فهرست مطالب

Stress Analysis - Volume:4 Issue:1, 2019
  • Volume:4 Issue:1, 2019
  • تاریخ انتشار: 1398/06/10
  • تعداد عناوین: 12
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  • A.R. Hosseinzadeh *, M. Ebrahimi, H. Sodagari, A.R. Abedian Pages 1-9
    An efficient design is a key factor in final expenditure of a certain construction. Pressure vessels are structures that play an indispensable role in different industries such as petroleum, power plants etc. Pressure vessels are receptacles often used to keep gases or liquids at a pressure typically different from what atmospheric pressure is. End caps which close the end of vessels can be formed in  different shapes. Thus, end cap design also has a significant role in the integrity of vessels to prevent fatal accidents that are frequent in the pressure vessel’s history. In this study, an extensive investigation of huge-flat end caps under external pressure was carried out to extract the most efficient geometrical layout. This kind of flat end cap is an essential part of the designed main duct in the Air Cooled Condenser (ACC) systems as a configuration that renders steam to condensed water inside a definite arrangement of finned tubes in a hybrid thermal power plant. To determine an optimized state of stresses considering weight limitation, a number of finite element models were simulated. The simulations were performed in a relatively wide domain of two geometrical variables, namely thickness and height of stiffeners. By constituting a comprehensive data library, an objective function was formed using the results of finite element. The procedure was followed through a genetic algorithm to find an optimized stress state. An analytical study was also accomplished to reach an optimized end cap resulting in the lowest stress level. The findings showed very similar results for the two methods. Furthermore, a profound observation of the influence of two geometrical parameters was conducted in different weight limits. Although this study is based on a particular actual-industrial problem in an implemented power plant, the proposed method and results are applicable to a great number of similar cases.
    Keywords: Flat end cap, Pressure vessel, Finite element, Kuhn-Tucker, Genetic Algorithm
  • S. Chahardoli, N. Vahdat Azad * Pages 11-25
    In the present research, the influence of cap geometry on the collapse of thin-walled aluminum-made energy absorbers with various section geometries was investigated. For this purpose, a total of 35 different absorbers were subjected to axial quasi-static loading. In this respect, five different section types and seven different cap configurations were considered for the absorbers and their caps, respectively. The analyses were performed in both experimental and numerical methods. The numerical simulations were conducted using LSDYNA Software and experimental tests were performed to verify the numerical investigations. Good agreement was obtained between the experimental data and numerical results. The results indicated that, in all cases, the application of the cap enhanced the crush force efficiency while lowering maximum force at collapse. In the final stage of the research, optimal absorbers for the cases with open-ended and close-ended caps were proposed using Minitab Software based on the response surface methodology.
    Keywords: Energy absorber, LS-DYNA, Quasi-static loading, Optimization
  • M. Saadatfar * Pages 27-41
    In this paper, hygro-thermo-magneto-electro-elastic creep stress redistribution of a functionally graded magneto-electro-elastic (FGMEE) hollow sphere is examined. It is supposed that all material properties are a power-law function of radius. Temperature and moisture concentration functions are obtained analytically and then, a differential equation with creep strains is obtained using equations of electrostatic, magnetostatic and equilibrium, At first, ignoring the creep strains, a solution for the initial hygro-thermo-magneto-electroelastic stresses at zero time is achieved. Subsequently, creep strains are considered and creep stress rates are obtained. The Prandtl-Reuss equations and Norton’s law are taken for the creep analysis. Finally, time-dependent creep stresses as well as magnetic and potential field redistributions at any time are obtained using an iterative method. Results show that the radial stress, radial displacement, electric potential and magnetic potentials increase as time goes by at a decreasing rate. Also, the grading index and hygrothermal condition have more considerable effect on the radial stress after creep evolution rather than initial case. Thus, their effects must be considered in creep evolution analysis.
    Keywords: FGMEE, Hygrothermal loading, Time-dependent creep, Hollow sphere
  • H. Dabbagh *, K. Amoorezaei Pages 43-52
    The current study presents a series of tests on steel fiber-reinforced lightweight aggregate concrete (SFRLWAC) cylinders in order to develop a stress-strain model for SFRLWAC subjected to compressive monotonic loading. In this experiment, steel fiber ratios of 0, 0.5, 1, and 1.5 percent by volume of the sample were used in the mixtures. The findings show that adding steel fiber to the lightweight concrete has a slight impact on the ascending branch of the stress-strain curve; however, it has a noticeable influence on the descending branch. The peak stress, strain at peak stress, and modulus of elasticity were investigated. To this end, some equations were established. To predict the complete SFRLWAC stress-strain curve, a stress-strain model was introduced and the validity of the model was explored. There was a good agreement between the proposed model data and experimental findings. Using ABAQUS software, numerical simulation of the SFRLWAC beams subjected to monotonic loading was conducted; the simulated results had an acceptable agreement with the experimental data.
