JOURNAL OF COMPUTATIONAL METHODS IN ENGINEERING (ESTEGHLAL)
Year:2009 | Volume:28 | Issue:1|Papers Count:9

In this paper, the harmonic balance technique is used to develop a new computer-aided design method for analyzing nonlinear behavior of log domain filters. Drawing upon the small signal linear model of circuits and power series expansion, previous methods commonly used the second and third terms of the series to analyze distortion. Contrary to conventional linear filters. log-domain filters have large signal behavior and, hence, fail to evaluate their distortions accurately. Compared with the techniques proposed elesewhere, the method proposed in this work, which is based on frequency domain analysis, is not limited by the large signal behavior of log domain filters, and is thus capable of evaluating distortions of log-domain filters more accurately, in a greater number of harmonics, and for circuits with higher complexity levels.

Analysis and interpretation of medical images are of clinical importance for medical diagnosis and treatment while they also have technical implications for computer vision and pattern recognition. In this context, one of the most fundamental issues is the detection of object boundaries, which is often useful for further processes such as organ/tissue recognition, image registration, motion analysis, measurement of anatomical and physiological parameters, etc. Although one of the best methods of edge detection is based on wavelet transform, the standard wavelet transform has its own shortcomings such as lack of shift invariant and lack of directional selectivity in sub-bands in multidimensional applications. The discrete complex wavelet transform, which is based on complex mother wavelet, not only overcomes these shortcomings but has acceptable redundancy and complexity as well. It is especially useful for multidimensional situations and for high accuracy applications such as medical image processing. In this paper, the shortcomings of ordinary wavelet transform are initially investigated and comparisons are made between the standard wavelet and the complex wavelet. Then, the discrete complex wavelet domain is applied for image enhancement and edge detection of noisy images. The simulation results show that our method exhibits a better performance, especially in noisy cases, as compared with the standard wavelet and spatial methods.

Eccentric Braced Frames (EBF) are widely used in steel structures in which the beam-column connections usually have a nonlinear semi-rigid behavior.This behavior greatly influences that of the Eccentric Braced Frame. In this research, all parts of the steel frames including beam-column connections were modeled three-dimensionally using shell elements. The nonlinear static push over analysis was used for these frames in ANSYS. In the first stage, the effects of the connections on lateral ultimate strength, capacity of energy dissipation, elastic stiffness, and EBF ductility factor were investigated. The results showed that the seismic behavior of the semi-rigid EBF was better than both the simple and the rigid EBFs. In the second stage, the effect of the link beam length was studied and the nonlinear and seismic behaviors of EBF were compared for a variety of situations. Finally, the best length of the link beam in the semi-rigid EBF was determined.

In the new generation of RC buildings, three performance levels, namely serviceability, life safety, and collapse prevention, are accepted for designing the concrete structure. Depending on the magnitude of the earthquake, cracking and plastic deformations are permitted. In this study, 8 low- to mid-rise (one to eight stories) intermediate moment-resistant RC frames were chosen. The frames were analyzed and designed based on the current seismic design code. Using the nonlinear pushover analysis, strength/stiffness degradation and residual strain and deformation of the damaged frame at the performance point of the frames were studied. In this study, the damaged frames were modeled with respect to the damage level. The damage levels of the frames depended on height and the period of the frames. The stiffness of the damaged frame degraded by about 30% compared to the original but strength deteriorated by about 10%. The ultimate displacements of the damaged frames reduced by about 50% compared to the original. X bracing system was used for repairing and rehabilitating the damaged frames for suitable performanceduring future earthquake events. The bracing increased stiffness and strength but decreased period and displacement of frames.

Based on the studies over the recent decades, steel plate shear walls, SPSWs, have been introduced as appropriate seismic load resisting systems, which have come to be used in structural design and in retrofitting of existing buildings with different configurations, both stiffened and un-stiffened. The few investigations carried out on SPSWs have shown that stiffening of a thin steel plate shear wall with the conventional horizontal and vertical stiffeners improves its non-linear behaviour.However, application of conventional stiffeners, commonly used for precluding incidence of shear elastic buckling in infill plates, is time-consuming and imposes high-fabrication costs. This study was, therefore, carried out to investigate the stiffening of SPSWs with diagonal stiffeners and their non-linear behaviour. The theoretical formulas validated against analytical results are presented that are used to estimate the shear strength capacity of the system and to size the diagonal stiffeners. The results show that diagonal stiffeners reduced the effective lengths of buckling in the inclined infill steel plate strips and, consequently, increased the elastic shear strength of the infill steel plate as well as the ultimate shear capacity of the system when compared to un-stiffened steel shear walls. Good agreement is also observed between theoretical and analytical results.

In this paper, an uncoupled thermo-mechanical nonlinear FE analysis of submerged arc welding process for straight seam large diameter oil and gas pipes to determine residual stresses is presented. The analysis starts with thermal part where, bi-elliptical Goldak equation is used for applying heat input distribution. Afterward, a certain code checks the elements one by one in order to determine the metallurgical phase transformations during cooling process based on cooling time from 800ºC to 500ºC (t8/5). Then the associated mechanical properties of each element is calculated and assigned to the corresponding element to execute the stress analysis phase. A good agreement is observed between simulation results and experimental results obtained from metallography and hole drilling test method. In addition to the results related to the special case studied in this research, undertaking severe nonlinearities due to phase transformation of the API X70-5L material and defining the equivalent material properties during phase transformation are of the new aspects of this research..

This paper explores the effects of three of the most important post-welding processes on the residual stresses of large-diameter oil and gas pipes. The analysis is one of uncoupled thermo-mechanical applied for both 2D and 3D models of the pipe. The pipe body is assumed to be prestressed due to prior submerged arc welding of its seam. To analyze the processes, parametric programming in the ANSYS commercial software is utilized. The processes are hydrotest, quenching by Houcsh method, and quenching by the last pass heat sink welding, which is the most cost-effective technique. The processes are, respectively, performed by applying very high internal pressure to the pipe, induction heating plus spray cooling of the pipe, and cooling the weld zone simultaneously with the last past welding. The novel aspects of the study include investigation of the above processes for API 5L-X70 pipes and application of FE analysis to simulate the quenching of pipes for both the Houcsh method and the last past heat sink method. The effects of the processes on the residual stresses are compared. The simulations are verified qualitatively using experimental results reported in the literature.