Sharif Digital Repository

Fatigue failure is deemed as one of the most dominant failure mechanisms of metal components applied in the industry, as in more than 90% of cases which have led to failure, fatigue mechanism has played the major role. Fusion welding is one of the most widespread joining techniques and nowadays is extensively used in various industries including transportation, machine construction, petroleum, and offshore structures. Material properties undergo inevitable changes during welding due to non-uniform heating and cooling as well as addition of a new material and, as a result, an inhomogeneity in the base metal. That being so, investigation of welded joints subjected to cyclic loading that may... 

In recent years, investigation of mechanical properties of micro- and nano-structure materials, and peculiarly nanocomposites have been attracted a wide variety of researches. In this regard, friction-stir-processed (FSPed) metal matrix nanocomposites have been considered an important case with appropriate mechanical properties especially for fabrication of ultra-fine grained Al alloys and Al-matrix nanocomposites. In this research, Al matrix hybrid nanocomposites were fabricated via multi passes-friction stir processing (FSP) of Al-Mg alloy (AA5052) with different amounts (~2 and 3 vol%) of pre-placed TiO2 (30 nm) particles nanoparticles. It was revealed that employing of cumulative FSP up... 

Special features of magnesium such as low density, perfect bio-compatibility in the human body, and close elastic modulus to the human bone have increased the use of Mg alloys in the bio-medical applications. Nevertheless, low mechanical properties and poor corrosion resistance of Mg alloys have limited their bio-medical applications. The addition of alloying elements together with the use of precipitation hardening and plastic deformation are the most effective methods to improve the mechanical properties and increase the corrosion resistance of Mg alloys. Thus, the present research with the aim of improving the mechanical properties of an Mg-Zn alloy by Ca and Gd additions together with... 

One of the Friction Stir Welding (FSW) limitation is thinner sheets joining. To solve this limitation, sheets were welded through FSW at upper thickness and then reduced welded samples thickness. In this research, 3-mm-thick AA1050 aluminum plates were joined by FSW.For this purpose, the effect of welding parameters on microstructure and mechanical properties of the welded samples were investigated by changing the ratios of rotational speed (900,1200 rpm) to travel speed (50,90,150 mm/min) (ω/v). It was revealed that the grain growth rate was strongly increased with the increase of the heat input by rotational speed at constant travel speed. Subsequently, hardness reduction was observed in... 

After the discovery of high temperature superconductivity in (LaM)2CuO4, different type of cuprate superconductors have been synthesized. Among them, YBCO compounds (e.g. YBa2Cu3O7-δ) have drawn more attention by researchers due to some advantages such as easier conditions of synthesis high quality samples in comparison with the other cuprate superconductors. However, superconductivity properties of these compounds are very sensitive to structural imperfections; hence it is important to study the effect of the microstructure on the behavior of the compounds. The compound YBaCu2Ox (Y112) is one of these microstructures that may form in YBa2Cu3O7-δ (Y123) samples. On the... 

Loess soil layers in semi-arid to arid areas have been deposited through the accumulation of aeolian sediments, resulted in a special metastable and open structure being usually unsaturated in nature. Since naturally occurred loess is widespread, it has been recently considered as a promising resource for constructing earth-fill structures. However, due to different microstructures of intact and recompacted loess, different responses to hydraulic and mechanical loading would be expected. Up to now, the hydro-mechanical behavior of loess from different perspectives has not been linked to its microstructural characteristics. The current research, therefore, aims to provide insight into the... 

Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and ‘‘reverse’’ shape memory in the high symmetry phase. Processing can change these features: repeated cycling can ‘‘train’’ the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to formations of localised defects and these can be studied by atomistic methods. Here we present a molecular dynamics simulation study of such behaviour employing a two-dimensional, binary Lennard–Jones model. Our atomistic model exhibits a symmetry breaking,... 

In this study, the effect of temperature on the microstructure and electrochemical properties of the NMC batteries has been studied. An extensive set of accelerated aging tests has been carried out employing a Li-ion high energy 18650 system (2.2 Ah, negative electrode: carbon, positive electrode: Li(Ni0.5Mn0.3Co0.2) O2. Different influence factors on cycle aging, such as temperature and discharge rate have been investigated. The aged 18650 cells have been inspected via scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), impedance spectroscopy, and X-ray diffraction (XRD) after disassembly to find more information about the chemical reasons of the degradation and... 

Nowadays filtration process is increasingly used in various areas such as water purification, food industries, filtering the air dust and other separation applications. In this work, the HDPE microporous filters have been fabricated at different temperature, pressure and time conditions via sintering process and then were characterized by different techniques. It can be expected that microstructure and mechanical properties of the samples could be controlled by changing the fabrication parameters of the process and also by changing the properties of the resin such as powder shape, particle size and rheological properties. In the first step, by using DSC, MFI, rheology test and optical... 

Conventional MRI has been used to diagnose different types of brain injuries. However these methods have generally failed to diagnose mild types of injury. We are working on the specification of mild traumatic brain injury, using diffusion MRI data, based on a multi compartment simulation of white matter tissue. This effort is essential for better understanding of underlying tissue micro-structure changes in patients with trauma. Some studies have been used in similar data fitting approaches in order to estimate axon diameter distribution. Specifically, a comparative study between different Compartment Models has shown that “ActiveAx” model has the best agreement with underlying tissue... 