    Keywords: Lightweight concrete, Steel fiber, Compressive behavior, Stress-strain model
  • N. Habibi *, M. Amoorezayi Pages 53-63
    Bolted joints are one of the most common joints in the industry and assemble the most of the machine elements and segments together. Majority of structures are affected by fluctuating forces, therefore there is the risk of fatigue failure that causes countless damages, thus fatigue life estimation of bolted joints have always been important. The value of high stress concentration at the threads root especially first engaged thread causes problems for fatigue life estimation, since by applying stresses lower than yield stress of the bolt material, plastic deformation occurs at zones of thread root that reach to ultimate stress but fracture does not happen and in some cases bolt-nut joints have infinite life, so that maximum stress at thread root is not fatigue life determinant. The modified critical distance technique and expressed stress at this distance were used for determination of fatigue life in joint. In this study, the bolted joint fatigue life prediction using critical distance technique was compared to experimental results. The three-dimensional finite element analysis for bolted joint was performed. Pre-tightening process and tensile axial force were simulated in ABAQUS software after applying two steps of force including rotation displacement to the center of the nut due to clamping joints (applied torque) and tensile force, the stress distribution resultant of different tensile forces by application of the critical distance technique and mechanical properties fatigue life were determined, and S-N curve prediction matched well with experimental data.
    Keywords: Critical distance technique, Bolted joints, Three-dimensional simulation, Fatigue life estimation, Preload, S-N curve
  • R. Moharrami *, V. Jafarpour Pages 65-71
    Creep-feed grinding is an accurate and efficient machining method. In this study, the effects of the cooling condition on surface residual stresses distribution in the creep-feed grinding of Inconel X-750 superalloy have been experimentally investigated. Some test samples were prepared and subjected to creep-feed grinding with dry and flood grinding at different flow rates. The variation of residual stresses in depth was obtained by the electropolishing layer removal technique. Results were shown highest creep-feed grinding forces were developed in dry grinding condition and these forces were declined by increasing the coolant quantity. According to results, by increasing about 71% of fluid flow under flood cooling, the normal and tangential forces decreased by roughly 30%. The results also demonstrated that the measured residual stresses on creep-feed grinded specimens are in the tensile form and using the coolant led to an overwhelming decrease in magnitude and depth of penetration of these stresses.
    Keywords: Residual stress, Measurement, Creep-feed grinding, Electro polishing, Layer removal
  • Z. Hosseini Tabar, F. Barati * Pages 73-88
    In the present study, a micromechanical modeling approach based on volumetric element was considered from a composite consisted of three components: matrix, particle, and particle-matrix intermediate phase. In order to predict the behavior of the damage evolution in the composite, the particle-matrix intermediate phase was modeled based on the cohesive zone model and disruptive elastoplastic behavior was considered for matrix. In order to study the efficiency of the implemented model, at first, modeling processes were conducted using the USERMAT code in finite element ANSYS software, and then the growth of fatigue damage was investigated in the AL composite reinforced with SiC particles. For this purpose, after the study of characterization static constant of cohesive zone model, validation of the static model was approved. S-N curve obtained from experimental results for pure AL were used for  Characterization fatigue constants of the matrix. Comparison of the obtained results from finite element analysis with that of experiment, justifies the capability of the employed model to predict the fatigue life of metal matrix composites reinforced with particles in other conditions and is able to consider the effect of volume fraction in predicting fatigue life while the modelbenefits from the lowest tests for the characterization constants of model.
    Keywords: Fatigue life, Cohesive zone model, Metal matrix composite, Particulate reinforcement
  • K. Aliakbari * Pages 89-98
    In this study, the failure analysis of base plate bolts of radial forging machine is investigated. Premature failure had occurred from the bolts shank-head fillet and threads zones. Hardness, impact and tensile tests are carried out to investigate the mechanical properties and spectrophotometer is used to evaluate the bolts chemical composition. Optical Microscope (OM) and Scanning Electron Microscope (SEM) are used for the investigation of microstructure, defects, fracture surface and failure causes. The fracture surface morphology shows that the crack growth consisted of bolts shank-head fillet and threads zones including the initiation zone, fatigue crack growth zone along with the beach marks and ratchet steps and the rapid final fracture zone. Stress analysis shows that the amount of pre-tightening selected lower than the proposed value leads to the joint loosening and shortens the bolt’s fatigue life. In addition, based on the paper results, the existing flowchart for component fabrication is analyzed and a flowchart based on research field is presented to enhance the quality of radial forging machine parts.
