New parametric study of nugget size in resistance spot welding process using finite element method

Resistance spot welding process (RSW) is one of important manufacturing processes in automotive industry for assembling bodies. Quality and strength of the welds and therefore body mainly are defined by quality of the weld nuggets. The most effective parameters in this process are: current intensity, welding time, sheet thickness and material, geometry of electrodes, electrode force, and current shunting. In present research, a mechanical–electrical–thermal coupled model in a finite element analysis environment is made using. Via simulating this process, the phenomenon of nugget formation and the effects of process parameters on this phenomenon are studied. Moreover, the effects of welding parameters on temperature of faying surface are studied. Using this analysis, shape and size of weld nuggets are computed and validated by comparing them with experimental results from published articles. The methodology developed in this paper provides prediction of quality and shape of the weld nuggets with variation of each process parameter. Utilizing this methodology assists in adjusting welding parameters so that costly experimental works can be avoided. In addition, the process can be economically optimized to manufacture quality automotive bodies.     

Numerical simulation of tensile strength of upset welded joints with experimental verification

Resistance upset welding (UW) is a widely used process for joining metal parts. In this process current, time and upsetting force are three parameters that affect the quality of welded products. This paper considers numerical simulation and experimental investigation of UW process parameters. The investigated parameters include heating and post-weld heating current and their corresponding duration as well as interference of the part features that form the joint. In this study, evaluation of tensile strength of the welded joint with variation of the process parameters is also reported. For numerical analysis, a two-dimensional axisymmetric model using a coupled electro-thermal finite element method is developed to study the thermal behavior of the welded joints. The results of this numerical simulation are used to determine the status of the weldment and therefore evaluate the quality of the weld at the joint. Both numerical and experimental results suggest an optimum set of welding parameters, i.e. time and electrical current that yields a maximum value for the tensile strength of the joint. Also the effects of post-weld heating time and current on the tensile strength are evaluated and show that these parameters have a remarkable effect on improving tensile strength of the weldment.     

A waste heat recovery system development and analysis using ORC for the energy efficiency improvement in aluminium electrolysis cells

The present work investigates an active waste heat recovery system for the side walls of the aluminium electrolysis cells, enabling utilization of the extracted heat in power generation. This will potentially lead to energy efficiency improvement in the primary aluminium production industry and an enhanced aluminium production rate. An experimentally validated loop thermosyphon heat pipe model was used for heat extraction from the cell side wall. Boosting system thermal efficiency through waste heat recovery, by means of a heat utilization system, and increasing the level of control, as well as thermal equilibrium, stand as the main addressed objectives of the current study, which consequently result in an increased aluminium production rate. An organic Rankine cycle is incorporated into the system, and its performance is evaluated, taking into consideration the operating situations in terms of available temperature and thermal power range.     

An Iterative Receiver for Uplink of Coded MIMO DS-CDMA System Employing Layered Space-Time Transmission

We consider a coded multiple-input multiple-output (MIMO) DS-CDMA system using layered space-time transmission in multipath wireless channels, where space-time signals from multiple antennas of multiple users propagate through rich scattering multipath fading. We propose a receiver employing iterative joint detection and decoding with a reduced-complexity detector using linear minimum mean squared error filtering with a priori information and parallel soft-input soft-output (SISO) decoders. Computer simulation results show that the proposed receiver for multi-user MIMO transmission provides high-spectral efficiency and performance approaching to single-user bound. Furthermore, the reduced-complexity receiver outperforms an iterative soft decision-directed maximal ratio combining (DD-MRC) receiver, RAKE receiver as well as a conventional non-iterative receiver.     

Fast decomposition of matrices generated by the boundary element method

A new method to reduce the solution time of matrices generated by the Boundary Element Method is presented here. The method involves converting the fully populated system into a banded system by lumping certain coefficients of the matrix into fictitious nodes and then constraining these nodes to accurately represent each coefficient. The major advantages of lumping over the substructuring method are that lumping can be applied to arbitrarily shaped geometries and infinite-domain problems and that it preserves the diagonal-dominance of the matrix. It is shown here that the proposed algorithm reduces the rate of increase of solution time t of an n-degree-of-freedom problem from t ∝ n³ to t ∝ n². Although the algorithm is for thermal problems, its extension to mechanical problems is straightforward. The procedure can easily be incorporated into existing boundary-element-based packages.     

