Increasing use of aluminum alloys in the automotive industry increases the importance of the production of sheet aluminum. To provide cost effective sheet aluminum to the industry, twin-roll casting (TRC) is becoming more important compared to DC casting. Demand for usage of different aluminum alloys in sheet form introduces some difficulties that should be considered during their applications. The main problem encountered during the welding of aluminum alloys is hot cracking. The aim of this study is to understand the difference in hot cracking susceptibility of two twin roll cast (TRC) aluminum-magnesium alloys (5754 and 5049 alloys) during welding. Varestraint test method was used to evaluate the effect of welding parameters, strain levels, filler alloys and mid-plane segregation on hot cracking susceptibilities. Hot cracking susceptibility of both 5049(Al-2wt%Mg) and 5754(Al-3wt%Mg) alloys increased with increasing strain level. Also, it was observed that hot cracking susceptibility was higher for the alloy having higher magnesium content. Thermal analysis results verified that hot cracking susceptibility indeed can be related to the v solidification range. As is suggested in the solidification range approach, the results of the present study confirm that the extent of solidification and liquation cracking depend on the magnitude of solidification range and the strain imposed during welding. Hot cracking susceptibility of 5754(Al-3wt%Mg) alloy has shown slightly decreasing behavior with addition of 5356 filler alloy. On the other hand, addition of 5183 filler alloy has increased solidification cracking susceptibility of two base alloys. The fracture surfaces of liquation and solidification cracks were investigated by scanning electron microscope with EDS. Liquation crack surfaces of the 5754(Al-3wt%Mg) alloy were found to have high Mg and Si content. For the 5754(Al-3wt%Mg) alloy, a quench test was designed to observe the effect of mid-plane segregation zone. It was observed that there was a eutectic reaction resulting in formation of liquid phase below solidus temperature of 5754(Al-3wt%Mg) alloy. Moreover, internal cracks have formed at the mid-plane segregation zone after Varestraint test. Results show that 5049(Al-2wt%Mg) alloy should be chosen compared to 5754(Al-3wt%Mg) alloy for welding. Moreover, low line energy should be applied and filler alloys with high magnesium content should be used during welding to decrease hot cracking tendency of welds.

In this study, the Element-Free Galerkin (EFG) method has been used for the analysis of plane stress problems. A computer program has been developed by using FORTRAN language. The moving least squares (MLS) approximation has been used in generating shape functions. The results obtained by the EFG method have been compared with analytical solution and the numerical results obtained by MSC. Patran/Nastran. The comparisons show that the mesh free method gives more accurate results than the finite element approximation with less computational effort.

In order to understand the tectonic evolution of the Kalecik region (Ankara, Turkey), a structural field study was performed in a selected area located in the east of Kalecik, where mostly imbricated thrust sheets of the Cretaceous Ophiolitic melange crop out. In the study area, the Cretaceous Ophiolitic melange, Cretaceous radiolaria-bearing sequences and the Paleocene units are all intruded by sub-vertical dykes. The attitudes of planar structures (dykes, beds and faults) and the kinematic data measured on faults were analyzed by using &ldquo / ROCKWORKS 2002&rdquo / and &ldquo / Angelier Direct Inversion Method (version 5.42)&rdquo / softwares, respectively. A major trend of NE-SW (045&deg / N) direction and relatively a post-Paleocene &ndash / pre-Miocene age was determined for the dykes indicating an extension in the NW-SE direction during post-Paleocene. The dykes cut bedded units displaying a dominant set trending in WNW-ESE (297&deg / N) direction and mostly dipping towards NE with moderate dip amounts. But at the same time, the Upper Cretaceous units were observed as intensely folded, faulted and thrusted due to the compressional regime that acted in Central Anatolia during Late Cretaceous. The angular difference between the major trend of dykes and the dominant trend of stratification was found as approximately 108&deg / ., which may also indicate that the dykes and beds were evolved during different deformation periods. The results of the kinematic analyses of different age faults revealed that the post-Paleocene &ndash / pre-Miocene Kalecik basaltic dykes are deformed under a continuous NW-SE-oriented post-Paleocene compressional to strike-slip tectonic regime which was followed by a NNW-SSE oriented post-Miocene extensional-transtensional regime.

