta, and the whirlpool forms at the obstructed and downstream unobstructed regions. The maximum wall shear stresses appear at the throat of the stenosis. Downstream region appears low and oscillated shear stresses. In addition, along with the increase of the stenotic ratio, the amplitude of the maximum shear stress will be intensively increased and localized, and the sensitivity is also increased. In the aorta with unsymmetrical stenosis, the Von Mises stresses reach the peak value at the side with the surface protuberance, but they are reduced at the side with no protuberance. The sign variation of the layer interface shear stresses near the throat indicates the variation of the shear direction which increases the opportunity of shear damage at the transition plane. Moreover, the shear stress levels at the fluid-solid and intima-media interfaces are higher than that at the media-adventitia interface. The unsymmetrical stenosis causes higher stresses at the side with the surface protuberance than symmetrical one, but lower at the side with no protuberance. These results provide an insight in the influence of the stenosis, as well as its morphology, on the pathogenesis and pathological evolution of some diseases, such as arteriosclerosis and aortic dissection
Review of "The Theory of Materials Failure" by Prof. Richard M. Christensen, Stanford University Published by: Oxford University Press, 2013, 277 pages
No full textA Novel Approach to Identify the Thermal Conductivities of a Thin Anisotropic Medium by the Boundary Element Method
Get PDFA common difficulty arises in characterizing the anisotropic properties of a thin sheet of anisotropic material, especially in the transverse direction. This difficulty is even more phenomenal for measuring its mechanical properties on account of its thickness. As the prelude of such investigation, this paper proposes a novel approach to identify the thermal conductivities of an unknown thin layer of anisotropic material. For this purpose, the unknown layer is sandwiched in isotropic materials with known conductivities. Prescribing proper boundary conditions, one may easily measure temperature data on a few sample boundary points. Therefore, the anisotropic thermal conductivities can be calculated inversely. For the inverse analysis, the boundary element method (BEM) is employed to combine with the conjugate gradient method (CGM). For verifying our analysis, numerical experiments were carried out. The obtained results have shown great computational efficiency and accuracy in identifying the thermal conductivities of the thin anisotropic layer
Fast and High-Resolution Optical Inspection System for In-Line Detection and Labeling of Surface Defects
Get PDFAutomated optical inspection systems installed in production lines help ensure high throughput by speeding up inspection of defects that are otherwise difficult to detect using the naked eye. However, depending on the size and surface properties of the products such as micro-cracks on touchscreen panels glass cover, the detection speed and accuracy are limited by the imaging module and lighting technique. Therefore the current inspection methods are still delegated to a few qualified personnel whose limited capacity has been a huge tradeoff for high volume production. In this study, an automated optical technology for in-line surface defect inspection is developed offering high performance in spatial resolution and detection speed for any surface. The inspection system consisting of an LED array which illuminates a wide inspection area on the test object captures scattered light from surface defects using a 12288-pixel line CCD at 12 kHz acquisition rate. A 3.5 m per pixel resolution of the line CCD provides a detection width capability of at most 43 mm which is equivalent to 147 megapixels image data acquired per second. To handle the large volume of data per acquisition cycle, the data are transmitted from a host CPU to multiple GPU devices where CUDA-based image processing kernels are adopted to perform detection and labeling of surface defects in parallel. The processed data is sent back to the CPU to display user-defined defect maps. 2-D inspection of back-coated flat mirrors, 43 mm x 70 mm in size, using a single CCD module and multiple GPU reveals that surface flaws such as bubbles, cracks, and edge defects are detected accurately. The acquisition time to capture and load the data to a CPU is 1.7 s while the processing time to transmit the same data for surface defect detection in a GPU is 248 ms. The latter time scale is considerably faster compared to minute-long computations in solely CPU-based processing algorithm of the same test object. The minimum width of detected surface defects is about 10 m with true detection rates above 94%. Moreover, the inspection system is easily configurable by tasking multiple CCD imaging modules to different GPU devices to allow inspection of larger test objects. This flexibility can improve both acquisition and detection speeds to boost in-line circuit chips, packaging, and touchscreen panel inspection systems
An Integrated Fracture Mechanics Based Approach for Non-Linear Analysis of Lightly Reinforced Concrete Beams
Get PDFA non-linear fracture mechanics based approach is proposed to depict a typical fracture mechanism from initiation to growth, eventually leading to failure. This concept is developed for a lightly reinforced beam in flexure. The proposed model integrates the existing methodology of a Stress Intensity Factor equilibrium equation with the bridging forces developed in concrete cover and rebar. The model and solution algorithm outlined presents an elaborate understanding of the mechanism involved and is significant in predicting the behaviour of flexural members. The analysis is performed using MATLAB programming. The proposed approach ensures a maximum tolerable crack length and crack width for flexural members to prevent a catastrophic failure. Such an approach has the potential to serve as an analysis and design tool for reinforced concrete components subjected to normal conditions and towards deciding rehabilitation and strengthening measures
Sound Propagation Analysis on Sonic Crystal Elastic Structures using the Method of Fundamental Solutions (MFS)
Get PDFThe study of periodic structures, namely sonic crystals, for sound attenuation purposes has been a topic of intense research in the last years. Some efficient methods are available in literature to solve the problem of sound propagation in the presence of this kind of structures such as those based in the Multiple Scattering Theory (MST) or the Finite Element Method (FEM). In this paper a solution based on the Method of Fundamental Solutions (MFS) which presents advantages, namely in computational discretization and calculation costs, is presented. The proposed formulation considers the presence of elastic ring shaped scatterers, correctly accounting for their interaction with the host fluid. The model accuracy is tested by comparing it to FEM in an exemplificative case. A set of numerical examples is then presented and the results compared with the ones given by a MFS approach where the structure is considered as rigid. The results showed that in the case of low-contrasting between solid/fluid properties, the elastic behavior of the structure could not be neglected
Modeling of Hydro-Viscoelastic State of Deformable and Saturated Product During Convective Drying
Get PDFA mathematical model was developed to simulate in 2D the spatiotemporal evolution of the moisture content, the temperature and the mechanical stress within a deformable and saturated product during convective drying. A comprehensive hydro-thermal model had been merged with a Maxwell model with two branches, assuming a viscoelastic material, a plane deformation and an isotropic hydric-shrinkage of the sample. A long sample of clay mixture with a square section was chosen as an application case. The transport and equilibrium properties of the product required for the modeling were determined from previous experiments which were independent of the drying trials. In order to validate the hydro-thermal part of the model, several drying tests were carried out for different values of temperature, relative humidity and air velocity in a vertical drying tunnel (designed and constructed in the LETTM laboratory). The theoretical and experimental results appeared in good agreement. The simulations of the spatio-temporal distribution of mechanical stress were performed and interpreted in terms of product potential damage. The sample shape was also predicted all aver the drying process with reasonable accuracy