مجله مهندسی مکانیک
سال سی و نهم شماره 1 (پیاپی 57، بهار و تابستان 1388)

Radial forging is an open die forging process for converting ingot into the hollow and solid cylinders. Deformation in this process results from a large number of short strokes and high speed hammer blows on the workpiece. Because of maximum forging force per die is constant, determining of die force before process can prevent die damage and wastage of material. In this research a hot radial forging process is simulated thermo mechanically through 3D finite element method. The behaviour of material is considered as an elastic-visco-plastic. A mixture of coulomb law and constant limit shear is applied to simulate workpiece-die contact. As determination of radial forging force by simulation, is time consuming, so in this paper for prediction die force, response surface method (RSM) is used. RSM is the combination of mathematical and statistical techniques used in an empirical study of relationships and optimization, where several independent variables, influence a dependent variable or response. RSM, the relationships between the responses and the variables investigated is commonly approximated by polynomial functions, whilst the model parameters are obtained by a small number of experiments utilizing a design of experiment. In this paper four parameters such as die inlet angle, billet temperature, work piece feed rate and reduction used as input parameters and die force used as output parameter.

Artificial Neural Networks (ANNs) are information processing systems which in recent years have attracted the attention of many researchers for modeling of the systems and also finding solution for linear and nonlinear problems in various fields. In the present work, using ANNs method, the performance of two lobe gas lubricated noncircular journal bearing are investigated. For this purpose, the effect of compressibility number (bearing number), mount and tilt angles on the static and dynamic performance characteristics of these bearings, such as attitude angle, load carrying capacity, power loss and stability margines, are studied. The present results of ANNs method compared with earlier reported results obtained by finite element method (FEM), are in good match. It is while the ANNs method is simple and consumes not much computer time. The results also show that tilling of the two lobe bearings can improve the stability margines and this effect is more at higher compressibility numbers.

In this paper, a new approach to free vibration analysis of a cracked cantilever beam is proposed. By considering the effect of opening and closing the crack during the beam vibration, it is modeled as a fatigue crack. Also, local stiffness changes at the crack location are considered to be a nonlinear amplitude-dependent function and it is assumed that during one half a cycle, the frequencies and mode shapes of the beam varies continuously with time. In addition, by using the experimental tests, it is shown that the local stiffness at the crack location varies continuously between the two extreme values corresponding to the fully closed and the fully open cases of the crack. Then, by using the mechanical energy balance the dynamic response of the cracked beam is obtained at every time instant. The results show that for a specific crack depth, by approaching the crack location to the fixed end of the beam, more reduction in the fundamental frequency occurs. Furthermore, for a specific crack location, the fundamental frequency diminishes and the nonlinearity of the system increases by increasing the crack depth. In order to validate the results, the variations of the fundamental frequency ratio against the crack location are compared with experimental results.

Internal combustion engines produce exhaust gases at extremely high temperatures and pressures. As these hot gases pass through the exhaust valve, temperatures of the valve, valve seat, and stem increase. To void any damage to the exhaust valve, heat is transferred from the exhaust valve through different parts, especially the valve seat contact. Because of exposure to hot exhaust gases, the valve burning of an internal combustion engine is a critical issue. In this study, for thermal analysis the exhaust valve of the XU7 engine at the maximum power is considered. Modeling and analysis is performed using axisymmetric, at two steps in FLUENT and ANSYS software and the exhaust valve, seat, guide and spring are modeled. At first step taking into consideration exhaust gas flow from exhaust valve, transient thermal analysis using FLUENT software is performed from an initial conditions and followed until the steady state condition. The maximum temperature of the valve is obtained in steady state condition and it’s value is 700 ºC. At two steps using ANSYS software and temperature distribution as a result of previous step, thermal stress analysis is performed and the result shows that the maximum stress happened at the edge of the head, and with the value of more than 100 MPa. Also, the structural analysis is done where the structural stresses is smaller than the thermal stresses.

In this paper, autofrettaged process is investigated using elastic-linear strain hardening plastic material model. This analysis is based on Tresca and Von-mises criteria so that formulation can be expressed as a equation for both of criteria.The aim of this paper is determination of an applied and simple formulation to specify optimum autofrettage radius and optimum autofrettage pressure. In addition, the equation has two properties: 1- because of using elastic-linear strain hardening plastic material model, has high accuracy. 2- It can be expressed for both of Trecca and Von-mises criterion. Therefore, formulations is provided for optimum autofrettage radius and optimum autofrettage pressure so that using a factor can be applied for both of Trecca and von-mises criterion. Also, using defention of a new parameter, that is criterion of relatively hardening which is between 1 (for elastic-perfect plastic) and smaller than 2 (for elastic-linear strain hardening plastic), the effects of hardening is prescribed. Verification of results is evaluated in by comparison with the results of previous researches.

In this research forming of CUO large diameter pipes is simulated by MSC.Marc. The aim of the present work is the analysis of forming process and to determine the effects of different forming parameters on geometry and properties of final product. A plane strain elastic-plastic analysis is presented and the distribution of equivalent stress and equivalent plastic strain at each increment as well as required force for each forming part are calculated. Results of the analysis compare well with experiments.

In this research the effect of heat treatment temperature, dispersant, pH of initial sol and withdrawal speed on microstructure of prepared anatase coating was studied. The uniform coating of TiO2 was prepared on 316L stainless steel by sol-gel method. In this process TiO2 stable sol was obtained by mixing of Tetra-η-Butyle Titanat, Ethyl Aceto Acetate and Ethanol. Optimized conditions to achieve uniform nanostructure coating have been examined by XRD and SEM tests. Results indicated that heat treatment in 400 °C resulted in anatase phase with 8.5 nm crystallite sizes. Dispersant addition in optimized amount promoted formation of a uniform film by prevention of agglomeration. By decreasing pH of sol, crystallite sizes were decreased. Increasing withdrawal speed affected the thickness of film directly.