فهرست مطالب محمد زهساز

One of the most important topics in the context of plasticity is obtaining the material hardening properties to be used in performing numerical analysis, especially when the applied load is cyclic and its value is higher than elastic limit. In this case, investigation of the kinematic hardening, which considers bauschinger effect, is very important. Since obtaining hardening coefficients of the materials using analytical methods, not only is difficult but also it usually provides significant error in the results. Hence, a computer code is developed for calculating these coefficients. In this study, to validate the calculated coefficients, the associated parameters are obtained for SS 304LN using both numerical and analytical methods and the results are compared with the experimental data. Then, these coefficients are used to study the ratcheting behavior of a SS 304LN elbow which is subjected to constant internal pressure and cyclic bending moment The results of this case study are also compared with the experimental data provided in the literature and good agreement is obtained.

In this paper, the fracture of V- notches with end holes (VO- notch) in tungsten-copper functionally graded material (W-Cu FGM) under the prevalent mode II loading is studied experimentally and theoretically. W-Cu FGM specimens are made by powder metallurgy technique in the experimental part. Several fracture tests are done on VO- notched W-Cu FGM specimens under prevalent mode loading for different notch tip radii and notch depths. To predict the fracture loads of VO- notched FGM specimens under prevalent mode loading, the Averaged Strain Energy Density (ASED) over a well-defined control volume is applied in the theoretical section. Moreover, the effect of notch tip radius, notch depth and notch opening angle on the fracture loads is investigated. In addition to good agreement between the numerical and experimental results, this study shows that Strain Energy Density (SED) theory works well on VO- notched FGM specimens under prevalent mode loading.

Friction stir welding (FSW) is one of the solid state bonding methods. One of the influential factors in the mechanical and metallurgical properties of the final joint is the tool shape, which, with its proper design, can ultimately achieve optimal joint quality. Among the improvements and modifications to improve the mechanical properties of the final joint is the out-centering of the tool pin against the tool shoulder. Off centering the pin will cause the heat generated in the larger area to be distributed and the volume of the deformed material will increase with the eccentric tool pin. In the present study, the development of a thermal model for tool with an out of center pin has been addressed. The modified model is a function of the temperature dependent friction coefficient and the temperature dependent yield stress, in which the effect of key parameters on the amount of heat generated in the FSW process is considered. In order to validate the developed model, three-dimensional simulation using ABAQUS package and two subroutines of USDFLD and DFLUX have been used. The results of the temperature distribution of the developed model have been compared with the experimental and analytical results of previous studies. Based on the results, the present thermal model with high precision is able to predict the maximum temperature and temperature distribution in the FSW process with an out of center tool pin.

In most industries, such as Petrochemical and Nuclear power plants design of pipelines which operate under high pressure is important. Especially when the pipelines are subjected to external cyclic loads for example during earthquakes. The situation becomes more severe when chemical reactions cause corrosion in pipe walls. In this case, ratcheting effect due to the cyclic mechanical load is magnified due to the corroded region and it becomes a major concern. In this paper the ratcheting behavior of corroded elbows have been investigated using finite element based software ABAQUS and numerical solution method. Two forms of corrosion: spherical and cubic geometric shapes are used with two depths of one and two millimeters on the sensitive areas such as symmetry axes in the intrados, crown and extrados of the elbow Finally results suggests the corrupting influence of corrosion in term of ratcheting and consequently increase this corrupting influence with increase depth of corroded and also its reliance to the geometry of applied corrosion. So the ratcheting on corroded place with cubic shapes is larger than spherical shapes. The results show that the ratcheting in the circumferential direction is too large compared with the axial direction.

Currently there are just a few papers about fault detection of tubes and cylindrical shells. Recently, methods based on modal curvatures have gained great attention due to their sensitivity to defects. But the methods require dense number of data extraction points which limits their industrial application. In this research a new method for damage identification of cylindrical shells has been introduced. The method does not need intact structure's data and in contrast to other methods requires a few data extraction points. By transferring the modal information of the structure to the ANN, output of the network is exact position of the defect on the structure and the method does not need skilled technician to interpret the data. Performance of the network is validated by unfamiliar data for the network and 0.97 regression is obtained

