مجله مکانیک سازه ها و شاره ها
سال دوم شماره 4 (زمستان 1391)

Numerical problems arising from lack of suitable selection of coordinates in substructures’ modal models lead to absurd results and gross the errors. In the present paper numerically problems that are normally associated with coupling procedures when experimentally approaches employ، are discussed. For this purpose، free interface component mode synthesis (CMS) method is described. CMS is a coupling approach that employs experimentally generated data. Redundancy in modal model is also expressed. Afterward، to recognize proper modal coordinates that required in coupling process، singular value decomposition is used as a most powerful mathematical tool. In addition، to decrease the redundancy errors an algorithm is employed that eliminates extra degrees of freedom (DOFs). Using two continues model (strip of sheet and plate) and one empirical plate model، the efficacy of these methods and their eliminations are estimated. According to the results، modal redundancy errors are evident well in plates and effects of methods which employed to decrease these errors can be observed، easily.

In this paper، annealling processes for stress relaxation and removing residual stresses from polycarbonate sheet are studied and evaluated using experimental tests. Then، by making some alterations and modifications in the prevous heating cycles، thermal operations in the furnace on different samples are performed. By appraising the obtained results، a modified thermal process is presented which can remove the residual stress from the polycarbonate sample، while reducing the time and cost of the annealing process. According to this process، the temperature is raised to 100˚C at the heating rate of 20˚C/h. The samples are kept at this temperature for 3 hours and then، the heating is continued up to 140˚C at a slower rate of 5˚C/h. Then، the specimens are heated at the rate of 1˚C/h until maximum temperature of Tmax = 156˚C. After reaching this point، the samples are cooled immediately، and the cooling rates are same as the heating rates.

Assessment of fracture toughness in pipe weldment is important for pipeline engineers and designers as welded zone is the most critical site for crack initiation and propagation. This research reports the determination of fracture toughness in spiral seam weld of API X65 pipe steel. This steel pipe has vast usage in Iran’s gas transportation projects. To measure the material fracture toughness، single edge notch bend (SENB) specimens were machined from the pipe. For evaluating the fracture toughness in test material، two techniques were used (based on multi-specimen tests in ASTM E 1820 standard). These techniques were based on determination of crack tip opening displacement (CTOD)، and JIC. A value of 0. 23 mm was obtained for CTOD، and its equivalent KIC was 265 MPa√m from the first method. The second test method gave a value of 396 KJ/m2 for JIC، whose equivalent KIC was 332 MPa√m. A discussion on the reasons for KIC difference from the two test techniques، and comparison between the obtained results and similar data from the literature conclude the paper.

In recent years the analysis of the constrained mechanical system by using singular models has been established. Singular models consist of both differential and algebraic equation so they are very useful for modeling constraint mechanical systems where the restrictions are usually introduced by algebraic equations. Due to the importance of singular systems، control approaches of such systems have attracted many interests. In this paper، we address an adaptive state feedback control approach for MIMO singular systems that makes the closed loop dynamics of this class of systems regular، impulse free and stable. The proposed controller has been implemented on a 3-link constrained robot manipulator which is modeled by singular systems. The control objective is that the Robot end-effector moves on a special surface with determined force applying to that. The wall surface limit the robot manipulator moves. The stability of proposed controller has been proved using Lyapunov theorem. Simulation results illustrate the effectiveness of presented controller

The purpose of this research was to develop an approperiate thermodynamic model for beta-type stirling engine via changing the form of heat regenerator. In conventional beta-type stirling engines، power and displacer piston are placed in one cylinder and the operating fluid passes between the compression and expansion chambers via the bypass of the main cylinder. In the present study، a new form of heat regenerator was proposed for the beta-type stirling engine. In this new form، successive homogeneous layers of square wire meshes filled the space of displacer piston so that the displacer piston took the role of heat regenerator and displacer simultanously. To this end، modeling was done using MATLAB software and the obtained results were compared with the published values. Considering the obtained results in comparison with the conventional engines، the proposed engine had decraesed geometrical size in addition to 6. 1% higher efficiency; also، it generated 16% higher power.

In this study a fin-tube heat exchanger with different kinds of vortex generator has been studied experimentally، using exergy method. Three kinds of vortex generator have been employed: 1-Tetrahedral wedge shape 2-Rectangular block shape 3-Triangular winglet shape. In this research we used a wind tunnel to produce wind flow over heat exchanger. hot water was circulating with the steady volume flow rate and temperature range of 44 to 68 celsius in the system. The results shows that use of these VGs will decrease the air side irreversibility to heat transfer ratio (ASIHR) in the air region and for a heat exchanger with block shape VGs، this reduction is more than the other two types. This could be due to the irreversibility decline in the air region because of lower mean temperature difference of water and air and the heat transfer conditions improvement. To reveal the effects of various VGs on heat exchanger performance with respect to reducing ASIHR، a quantity is used namely performance of vortex generator (PVG). The results showed that PVG values are in the range of less than 5% for wedge-shaped VG''s to over 35% for block-shaped VG''s. Which represents the good effects of VGs especially block-shaped ones on the heat exchanger performance.

Electro-osmotic flow of an aqueous electrolyte solution between two parallel silicon walls was studied by molecular dynamics simulation. Also، effects of the electrolyte solution on the electro-osmotic flow properties were examined by varying type of the dissolved salt. To achieve this goal، two mono-valent salts (NaCl & KCl)، a divalent salt (MgCl2)، and a trivalent salt (LaCl3) were simulated and the obtained results were compared together. The unique point of the present study is that all of the events happened in the flow properties are analyzed by investigating the changes made in the electrical double layer (EDL) properties. It was found that flow velocity rises from zero to a larger value in the EDL، and it remains constant in the bulk flow. Obtained profiles showed that the electro-osmotic flow formed by a mono-valent salt is stronger than the flow formed by divalent and trivalent salts. Also it was observed that the flow velocity suddenly decreases، immediately after the EDL''s boundaries. Results showed that by increasing the valence of the salt، the number of cations attracted in the stern layer increases and so the ability of the EDL to form the electro-osmotic flow decreases. In addition، it was observed that physical properties of the driving ions (which are the cations in this work) like the ionic diameter; affect on the electro-osmotic flow properties.

As a way to meet the challenge of developing more fuel efficient and lower emission producing vehicles، auto manufacturers are increasingly looking toward revolutionary changes to conventional powertrain technologies as a solution. A Hybrid hydraulic Vehicle combines some of the benefits of conventional vehicles with mechanical energy storages. The presence of the hydraulic powertrain is intended to achieve better fuel economy than an conventional vehicle. The complexity of an Hybrid Vehicle Powertrain together with the vehicle''s many operating modes demand that intelligent controller is developed at the vehicle to guarantee stable and consistent operation. So، in this paper fuzzy control strategy has been considered for energy management strategy. The objectives of this paper are improving driving conditions and improved fuel consumption، control for hydraulic hybrid vehicles. Simulation Results like energy storage’s state of charge، ICE torque، vehicle speed and fuel economy، show that the effectiveness of the proposed control strategy during the standard driving cycle.