Sharif Digital Repository

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,... 

Most engineering components must be manufactured so they are safe to use and are ‘‘fit for purpose’’. However despite all the efforts, cracking of the engineering components is inevitable. Residual stresses play an important role in either increasing or decreasing the possibility of failure. For instance, enhancement in apparent toughness following pre-loading arises principally because of the creation of local crack tip compressive residual stresses. Introducing compressive residual stresses can also slow the fatigue crack growth. Yet, the dilemma is choosing an efficient method growth to introduce such stresses for retardation of fatigue crack growth. There are many methods available in... 

Offshore structures are valuable structures used in the offshore petroleum and gas fields for the purpose of hydrocarbon production operation. These structures are influenced by wave loads. Sea waves are inherently alternative and for this reason fatigue failure is considered as one of the most important modes of failure in these structures. Joints in these structures are made by tubes and they are called tubular joints. The joining process in manufacturing of these joints is welding. Welding process induces residual stresses, mostly tensional, in the substructure of materials. This tensional residual stress has a destructive effect on fatigue life of the joint and increases probability of... 

Residual stresses are one of the influencing factors on fatigue life. These stresses may cause cracking and failure of the element even in the absence of any applied stress or any external load. For this reason they are very important, and their measurement is crucial. The methods for measuring the residual stress in spite of their name often measure the strain and by analyzing the measured strain, determine the residue stress of the element. The important problem about residual stresses is that their values in the element often remain unknown and usually no simple and non destructive method for measuring is available. Usually finite element analyzing is used for modeling the production... 

According to the widespread application of shape memory alloys, their study in the presence of cracks has received lots of attention in the last two decades. The phase transformation ahead of the crack tip may change the stress field and the fracture behavior, especially at the nanoscale. Thus, modeling this phenomenon helps to recognize the affecting factors on crack propagation and prediction of material behavior when cracks exist. In the first step of this thesis, a phase field approach for crack propagation was presented, first by considering fracture as phase transformation and then as bond breaking. According to the scaling-up strategy, the developed theory is applicable from the... 

Shot peening is a cold-working process commonly used in industry to improve the fatigue performance, stress corrosion resistance and surface nano-crystallization of metallic parts. This process extends fatigue life via two mechanisms: Preventing the crack growth due to compressive residual stresses and, preventing the crack initiation because of increased material hardness. These mechanisms are the results of the bombardment of the component's surface with small spherical particles. In this research, an analytical model is developed for estimating the residual stress distribution induced by shot peening process. The modifications of the developed analytical model are related to... 

Residual stresses are stress fields that exist in the absence of any external loads. All mechanical processes can cause deformation that may lead to residual stresses. It is well established that virtually no material, no component and no structure of technical importance exists free of residual stresses. However, in practice, the knowledge of the true distribution of exiting residual stress is rare. The residual stress fields are very challenging to evaluate, independent of the measurement techniques. The important aspect about the residual stress determination is that often true distribution is unknown in the engineering specimen. A simple and non-destructive method to measure residual... 

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... 

Laser cladding is a processing technique of layer-by-layer material deposition which is widely used to repair important components. However, tensile residual stresses generated in this process may result in detrimental effects such as crack, undesirable deformation and reduced fatigue life. A thermo-mechanical finite element model is developed to accurately predict residual stresses of the multiple beads laser cladding process with incorporating the cyclic plasticity into the analysis by utilizing the nonlinear kinematic hardening behavior. FE results are presented for Inconel 718, a material for which the laser cladding process is widely used. The FE results are compared with those of... 

Prediction and calculation of shielding tensors of H, C, F and P nuclei of different molecules has been the subject of much research, because these nuclei have the greatest importance in NMR experiments. The optimum wave functions and calculation method were obtained using empirical models and factorial design. Based on preliminary experiences, the following four factors at two levels were selected: electron correlation, triple-ξ valence shell, diffuse function and polarization function. The wave functions for calculating gas phase 1H and 13C chemical shifts of various types of alcohols, amines and simple hydrocarbons were optimized using empirical model. The B3LYP/6-311+G wave function is... 

Keywords or keyphrases are of importance as the smallest unit of representing the meaning of a text. Automated Keyword Extraction (AKE), as one of the natural language processing tasks is used in various applications such as searching, indexing and information retrieval. Keywords of scientific articles are basically specified manually by their authors, whereas most of the information available on the internet lack such keywords. In this research, we endeavor to automatically extract keywords of a set of Persian paper abstracts using an unsupervised machine learning method. The method used is to extract a set of candidate phrases from the text, and to cluster the document words to find a set... 

The objective of this study is to explore the relationship between the stock price volatility and dividend policy (dividend yield and dividend payout ratio) for the Iranian stock market. According to the studies of Baskin (1989) and Allen and Rachim (1996), the fixed effect and random effect regression models are applied in this research. The sample of data is composed of 200 public firms which are listed on the Tehran Stock Exchange (TSE) and paid dividends continuously from 2010 to 2020. The results indicate that the dividend policy has a significantly negative relationship with the stock price volatility. In addition, the Size of the company is negatively related to the stock price... 

Accurate description of kinematics of continuum mechanics is essential in simulation of large deformation problems in solid mechanics. From the numerical viewpoint, two main approaches have been used for such description; the Lagrangian approach and the Eulerian approach. However, each of these approaches suffer from shortcomings which hinders their application in large deformation problems. A more general approach called the Arbitrary Lagrangian-Eulerian method (ALE) provides an opportunity to exploit the advantages of both Lagrangian and Eulerian approach, while avoiding their shortcoming. In an ALE analysis, the FE mesh is neither attached to the material nor fixed in space, necessarily.... 

A general regularity has been found based on an effective pair potential of Lennard-Jones LJ (12, 6) for both dense nonmetallic and nonionic fluids and solids; namely, (Z-1)V2 linearly varies with respect to ρ2, this equation of state (EoS I) is known as LIR. The other equation of state (EoS II), according to wich the isotherm of (Z-1)V2 is linear in term of 1/ρ, wich gives a good description for the metallic and ionic fluids and solids. This equation of state was suggested based on an effective pair potential of LJ (6, 3). Unexpectedly, solid and liquid Ne indicates a significant deviations from EoS I. Recently, a general equation of state (EoS III) based on an effective near-neighbor pair... 

In this thesis, the molecular dynamics simulation is used to investigate the melting transition of B-DNA molecule, via of configurational entropy, the fraction of broken hydrogen bonds (f-curve) and hydrogen bonding energy.We have performed molecular dynamics simulation on Drew-Dickerson oligomer with sequence of (CGCGAATTGCGC) at different temperatures, within the range of 280-400 K with the 20 K intervals. The simulation was done in two different mediums (pure water and 1 M NaCl), to see influences of water and salt in stabilizing the DNA molecule. At each temperature, configurational entropy is calculated by the Schlitter’s formula, using the Cartesian coordinate of all atoms. So, in each...