In this paper, hydrodynamic behavior of 1800-2000 sleeve valve, which is the main control valve of Talvar dam, has been studied using experimental tests and numerical solutions. Firstly, 10:1 and 15:1 models of the valve have been produced to carry out necessary tests. Then, using computer software, hydraulic models of the valve have been developed and their hydrodynamic behavior has been investigated using Computational Fluid Dynamic (CFD). Then, a numerical model of the valve has been developed based on Finite Element (FE) method and the hydro-mechanic behavior of the valve has been analyzed. Major parameters such as stress distribution in the main components of the valve and their safety factor, distribution of the velocity and pressure for different upstream head and discharge rates together with the valve’s dynamic response have been obtained. Comparing the results of the numerical analysis with the experimental data shows that analytical results have good agreement with experimental data. Results also show that stress level in the hydrostatic solution is lower than the yield limit. Also, the maximum stress level obtained from the dynamic response of the valve is compared with the yield stress of the major parts and a safety factor of 2.5 has been obtained. In addition, the results show that in practice, the nominal discharge of 23.5 m3/s (at water level of 55 meters from the valve axis with 100% opening) and 12.7 m3/s (at water level of 15 meters from the valve axis with 100% opening) can be achieved. In general, a methodology is developed to design the same large size valves which can be implemented in future products.

The present paper studies the effect of creating a fillet on fretting fatigue strength of an axle, using different multiaxial fatigue criteria and fretting fatigue damage parameter. Finite element method is applied to obtain stress and strain in the axle and the results obtained from modeling are compared with the experimental test data available in literature. The results show that fretting fatigue strength of the axle and fracture area depends on the ratio of diameter to fillet radius. Multiaxial fatigue criteria is almost capable of estimating the fatigue strength of the axle . However; the results obtained from investigating fretting fatigue damage parameter are almost in accordance with the results of experimental data and consequently fretting fatigue damage parameter can be appropriately used as a criterion for estimating fretting fatigue strength of the axles.

In this paper, c. For this purpose, stress distribution and deformation of the major parts of the valve; and also, the hydrodynamic torque, which is applied to the valve-disk, have been calculated at different opening angles of the disk. To carry out the numerical analysis, coupled Computational Fluid Dynamics (CFD) and Finite Element based solid mechanics solution methods have been used. To validate the numerical results, the displacements of some critical points on the valve-disk have been measured and the results are compared with those obtained using numerical solution. Also, the hydrodynamic torque of a model valve has been measured using an experimental rig and the data are compared with the numerical results. Both comparisons show good agreement and approve the accuracy of the numerical solution method. The results show that maximum stress occurs at the valve shaft when it is fully closed; and, hydrodynamic torque exerted on the disk becomes maximum when the valve is at almost fully open angle. The effect of different parameters such as disk-off set, flow direction and shape of the valve-disk on the hydrodynamic torque of the valve has been investigated. In this process, a modeling method to study the hydro dynamic behaviour of large size butterfly valves has been developed which can be implemented in other case studies by the manufacturers.

In this paper the creep behavior of a functionally graded (FG) rotating disc made of Aluminum 6061 and Silicon Carbide is investigated and the optimum volume fraction of FG disc and its profile has been obtained. For this purpose, the temperature gradiant along the disc radius is obtained by solving the govering heat transfer differential equation. All the thermal properties of the material are assumed to be the function of temperature and volume fraction. To obtain material properties, two models of Mori-Tanaka and Hashin-Schtrickman are used. To validate the results, they are compared with those given in the literature. Two solution semi-analytical and closed form are employed and the results are compared. The optimum design is carried out with one, and multi-objective methods which are based on genetic algorithm. The objectives are increasing the factor of safety, reducing the weight of the disc and reducing the range between minimum and maximum safety factors. The design variables are percentage of volume fraction, the power of material distribution formula, and the thickness of the disc. The results show that two solution methods compare well. Also, it has been shown that high fraction of Silicon Carbide in the outer side the disc provide optimum results. Also, contradiction of the objectives is reviled, hence the results are presented as Pareto front.

The main goal of this investigation was to study the influence of sheet spacing on the fatigue life of 5083 O aluminium alloy spot-welded joints. The values of notch strength reduction factors were obtained for all kinds of spot-welded joints with different gap distances between sheets based on volumetric approach. The fatigue lives of spot welded joints were then determined using the notch strength reduction factors and the available smooth S-N curve of 5083-O aluminium alloy sheets. The results were compared with the experimental fatigue test data, showing a very good agreement between numerical predictions and experimental results.