Sharif Digital Repository

Recent research studies on ductile fracture of metals have shown that the ductile fracture initiation is significantly affected by the stress state. In this study, the effects of the stress invariants as well as the effect of the reverse loading on ductile fracture are considered. To estimate the reduction of load carrying capacity and ductile fracture initiation, a scalar damage expression is proposed. This scalar damage is a function of the accumulated plastic strain, the first stress invariant and the Lode angle. To incorporate the effect of the reverse loading, the accumulated plastic strain is divided into the tension and compression components and each component has a different weight... 

Welded joints are the most used joints in industrial applications. Fatigue analysis of welded joints is of great importance. Residual stresses produced during welding process play an important role in fatigue failure of welded joints. Residual stresses’ effect on fatigue life of the structures is similar to mean stresses and can be accounted for like mean stress effect. However residual stresses are not constant and do change in service. In this thesis, welding process has been modeled using FE software ABAQUS in order to predict residual stress distribution due to welding. The predictions show good agreement with measurements especially for longitudinal residual stresses. Then cyclic load... 

Components in cryogenic milieu with relative motion, like seals and pumps cannot be lubricated by using oils or other lubricants. Therefore materials wear in cryogenic condition is a great concern. To earn valid data about the wear properties of common and super useful material in cryogenic tribosystems, reconstruction wear test apparatus at cryogenic temperature is needed. Investigations on metal-metal couple show that austenitic stainless steels have favorable and auspicious sliding properties at cryogenic temperature. This persuades us to survey the wear parameter of these prolific materials like austenitic stainless steels specifically. This research provides the wear parameters of... 

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... 

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... 

Wear is the most important cause of surface damage occurs by direct contact of surfaces. Increasing the quality and strength of surface against different kinds of destructive phenomena is significant in manufacturing of mechanical parts. Surface nanocrystallisation can improve the surface protection against wear and can be done by lazer beam and shot peening. In this investigation by sever shot peening process the surface of A356 Aluminium alloy transforms to nanocrystal structure. The dry sliding wear and friction behaviors of A356 Aluminum were evaluated using a pin-on-disk apparatus at ambient conditions. The stationary diameter of 5mm stainless steel pin produced a wear track (scar) on... 

The most failures caused by wear, fatigue and corrosion are very sensitive to surface of material. Since the most failures starts or occurs at the surface of material, so control and improvement of the surface properties can have a significant effect on increasing the service life and improve the overall behavior of the material. Reforming of the structure and grain size is one of the methods to improve elastic properties, hardness, softness, toughness, etc. Also the macroscopic residual stresses in fracture toughness test specimens, can have tremendous effect on fracture toughness measurements. Many methods are used to modify the surface structure and to create a compressive residual stress... 

In this study a layer with compressive residual stress was created by severe shot peening and hence surface grains was changed to nanostructure. Fatigue life will be increased as a result of compressive residual stress. Surface roughness increase, is one of the disadvantages of severe shot-peening process. Re-shot-peening with the smaller shots and lower intensity is proposed for reducing surface roughness. Numerical simulation as a powerful method, used to investigate the effects of shot-peening parameters on the residual stress distribution, surface roughness and grain size. Obtained results indicate that more energy in process can improve residual stress distribution and grain size unlike... 

Repair welding is one of the most prevalent processes in repair and maintenance part of all industries. Amongst the issues of welding processes are residual stresses and distortions. Thus prediction of residual stresses and their distribution are important for designing welding procedure specifications. Circumferential butt weld is often used for joining steam and hot water lines in plants. Due to the intense concentration of heat in the welding, the regions near the weld line undergo severe thermal cycles. The thermal cycles cause non-uniform heating and cooling in the material, thus generating inhomogeneous plastic deformation and residual stresses in the weldment. The presence of residual... 

Nanocrystalline metals and alloys have some appealing characteristics with significance potential compared to their microcrystalline counterparts for engineering applications. These include ultra-high yield and fracture strengths, decreased elongation and toughness, superior wear resistance, and the promise of enhanced superplastic formability at lower temperatures and faster strain rates. This leads us to study the effects of different nanocrystalline parameters on crack propagation process in these materials. In the present study, the behavior of a crack in a columnar nanocrystalline structure is examined. One of the methods of modelling nanocrystals primary structures is the Voronoi... 

Various methods are used to improve the behavior of steel parts. Cryogenic treatment is one of the prevalent methods used for this purpose. It is not a substitute process but rather a supplement to conventional heat treatment process. In this Study the effect of the minimum temperature and the time of cryogenic treatment on wear resistance and residual stress of stainless steel AISI 316 and AISI 304 were studied. For this purpose conventional heat treatment, deep cryogenic treatment (24, 48 and 72 hours) and shallow cryogenic treatment (5, 10 and 15 hours) were performed on the samples. The wear resistance were measured by Pin-on-disk test according to the standard G99-95 (under the load of... 

