فصلنامه مکانیک هوافضا
سال هفتم شماره 2 (پیاپی 24، تابستان 1390)

Resistance spot welding is widely used in industry (especially in automobile manufacturing industries). There are approximately two to five thousand spot welds in the body of a car and the strength of the body is very muchdependent on the quality of these spot welds. Therefore, it is very important to use a proper technique for assuring the quality of spot welds. In this paper, the effect of welding parameters, such as electrical current, welding time,pressure, and the number of pulses on the weld tension-shear strength, surface hardness, and the hardness of welding center were studied. The design of experiments (DOE) technique was used for modeling the dependence of the input parameters on the output values. The results indicate that the weld strength is highly dependent on the welding time. The hardness of the weld center is also very sensitive to the changes in electrical current-thickness.

CNC machine tools are widely used to produce precise parts through executing expensive operations. In precision machining, the machine tool's geometrical and kinematical errors cause geometrical and dimensional deviations in vital features of machined parts. Kinematic modeling of a machine tool structure can help to predict the real value of the overall tool positioning error. In this paper, modeling of three-axes CNC machine tools has been considered. The developed model can simply be used for all kinds of linear axes configurations, like three-axes CNC machine tools.The model can be useful for a large variety of applications, including software based error compensation systems and prediction of the real shape of parts after being machined or measured by error contained CNC or CMM machines.Experimental tests were conducted to validate the model. Analytical estimations were shown to be in good accordance with experimental tests.

The aim of this research was to measure and study the deformation of circular steel plates (St37) in explosive forming operation for increasing the strength of circular shutters in manufacturing processes. Thus, some tests on circular steel plates (St37) were performed and the maximum deformation and it's profiles produced by exploding of different masses of C-4 in various distances, were measured. The experimental results have been compared with the results calculated by our theoretical methods. The Rajendran formula was found the best option among the existing theoretical methods for calculating maximum displacement of the material in deformation process. Finally, it was found that the percentage of error between experimental and theoretical results was negligible.

An advantage of the flow-forming process is high accuracy of products. Most researches have studied product diametric accuracy, but they have rarely investigated thickness accuracy. In this paper, a procedure is proposed to predict the effects of unbalanced forces on the product thickness accuracy. For this purpose, affecting parameters on roller forces were distinguished and roller forces were assumed as functions of these parameters. Noting that during the process, affecting parameters only change with mandrel deflection, these parameters (and consequently rollers forces) were calculated as functions of mandrel deflections. Function's coefficients were calculated using finite element method. Considering the force equilibrium of rollers and mandrel and using the above-mentioned functions,the mandrel deflection was calculated. Consequently, product thickness error due to mandrel deflection was calculated. Experiments were also performed to validate the proposed theory. Theoretical and experimental thickness variations follow similar trends. Finally, it was demonstrated that unbalanced forces cause non-uniform thickness along the product's length.

This paper presents an artificial neural network simulation, based on 216 set of experimental data for prediction of SMAW bead geometry. Input parameters consist of electrode type, current, travel speed, arc length, transverse motion, and angle of the electrode with respect to the vertical plane along the weld line. outputs of the neural network were bead width, bead height, penetration, and the rate of electrode consumption. The Error analysis in the neural network was performed by well-known back-propagation error reduction method containing two hidden layers. The results showed that the results were basically in good agreements with the available experimental data,indicating a suitable technique for simulation of the SMAW. It is concluded that the established neural network system, in conjunction with back-propagation analysis, is able to determine the bead geometry for a set of specified input parameters, facilitating the design of special purpose arc welds.

In this work, the bending of a sheet on elastic pad has been analyzed. The governing equations for bending of plate were derived and analytically solved. A formulation was derived for the indentation depth at which the plastic deformation of the sheet initiates. The equations were also numerically solved using finite difference method to verify the analytic solution. Additionally, some experiments were performed to verify these results.

The standard XRD (X-ray diffraction) technique is one of the most useful non-destructive residual stress measurement methods, especially in military and in nuclear and aerospace industries. The XRD method uses the distance between crystallographic planes (d-spacing) as a strain gauge. According to the Bragg’s law, the precision of d-spacing measurement depends on the precision of measured diffraction angles (2θ). So the precision of the XRD residual stress measurement depends strongly on how precise the 2θ values are measured. In this paper, to show the importance of the peak positioning method on accuracy of the standard XRD method and to present how diffraction peaks are analyzed, three more common peak positioning methods, C.G(center of gravity), Gaussian and parabola are used to determine diffraction peak position of a two-layer Al/Cu cold rolled plate. Then, residual stresses and standard deviations are calculated for all mentioned methods. Also, the effects of various parameters, such as background correction, absorption coefficient, Lorentz polarization factor, and kα2 striping are considered on the results of all peak positioning methods. The results of XRD residual stress measurement technique are also verified by the drilling residual stress measurement method. The results show that the residual stress analyzing by XRD technique is very sensitive to the peak positioning method and for residual stress measurement by this method in rolled plates C.G. method has the highest accuracy and the lowest standard deviation respect to other peak positioning methods. Considering various parameters showed that kα2 stripping and background correction have the highest effects on the results of all peak positioning methods.