Sharif Digital Repository

One of the requirements of designing structures is the properties of their materials. The behavior of the material should be studied under the same conditions (loading regime) as it would be in practice. Regarding the structures which are subjected to cyclic loading, usually the results of monotonic tests on their material are used for their analysis under cyclic loads. Taking this fact into account that the behavior of materials under cyclic loading is normally much different from that under monotonic loading, one should doubt the results of the analyses of structures subjected to cyclic loading for which monotonic test results are used. In this project, the properties of steel rebars under... 

Aluminum alloy (A356.0) is used in the production of cylinder head. Automobile engines are often subjected to thermo-mechanical loadings. In order to investigate the material performance, it is crucial to obtain fatigue properties of the material under thermo-mechanical and isothermal fatigue tests. The specimen is subjected to tensile and compressive stresses in a constant temperature. But in thermo-mechanical tests, the temperature and the strain vary in a specific span (between maximum and minimum values). By using the strain- stress hysteresis loop (obtained by fatigue tests), we can determine fatigue properties. Then, these properties are used in the simulation. In this thesis,... 

Magnesium alloys have been used in the production of light weight components in automobile industries (for example: the ladder frame and the valve cover) due to a good combination of low density (1740 kg/m3), the tensile strength (160-365 MPa) and the elastic modulus (45 GPa). For the design and the optimization of these components under various loadings and different conditions, it is necessary to determine fatigue properties of the material by using thermo-mechanical and low cycle fatigue tests at different temperatures. In thermo-mechanical fatigue tests, the temperature varies in a specific range with the mechanical strain. The goal of this research is to investigate low cycle fatigue... 

The main purpose in this thesis is the finite element simulation of the stress-strain behavior in an aluminum-silicon-magnesium alloy, subjected to low cycle fatigue and thermo-mechanical fatigue loadings. This finite element modeling is performed in the ABAQUS software. Two time-dependent and time-independent models are used to simulate the behavior of the material. The time-independent model is the combined nonlinear isotropic/kinematic hardening model, which predicts the mean stress relaxation behavior, the cyclic softening or hardening and the translation of the yield surface, while it ignores the effect of the strain rate. Time-dependent models include Johnson-Cook and the two-layer... 

In sliding wear, the process of making debris takes place with different mechanisms. Previous works estimating the wear rate have been carried out based on contact mechanics and material failure mechanisms. In present work, the possibility of existence of low-cycle fatigue as the dominant mechanism in the wear of an as-cast eutectoid steel is investigated. Moreover, wear coefficient and the the volume loss per distance of that steel was predicted. At first, wear tests were done in order to obtain the wear trend of the steel in different loads. Then, regarding the roughness of the disk surface, a finite element model was built to simulate the sliding of the pin surface over the asperities... 

Application of light metals such as Magnesium alloys due to proper mechanical behavior is increasing in industries such as automotive, electronics and so on. In this project, ingot of Mg-9Li-1Zn alloy with 10 mm thickness were produced by casting, then hot rolled at 300oC to reach 2mm thickness. Annealing at 350oC for 3 hours resulted homogenous properties in the final sheet.Study of monotonic and cyclic behavior of Mg-Li alloy by experiments and FE simulation was the goal of this project. Experimental monotonic and cyclic test were carried out in order to obtain the hysteresis loops and cyclic stress-strain curve. The Chaboche constant were extracted from the experimental test and used to... 

In this thesis, based on the taking-off and landing of aircraft with GE 21-J85 turbine, the low cycle fatigue (LCF) test for superalloy used in first stage blades is considered. The LCF tests were performed on both coated and uncoated Rene 80 which are used in GE 21-J85 turbine at R = (min/max) = 0, strain rate of 2 10-3 s-1 and 760oC, 871oC and 982oC. To produce specimens, an investment casting method was used so that adequate amount of polycrystalline round bars could be obtained. The specimens were all homogenized and solution treated. After the inspection, some specimens were coated with pack cementation method in CODEP-B powder in H2 atmosphere. The coated and uncoated specimens all... 

Thermal barrier coating (TBC) systems have been widely used in gas turbines and diesel engines. Their advantages include reducing the fuel consumption, improving the performance and increasing the fatigue lifetime at high temperatures. In the present thesis, the fatigue behavior of an aluminum alloy (the A356.0 alloy), with and without a typical TBC system is studied. The application of this research can be introduced as coating the cylinder head in diesel engines. Initially, coating process parameters (including the feed rate and the nozzle distance) of the plasma spraying, with the objective of bending strength are optimized. The optimum value of the TBC thickness is obtained based on...