    Keywords: Radial forging machines, Bolts failure, Microstructure, mechanical properties, Stress analysis
  • M.H. Velayatparvardeh, A. Shooshtari * Pages 99-112
    In this paper, the vibration characteristics of GFRP-stiffened pipes, in intact and cracked conditions are investigated. The results have different applications, which the most important ones are optimized designs of such pipes and diagnosis of the damage in them. Therefore, by Love theory, governing equations of motion for the GFRP-stiffened pipes were obtained. Having obtained characteristic equation, the natural frequencies of the problem were calculated for intact case. Then by modeling a sample of these pipes in the ANSYS software and using Modal analysis, natural frequencies and related mode shapes due to finite element method were calculated in cracked and intact conditions. Then by using the experimental modal analysis method, the natural frequencies of a sample, which was built similar to these pipes, were obtained in cracked and intact conditions. The results of the analytical method, finite element method, and the experimental modal analysis were compared and it was shown that the results have a good compatibility. The same process was performed on carbon fiber composites.
    Keywords: Composite pipe, GFRP pipe, Love theory, Vibration, Natural frequencies, Mode shapes, FRF: (Frequency Response Function)
  • A. Nouri *, M. Kazemi Nasrabadi Pages 113-124
    With increasing applications of the Friction Stir Welding (FSW), a proper study of the fracture behavior is required. In this research, fracture behavior of AA7075-T6 alloy joint made by FSW is investigated by evaluat-ing a fracture test on the Diagonally Loaded Square Plate (DLSP) specimen containing a V-notch, under various loading conditions. Significant plastic deformation takes place around the notch tip at the propagation instance, which shows the elastic-plastic behavior of the welded joint. Ductile failure needs some elastic-plastic fracture mechanics criteria, which are complex and time-consuming. To deal with this, the Equivalent Material Concept (EMC) was applied via replacing a virtual brittle material with a ductile material by equating the tensile behavior of the welded material. In order to predict the Load-Carrying Capacity (LCC) of the FSW DLSP specimens, the EMC was used, which is in conjunction with two brittle fracture criteria called the Maximum Tangential Stress (MTS) and the Mean Stress (MS). Finally, results indicate that with a slight difference, two mentioned criteria could predict the LLC of the V-notched specimens.
    Keywords: Friction Stir Welding (FSW), Equivalent Material Concept (EMC), fracture toughness, Elastic-plastic behavior, Maximum Tangential Stress (MTS), Mean Stress (MS)
  • K. Khorshidi *, M. Ghasemi, M. Karimi, M. Bahrami Pages 125-136
    This work aimed to study the thermo-electro vibration of a piezoelectric micro-plate resting on the orthotropic foundation. To catch the small-scale effects of the structure, couple-stress theory was employed. Motions of the structure were modelled based upon different shear deformation theories including exponential, trigonometric, hyperbolic, parabolic, and forth-order shear defor-mation theories. These modified shear deformation theories are capable of considering transverse shear deformation effects and rotary inertia. Equation of motions are derived with Hamilton’s prin-ciple and to solve these equations an analytical approach is applied. Besides, Effect of different boundary conditions including SSSS, CSSS, CSCS, CCSS and CCCC are investigated. The pre-sent results are validated with the previously published results. In the result section, the influences of various parameters such as increasing temperature, boundary conditions, foundation parameters, thickness ratio, aspect ratio, external volatage, and length scale on the natural frequencies of the plate are illustrated in detail.
    Keywords: Couple-stress theory, Analytical approach, Orthotropic foundation, Piezoelectric micro-plate
  • M. Matinfar, M. Mahdavi Shirazi, M. M. Alipour * Pages 137-150
    In this paper, for the first time, displacement and stress analysis of bidirectional functionally graded (BDFG) porous sandwich beams are developed using the Chebyshev tau method. Based on the presented approach, sandwich beams under non-uniform load rested on Winkler/Pasternak foundation are analyzed. The material properties of core and each face sheet can be varied continuously in the axial and thickness directions, also the material properties are affected by the variation of temperature and moisture. To overcome some of the shortcomings of the traditional equivalent single layer theories for analysis of sandwich structures, governing equations are extracted based on the layerwise theory and five coupled differential equations are obtained. The resulting differential equations are solved using the Chebyshev tau method (CTM). The effectiveness of the CTM is demonstrated by comparing the obtained results with those extracted from the ABAQUS software. The comparisons indicate that the applied method to solve the systems of ordinary differential equations is efficient and very good accurate.
    Keywords: Chebyshev tau method, Bi-directional functionally graded, Porous sandwich beams, Elastic foundation, Layerwise theory