In most cases, material failures occur on their surfaces, such as fatigue fracture, wear and friction, fretting fatigue and corrosion, so surface modification is one of the most effective techniques to improve the strength of materials. Correction of near surface grain size to nanoscale is one of the effective ways to improve the strength, fatigue life, toughness and etc. With increasing evidences of novel properties in nanostructured materials, it is reasonable to propose to achieve surface modification by the generation of nanostructured surface layer so that the overall properties and behaviors of the material are significantly improved.By means of surface severe plastic deformation, the... 

Mode І critical strain energy release rate at crack initiation, Jci, of SAC305 solder joints between two copper substrates was measured using double cantilever beam (DCB) specimens at a strain rate of 0.03 s-1 as a function of bond-line width (i.e. joint size in the out-of-plane dimension), solder thickness, copper adherends thickness and suface roughness. Fracture force per unit width and Jci were relatively insensitive to width of the joint ranging from 8 to 21 mm. Variations in solder thickness (i.e. 150 µm, 250 µm and 450 µm) also had an insignificant influence on the fracture energy of solder joints. This behavior was explained in terms of stress distribution, plastic zone area and... 

Wire arc additive manufacturing (WAAM) is used to manufacture parts in large scale, low cost, and at high rate of production. One of the problems of this technique is the creation of residual stress in the produced components. The aim of this study is to investigate the efficiency of procedures reducing the amount of residual stress to ameliorate the WAAM technique. In this research, the WAAM method and the procedures for reducing the amount of residual stress are explained. Also the advantage of this technique is presented. Due to the enormous amount of parameters and for the aim of saving time and expense, the simulation method is used. The accuracy of the simulation was checked with... 

One of the most common nondestructive tests is ultrasonic testing which has been paid great attention from specialists of this field. Apart from being economical and efficient, ultrasonic waves are sensitive to small changes in the structure and thus have a high degree of reliability. The most common method of ultrasonic testing is manual single-element A-scan inspection, carried out offline using longitudinal waves with pulse echo technique which is used in this thesis. The purpose of this thesis is to monitor structural health of thin metal joints and estimate quality of resistance spot welds by simulating ultrasonic testing using a finite element software. Initially, acoustic properties... 

Theory of Critical Distances (TCD) is one of the methods that has attracted the scientists in life estimation of fretting fatigue problems in recent years. Less difficulty and less time consuming features of this method in comparison with other methods like fracture mechanics are the attracting aspects of this theory. In this research, the capability of the Theory of Critical Distances in life estimation of combined bending and tensile loading modes in fretting fatigue for the pads with sharp edges has been evaluated. This included the evaluation of this method in lifetime estimation of 316L stainless steel for fretting fatigue in two different types of heat treatments and the evaluation of... 

Processing an alloy can enhance its mechanical properties. Based on repeated radiation of high-power pulsed laser beam, laser shock peening (LSP) is one way to impart favorable residual stress in an alloy and to enhance its damage tolerance property. Study of overlapping LSP is possible through the simulation of limited laser shots. Still, its computational cost is higher than that of a single shot. This research, primarily, is aimed at improving a methodology for reduction of computations in the study of an overlapping LSP and its impacts on fatigue crack growth. Addressing research gaps by utilizing the methodology is the secondary goal. The methodology consists of plastic strain... 

Fretting fatigue occurs at the point of contact of two parts that are under dynamic loading. During this phenomenon, due to the force applied at the contact point, deformation occurs and causes slip-stick zones, which intensifies the effect of surface degradation and crack nucleation happens earlier. In this research, the improvement of fretting fatigue life under combined loading by induction of compressive residual stresses by laser pinning process in the sample has been investigated. The laser shock peening process is a method by which residual stress is created in the sample and can improve fatigue life. The material used is 316L stainless steel, which has many industrial applications.... 

Cracks significantly change the vibration response of structures. Cracks which exist in structures as a defect in the material or created by repeated loads, reduce the natural frequencies and change the vibration mode shapes. Therefore with measuring the vibration response of structures, the existence and position of cracks are detected. This research analytically investigates the free vibration and elastic buckling of a beam made of functionally graded materials (FGM) with open edge cracks using Reddy's beam theory and the rotational spring model. It is assumed that the material properties vary along the beam thickness according to a power law distribution in terms of volume fraction of... 

In the present work, the residual stress distribution in welded tubular X-joints of offshore platformshas been investigated. For this purpose a 3D thermo-mechanical finite element model for welding process has been proposed using Goldak thermal model. Residual stress of the joint is obtained using this model. The maximum temperature of the welding process and the maximum residual stress are 2050 °C and 370 MPa, respectively.
Then residual stress distribution of post weld heat treatment, fatigue loading and shot peening has been studied too. In order to simulate the post weld heat treatment, a decoupled thermo-mechanical analysis has been performed. The temperature of the furnace and the...