Meshfree methods have become strong alternatives to conventional numerical methods used in solid mechanics after significant progress in recent years. Radial point interpolation method (RPIM) is a meshfree method based on Galerkin formulation and constructs shape functions which enable easy imposition of essential boundary conditions. This thesis analyses plane elasticity problems using RPIM. A computer code implementing RPIM for the solution of plane elasticity problems is developed. Selected problems are solved and the effect of shape parameters on the accuracy of RPIM with and without polynomial terms added in the interpolation is studied. The optimal shape parameters are determined for plane elasticity problems.

No description available.

The finite-difference time domain(FDTD) method is formulated by discretizing Maxwell¡¦s equation over a finite volume and approximating the derivatives with centered difference approximation. The total-field/scattered-field formulation use for simulating the uniform plane wave and the added -source formulation use for simulating the plane wave,compare the result of the electric field within metallic enclosures through aperture excited by uniform plane wave with plane wave,The larger of the exciting plane of the plane wave the more approximate to the result of the uniform plane wave .It must be very large for the induced electrical field within enclosure with a slot which vertical to interference source polarization . Generally speaking, the aperture on the enclosures not the slot but small holes on the condition of don¡¦t know interference source polarization.

In this thesis, we study the electromagnetic effect of the high-speed digital PCB in three sections. In first section, based on the FDTD modeling approach, the bridging effect of the isolation moat on the EMI caused by the ground bounce noise is investigated. We find that isolating the noise source by slits is effective to eliminate the EMI, but bridges connecting between two sides of the slits will significantly degrade the effect of EMI protection. In second section, we investigate both in time and frequency domains the power plane noise coupling to signal trace with via transition in multi-layered PCB. Separating the power plane with slits is effective in reducing noise coupling in high frequency but a new resonant mode will be excited at lower frequency. Current distribution pattern of this new resonant mode between the power planes helps us to understand this phenomenon more clearly. In final section, by using FDTD link SPICE method, we can predict the electromagnetic behavior of the PCB with active device effectively.

<p>This thesis deals with numerical simulation of turbulentflows in geometrically simple cases. Both plane and cylindricalgeometries are used. The simplicity of the geometry allows theuse of spectral methods which yield a very high accuracy usingrelatively few grid points. A spectral method for planegeometries is implemented on a parallel computer. Thetransitional Reynolds number for plane Couette flow is verifiedto be about 360, in accordance with earlier findings. TurbulentCouette flow at twice the transitional Reynolds number isstudied and the findings of large scale structures in earlierstudies of Couette flow are substantiated. These largestructures are shown to be of limited extent and give anintegral length scale of six half channel heights, or abouteight times larger than in pressure-driven channel flow.Despite this, they contain only about 10 \% of the turbulentenergy. This is demonstrated by applying a very smallstabilising rotation, which almost eliminates the largestructures. A comparison of the Reynolds stress budget is madewith a boundary layer flow, and it is shown that the near-wallvalues in Couette flow are comparable with high-Reynolds numberboundary layer flow. A new spectrally accurate algorithm isdeveloped and implemented for cylindrical geometries andverified by studying the evolution of eigenmodes for both pipeflow and annular pipe flow. This algorithm is a generalisationof the algorithm used in the plane channel geometry. It usesFourier transforms in two homogeneous directions and Chebyshevpolynomials in the third, wall-normal, direction. TheNavier--Stokes equations are solved with a velocity-vorticityformulation, thereby avoiding the difficulty of solving for thepressure. The time advancement scheme used is a mixedimplicit/explicit second order scheme. The coupling between twovelocity components, arising from the cylindrical coordinates,is treated by introducing two new components and solving forthem, instead of the original velocity components. TheChebyshev integration method and the Chebyshev tau method isboth implemented and compared for the pipe flow case.</p>

