Year:2003 | Volume:37 | Issue:1 (79)|Papers Count:17

Fatigue analysis is one of the most important case studies in marine structures design in which stress concentration factor is used as a parameter including the structural specifications. Codes and standards present different formula for determination of stress concentration factors at offshore structures joints that their accuracies and validations are not clear. This study is an attempt to create a 3-dimensional model, using finite element method and employ appropriate elements, as to discuss stress concentration factors. At current research, the finite element model of unplanned and multiplanar K joints, including the weld, was produced using ANSYS-5.4 general-purpose Finite element software and related stress concentration factors were compared. A simple and relatively accurate method is used to simulate the weld. It was shown that the common methods for determination of stress concentration factors of multiplanar joints, in which an equivalent planar approach is employed, are not economical. Comparing the results of numerical method and those of parametric equations, some suitable equations were derived and their validation range was discussed. With respect to parametric studies of K & KK joints, optimum geometrical dimensions for an economical design were determined. At the end, two equations for determination of stress concentration factors of K & KK joints were presented as research result.

To evaluate the remaining load carrying capacity of Akbar-Abad Railway Bridge, dynamic and static load testing were carried out. The bridge included five identical 6-meter arches, made of mass concrete material, with more than 60-year old. In the dynamic test, a 120-ton locomotive passed over the bridge at different speeds and displacements and accelerations were recorded at critical sections. In the static test, 500 tons of rail weight was gradually applied to the middle span and deflections were measured. The bridge showed a relatively stiff response without any indication of resonance or big dynamic magnification effects during the tests. The response remained linear-elastic under maximum load and the results suggested a relatively large strength reserve, relative to the standard service loads, for the bridge. Overall, the study showed that, despite aging effects and experiencing deep and wide cracking at the crowns, the bridge still enjoyed relatively large strength reserve and safe performance.

Discrete Element Method can be used for numerical analysis of different geotechnical problems such as retaining wall earth pressure distribution. This paper is an effort to use the method in determining active and passive earth pressure distribution behind a retaining wall. Soil mass in the present method is treated as comprising of blocks, which are connected by elasto-plastic Winkler springs. The solution of this method satisfies all equilibrium and compatibility conditions. The method has the simplicity of the classic limit analysis and offers more ability in solving problems in complex geometry and loading conditions such as seismic impacts, pore water pressure, non-homogeneity of the soil reinforced backfill soils, etc. In this paper, the formulation of the method is briefly reviewed. Examples are shown to demonstrate the applicability of the method for analysis of earth pressure behind a retaining wall including dynamic lateral pressure of non-homogeneous soil. The results are compared with previous classic methods. Applicability of DEM for analysis of reinforced soil structure and advantages of this method over conventional methods are also discussed.

Calculation of impedance function is a mixed boundary value problem, which is changed to a Neuman problem by using a functional expansion of stress distributions. The problem is, then, solved using integral transforms (Hankel transform for circular foundations) and the impedance function is obtained in terms of an integral that is resulted from Hankel inverse transform. This integral cannot be solved analytically and, therefore, is solved numerically and the impedance function is obtained. The variation of impedance function with dimensionless frequency is presented for some isotropic or transversely isotropic materials with different poisons ratios. It can be seen that anisotropy has a great effect on impedance functions and this effect increases as the anisotropy ratio increases.

Knowing the nature of earth pressure behind the retaining walls under different conditions plays an effective role in safe design and construction of these structures. Many physical and analytical models have been developed to evaluate the earth pressures under static loading conditions. Although there are still some differences between the earth pressures measured by large scale physical models and those estimated by analytical models, even under static loading conditions, however the pressure distribution behind the retaining wall and its point of application are relatively specified and defined clearly in these conditions. The main difficulty rose when the dynamic conditions occurred. In spite of different methods and codes suggested to consider the pressure distribution and the point of load applications behind the walls in these conditions, to apply these methods correctly, one has to know the accurate assumptions upon which these models have been developed. In order to achieve an accurate and comprehensive understanding of the earth pressures under different conditions, a new physical model has been developed which is capable to study the earth pressures behind the model walls under different monotonic and cyclic overburden surcharges. The retaining walls in this model may set at rest condition or have any required displacement leading to active or even passive conditions. In this paper the design principles and construction of such model have been described in details and its capabilities in monitoring the conditions and measuring the developed pressures have been explained. Finally some important test results, achieved through extensive studies by this model, have been presented and discussed.

Analysis of amplification of base motion passing through the V_shape valises, filled with alluviums, is usually involved with uncertain geometrical parameters and geotechnical properties. It is also involved with complex and time consuming soil structure interaction algorithms. Therefore, a neuro_fuzzy system can result in a reasonable solution avoiding any interaction analysis for a particular geometry and geotechnical parameters. Defining such fuzzy system, of course, needs a large number of interaction analyses, each, participating in training of the system. In this work, it is shown that a midpoint result namely; an amplification curve with respect to dimensionless frequency, exists, which can brake the procedure into two parts, each using a small number of interaction analysis. The results of narrow valises are presented in two different ways: A verbal description type of results for quick reference, and a calculation method using any conventional fuzzy software.