Seismic qualification of motor operated valves—alternate approach

This paper presents a potential alternate method for determining operating capacity of motor-operated valves subjected to seismic and other applicable loadings. As a result of programs at nuclear facilities to ensure the operational capability of MOVs (under NRC GL89-10), extensive analytical focus to develop the structural capability of valves has ensued. In the past, seismic qualification of valves typically addressed the strength of the topwork structure to resist inertial loading from excitation of the large valve actuator mass. These evaluations paid little or no consideration to the loading resulting from valve closing forces. The focus of the recent efforts is to develop the maximum operational capability of the valve, in terms of thrust, with consideration of seismic and other services loading as applicable. The alternate method outlined in this paper presents a series of thrust capacity curves, with reduction factors for seismic loading which can be applied and developed to determine safe thrust loadings without performing extensive analytical effort. A similar approach was put forward by the SQUG GIP approach to MOVs to ensure the safe operation of valves based on past earthquake experience. However, the GIP approach cannot be used to determine safe operational loads and thus has limited use in the necessary analysis required for GL89-10 programs at nuclear facilities.     

Soil–structure interaction in fuel handling building

This paper presents an accurate three-dimensional seismic soil–structure interaction analysis for large structures. The method is applied to the fuel building in nuclear power plants. The analysis is performed numerically in the frequency domain and the responses are obtained by inverse Fourier transformation. The size of the structure matrices is reduced by transforming the equation of motion to the modal coordinate system. The soil is simulated as a layered media on top of viscoelastic half space. Soil impedance matrices are calculated from the principles of continuum mechanics and account for soil stiffness and energy dissipation. Effects of embedment on the field equations is incorporated through the scattering matrices or by simply scaling the soil impedance. Finite element methods are used to discretize the concrete foundation for the generation of the soil interaction matrices. Decoupling of the sloshing water in the spent fuel pools and the free-standing spent fuel racks is simulated. The input seismic motions are defined by three artificial time history accelerations. These input motions are generated to match the ground design basis response spectra and the target power spectral density function. The methods described in this paper can handle arbitrary foundation layouts, allows for large structural models, and accurately represents the soil impedance. Time history acceleration responses were subsequently used to generate floor response spectra at applicable damping values.     

A general boundary-integral formulation for zoned three-dimensional media

A new boundary-integral formulation is proposed to analyse the heat transfer in zoned three-dimensional geometries. The proposed formulation couples the boundary formula, the gradient of the boundary formula and the exterior formula. An advantage of this method over the traditional methods is that any linear condition at the interface between subdomains may be incorporated into the formulation at the outset. In addition, the new method provides a sparse and well-conditioned matrix of coefficients with a minimum number of equations.     

Aqueous preparation and evaluation of albumin-chitosan microspheres containing indomethacin

Controlled-release egg albumin-chitosan microspheres containing indomethacin as a model drug were successfully prepared by coacervation method. The proposed method can offer a simple method for microsphere preparation in an aqueous system with the elimination of the use of organic solvents that are usually needed in conventional techniques of microencapsulation. The interaction between negatively charged egg albumin molecules in phosphate buffer, pH 7.2, or sodium hydroxide solution and positively charged chitosan molecules dissolved in diluted acetic acid to form an insoluble precipitate was the principle for the formation of the microspheres. The effects of many process variables, such as amount of formaldehyde as a cross-linking agent, stirring time, final pH of encapsulation medium, initial drug loading, and albumin concentration or albumin-to-chitosan weight ratio, on the properties of the prepared microspheres were investigated. Incorporation efficiencies of the microspheres to the drug were high in most cases and ranged between 63.3 ± 3.6% and 92.39 ± 3.2%, while particle sizes were 435.2 ± 12.6 up to 693.9 ± 34.6 µm for the different tested batches. On the other hand, the values of angles of repose and compressibility indices were in the range of 23.5 ± 0.4 to 32.0 ± 0.7 degrees and 11.1 ± 0.7% to 23.6 ± 0.7% respectively, which indicate overall good free flowing nature of the microspheres of all batches. The maximum required amount of the cross-linking agent was determined to avoid excessive unnecessary chemicals. It was also noticed that excessive time of stirring and excessive initial drug loading are not recommended as it may lead to microspheres of low properties. The pH of the encapsulation media (pH 3.77 up to pH 4.91) significantly affected the properties of the microspheres. As the pH of the encapsulation media was increased, the incorporation efficiency, particle size, and flowability decreased, along with increase of drug release rate, which could be related to incomplete cross linking of the microspheres matrix. It was also observed that high concentration of albumin solution and accordingly the increase of albumin-to-chitosan weight ratio were accompanied with increases in incorporation efficiency and particle size with improved microsphere flowability and slow indomethacin release. Thus, the proposed microspheres showed the ability to control the release of indomethacin, and their properties were highly affected by many process variables that could be controlled to obtain an optimized system.