<p>After only two years of service, extensive cracking was found in the webs of two light-rail commuter line bridges in Stockholm, the Gröndal and Alvik bridges. Due to this incident it was found necessary to study the means available for analysing shear cracking in concrete structures subjected to in-plane stresses. The aim of this PhD project is to study shear cracking with these two bridges as reference. In this thesis, the first part aims to study the possibility of using finite element analysis as a tool for predicting shear cracking for plane state stresses. The second part is concerning how the shear cracks are treated in the concrete design standards.</p><p>Shear cracking in reinforced beams has been studied with non-linear finite element analyses. In these analyses the shear cracking behaviour was compared to experiments conducted to analyse the shear failure behaviour. Finite element analyses were performed with two different FE programs Abaqus and Atena. The material model used in Atena is a smeared crack model based on damage and fracture theory with either fixed or rotated crack direction. The material model used in Abaqus is based on plasticity and damage theory. The fixed crack model in Atena and the model in Abaqus gave good results for all studied beams. For the two studied deep beams with flanges the results from the rotated crack model were almost the same as obtained with the fixed crack model. The rotated crack model in Atena gave though for some beams a rather poor estimation of the behaviour.</p><p>The calculation of crack widths of shear cracks has been studied for the long-term load case in the serviceability state for the Gröndal and Alvik bridges, with the means available in the design standards. The methods based on the crack direction corresponding to the principal stress and do not include the effect of aggregate interlocking seems to be too conservative. Two of the studied methods included the effect of aggregate interlocking, it was made either by introducing stresses in the crack plane or implicitly by changing the direction of the crack so that it no longer coincide with the direction of principal stress. For calculations based on probable load conditions, these methods gave estimations of the crack widths that were close to the ones observed at the bridges. Continuous measurements of cracks at the Gröndal and the Alvik bridges have also been included. Monitoring revealed that the strengthening work with post-tensioned tendons has, so far, been successful. It also revealed that the crack width variations after strengthening are mainly temperature dependent where the daily temperature variation creates movements ten times greater than those from a passing light-rail vehicle. Monitoring a crack between the top flange and the webs on the Gröndal Bridge showed that the top flange was moving in a longitudinal direction relative to the web until the strengthening was completed. The crack widths in the sections strengthened solely by carbon fibre laminates seem to increase due to long-term effects.</p>

<p>The use of composite materials in vehicle structures could reduce the weight and thereby the fuel consumption of vehicles.</p><p>As the road safety of the vehicles must be ensured, it is vital that the energy absorbing capability of the composite materials are similar to or better than the commonly used steel structures. The high specific bending stiffness of sandwich structures can with advantage be used in vehicles, provided that the structural behaviour during a crash situation is well understood and possible to predict. The purpose of this thesis is to identify and if possible to describe the failure initiation and progression in in-plane compression loaded sandwich panels.</p><p>An experimental study on in-plane compression loaded sandwich panels with two different material concepts was conducted. Digital speckle photography (DSP) was used to record the displacement field of one outer face-sheet surface during compression. The sandwich panels with glass fibre preimpregnated face-sheets and a polymer foam core failed due to disintegration of the face-sheets from the core, whereas the sandwich panels with sheet molding compound face-sheets and a balsa core failed in progressive end-crushing. A simple semi-empirical model was developed to describe the structural response before and after initial failure.</p><p>The postfailure behaviour of in-plane compression loaded sandwich panels was studied by considering the structural behaviour of sandwich panels with edge debonds. A parametrical finite element model was used to determine the influence of different material and geometrical properties on the buckling and postbuckling failure loads. The postbuckling failure modes studied were debond crack propagation and face-sheet failure. It could be concluded that the postbuckling failure modes were mainly determined by the ratio between the fracture toughness of the face-core interface and the bending stiffness of the face-sheets.</p>