In this research work a shape from shading (SFS) technique which incorporates a Lambertian model is implemented for the automatic generation of a digital terrain model (DTM) using a single view aerial image. The developed algorithm is tested on both simulated and real data. The estimated accuracy of the generated DTM from the simulated data, which has a bilinear surface, is about ±3 cm. The real data is a scanned aerial photograph taken over a low texture hilly terrain. The generated DTM by the SFS technique is compared with a DTM acquired by the manual measurement of the stereo image of the same area using a photogram metric plotter. The estimated rimes of the discrepancies between the grid nodes of the measured and the automatically generated DTM is about ±4 meters. The unsuccessful reconstruction of the terrain surface for the real data is due to the fact that a simple Lambertian model does not take into account, in a perfect way, different nondeterministic influential factors such as the terrain alb Edo variations and the random noise. The influence of the latter case was reduced by a low pass filter applied as a preprocessing stage.

The direct application of finite element method to crack problems will produce errors, which cannot be neglected. One of the major sources of the errors is crack tip singularity. The numerical solution of path independent J-Integral method is considered to be based on conservation law of energy in conservative fields. Combination of such method with finite element method creates a very powerful tool to analyze cracks with complex shapes and model geometries. In the present research after a short review of fracture mechanics history in general and J-Integral in particular, cracks in composite materials which are divided into internal cracks and interlaminar cracks are analyzed using J-Integral. For numerical solution of J-Integral two computer programs are written using MATLAB software. Input data for these programs, stress-strains and coordinates of nodal points come from output files of finite element analysis. These programs read their input data directly from the output files of finite element program. Comparison between results of the present study and existing results in literature shows the ability of this method in analyzing plane crack in three-dimensional space.

Explosive forming of cone was investigated from some new analytical, numerical, and experimental aspects. In the analytical modeling, it is supposed that deformation profile of plate is a truncated cone during process, which will be deformed to a fully formed cone at the end of process. The variation of plate thickness was also neglected. Analysis required equations for determining vertical velocity and vertical displacement profiles during process; required pressure and final product dimensions are also given in this analytical model. ABAQUS/Explicit package was used for FEM simulation. The results of analytical model given in this paper are fully agreed with experimental and numerical (FEM) results.

In this paper relationships for resistance and trim of planning Catamarans in calm water are presented. This work is based on Savitsky empirical method for monohull planning crafts. According to the present method, a computer software is developed for calculation of resistance and trim of a Catamaran in different speeds. Moreover, the effects of different factors such as displacement, longitudinal center of gravity, deadrise angle, length to beam ratio, distance between demihulls are studied. The investigation of these factors can be very useful in preliminary design for designers and initial assessments of the craft.

Laminar two-dimensional flow of a viscoelastic fluid of the "Second-Order" type was investigated above a fixed rigid plate. Conventional boundary layer approximations were invoked to simplify the equations of motion which incorporate several elastic terms in addition to the familiar inertia, viscous and pressure terms. The governing boundary layer equation was realized to be a PDF equation which was further transformed into a nonlinear fourth-Order ODE through the use of the concept of a stream function together with introducing a similarity variable. The x-coordinate could not be eliminated from this equation, and so only local similarity solutions could be obtained for this flow field. Using a combination of finite difference and shooting methods, this equation was solved numerically for Deborah numbers as high as 1.0. The results showed that the wall shear stress scales with fluids elasticity, increasing for higher values of fluids elasticity. The boundary layer thickness was also found to decrease with an in crease in the elastic level of the fluid. Around a Deborah number of 0.4, velocity inside the boundary layer is predicted to exceed that outside the layer, a prediction which awaits experimental verification.

The present work addresses the validity of no-slip condition for water-based betonies drilling muds. The mud used for the investigation was a suspension of 10% solid (betonies) concentration and its wall slip effects was explored using viscometric techniques. A concentric-cylinder viscometer (model Fann 50C) was used for this purpose. Flow curves obtained using two bobs with different radii indicates that the viscosity measured is a function of the gap distance between the inner and outer cylinders confirming that slip has indeed occurred for this particular mud. A simple technique was explained which enables correcting the flow curve for the slip effects thus resulting in true viscosity function for the test fluid. Experimental results obtained show that slip velocity scales with wall shear stress inferring that for this particular fluid, wall slip effects are less sever as compared to other non-Newtonian fluids such as polymer melts and solutions

Honeycombs are used in many industries due to their individual properties such as energy absorption property. In this paper the "angle element" is introduced and its energy dissipation mechanisms under quasi-static loading are studied. The crushing load of honeycomb is calculated by energy method and describing Wierzbicki model for determining the mean crushing strength and half wavelength of folding of cell walls, a new model is presented which predicts better results than the Wierzbicki model. Based on this analysis the cell geometry has important role on mechanical behavior of honeycomb structures; so that decreasing the cell size increases the mean crushing strength and decreases the half wavelength of folding. On the other hand increasing the cell wall thickness increases these two parameters. The results of the new model are in good agreement with available experimental data.

An attempt, herein, is made to present a comprehensive solution of Jerk vs. Force- Optimal-Control of active suspensions. The comparison is made for a 1/4 car model; however, the body acceleration and jerk are directly from the system Hamiltonian. Also, in both J.O.C. and F.O.C. solutions, the force actuator is used to implement the controller. The results show that a reduction of jerk and acceleration is obtained against a higher rattle space and tire deflection. The compromise/tradeoff can be managed by setting appropriate weights in the cost function.

PAM (Principle Axis Machining) is introduced as an effective way of milling free-form surfaces on 5-axis machines. PAM matches the minimum principle curvature of Toroidal end mill to the maximum principle curvature of the surface at the machined points via tool inclination in two directions. An investigation is made, herein, for comparison of surface roughness in milling with 3 end mills (flat-bottom, ball-noise, and Toroidal). This is done, first, theoretically based on tool/surface geometry along and across feed directions, and then evaluated according to some typical test results.