In this research, low carbon steel rolled under 16%, 33% and 50% deformation and applied under electric pulses. Microstructure investigated by optical microscopy and scanning electron microscopy in order to observe grain size, precipitation size and distribution and hardness measured by Vicker's hardness test, after each electric pulse treatment. I order to investigate microstructure evolution as well as grain size, precipitates and hardness all the species with different deformation percentages applied under electric pulses with different total electric pulse duration, number of pulses and the off time between two successive pulses. To obtain required information about species with... 

Multi-directional forging is used as a severe plastic deformation process for producing ultrafine grained materials in which a sample undergoes consecutive compressive strains and as a result, coarse grains break to smaller grains. The strength of multi-directionally forged material increases because of grain refinement, although the high strain energy reserved in a sample after large plastic deformation lessens its ductility. Metals experience non-isothermal heating cycle during annealing till they reach the target temperature. Non-isothermal heating cycle can affect microstructure and mechanical properties of severely deformed material due to its high strain energy. Thus, investigation of... 

Composite production is an important method for increasing abrasion resistance in materials. Friction stir processing (FSP) is a solid composite production method which has eliminated the problems of liquid composite production. This research used FSP to study the possibility of in situ production of 1050 aluminum composite with a mixture of copper and iron powder with micrometer sizes of 25-14 and 10 μm, respectively, as reinforcing particles. The following weight percentages of reinforcing powder was added to the sample: 75% iron-25% copper, 50% iron-50% copper and 25% iron-75% copper. In addition, 3 and 5 process passes were considered as another variable in this study. Afterward, the... 

Ultra-fine and fine-grained aluminum matrix nano-composites were fabricated by friction stir processing (FSP) of 1050 Al alloy sheets containing 6 vol% volume fractions of pre-inserted MMO nanoparticles. Annealed sheets were processed at different rotational (w=800-2000 rpm) and traverse speeds (v-50-200 mm/min). A special design tool (concave shoulder and cylindrical pin with regular threads) without changing the rotation and traveling directions was utilized. It is shown that the best and highest desired mechanical properties of the base metal is attained at w=1600 rpm and v =100 mm/min through one single pass. The structure of grains are changed due to the applied severe plastic... 

Austenite in the 300 stainless steels is not a table phase in deformation processing during which martensite phase forms. This causes significant changes in the microstructures as well as the mechanical properties of deformed steel. In this work, the effects of rolling speed, reduction, temperature and strain pass on the strain-induced martensite during strip rolling operation of stainless steel 304L are studied. Cold rolling experiments under different speeds, reduction and temperature are conducted and then, the microstructural events and mechanical properties of the rolled steel are evaluated. For doing so, different testing techniques including tensile test, hardness measurement, optical... 

As one of the main features of Li-Ion batteries, internal resistance severely affects performance and characteristics of these batteries e.g. output power and working voltage. In this study, we tried to understand role of internal resistance on microstructure development and provide a correlative relation between microstructural features and performance of lithium-ion batteries. To catch this target, four types of Li-Ion battery chose, varying cathode type. These batteries were exposed to both normal and severe cycling conditions. Regarding to the key role of current rate on the performance of Li-ion batteries, an extensive research on different types of batteries at various current rates... 

Polyethylene, due to its low production and maintenance costs, light weight and excellent chemical resistance, is the most widely used material for production of water and gas transfer pipes. Pipe grade polyethylene (PE100) with bimodal molecular weight distribution has excellent mechanical properties, especially long-term creep performance. In order to prevent UV degradation, carbon black is added as masterbatch to PE100 to produce black compounds, which acts as most effective industrial UV stabilizer of polyethylene. Five PE100 black compounds and one white PE100 as reference sample were produced using a twin-screw extruder. Rheological, melt flow DSC, TGA, FE-SEM and tensile tests were... 

Today, in many industries polypropylene is used as a polyolefin with favorable properties such as chemical resistance, fatigue strength, electrical insulation, as well as good mechanical properties such as high Yang modulus, good toughness, and so on. On the other hand, polyolefins have poor thermal resistance and high flammability. Over the years, many researchers have been working to reduce their flammability through different methods and increase their thermal resistance. Among these methods, adding materials that delayed flammability or stoped it, can be noted. In the industry, these anti-flammable additives are called flame retardants. Flame retardant additives should be selected to... 

Upset Resistance Welding (URW) is a solid-state joining process which can be used to join a variety of parts and profiles including railways, wheel rims and pipes used in oil and petrochemical industries. This process has three primary parameters, welding current, current passage time and mechanical pressure. Martensitic Stainless Steels (MSSs) due to high strength and wear resistance, good atmospheric corrosion resistance and relatively low price and Duplex Stainless Steels (DSSs) because of higher strength and better corrosion resistance in chlorine environments, are widely used in different industries. Dissimilar MSS/DSS joints are of great importance for piping, specially in oil and... 

Titanium alloy and aluminum alloy 6061-T6 were successfully joined by resistance spot welding. The microstructure and mechanical properties of the lap joint were investigated. The microstructure of joints was inspected by OM, XRD, and SEM. The results reveal that the reaction between liquid aluminum and solid titanium were caused welding-brazing joints, which restricts the formation of IMCs. The microstructure in the aluminum side is a type of transition structure, which is considered to be the same microstructure that connects the columnar dendrite and Equiaxed dendrite in similar aluminum alloy RSW. The welding current has the most significant influence upon the tensile shear and cross...