Sharif Digital Repository

In this paper, using numerical solutions of bore and outer radius hoop residual stresses in various tube geometries for autofrettage prior to shrink fit and shrink fit prior to autofrettage design are investigated, and the results are compared with monobloc autofrettaged and shrunk fit tubes. Both of the former compound tubes induce more compressive bore hoop residual stresses than a monobloc autofrettage and shrunk fit case. Furthermore, in geometries in which maximum bore hoop residual stresses are induced, the outer radius experiences acceptably low tensile residual stresses. Bore hoop residual stresses increase with increase of tube outer radius; however the rate of increasing decreases.... 

This paper deals with development and qualification of a nodalization for modeling of the PSB-VVER integral test facility (ITF) by RELAP5/MOD3.2 code and prediction of its primary and secondary systems behaviors at steady state and transient conditions. The PSB-VVER is a full-height, 1/300 volume and power scale representation of a VVER-1000 NPP. A RELAP5 nodalization has been developed for PSB-VVER modeling and a nodalization qualification process has been applied for the developed nodalization at steady state and transient levels and a qualified nodalization has been proposed for modeling of the PSB ITF. The 11% small-break loss-of-coolant-accident (SBLOCA), i.e. rupture of one of the... 

Cylindrical shells constitute the main structural components in pressure vessels and pipelines. Cylindrical shells made of fiber-reinforced composites are now being considered in the design of many components due to their high specific strength and stiffness. Buckling is one of the main failure considerations, when designing the cylindrical shells. The buckling behavior of the composite cylindrical shells can severely be compromised by introducing defect in the structure, due to high stress field generated around these defects. Defects could be generated during service due to cyclic loading or during manufacturing. A reliable operation of these structures require to understand the effects of... 

In this study, residual stress distributions in autofrettaged homogenous spherical pressure vessels subjected to different autofrettage pressures are evaluated. Results are obtained by developing an extension of variable material properties (VMP) method. The modification makes VMP method applicable for analyses of spherical vessels based on actual material behavior both in loading and unloading and considering variable Bauschinger effect. The residual stresses determined by employing finite element method are compared with VMP results and it is demonstrated that the using of simplified material models can cause significant error in estimation of hoop residual stress, especially near the... 

Material modeling of high strength steels plays an important role in the accurate analysis of autofrettaged tubes. Although, the loading behavior of such materials is nearly elasticperfectly plastic, their unloading behavior due to Bauschinger effect is very complicated. DIN1.6959 steel is frequently used for construction of autofrettaged tubes in some countries such as Germany and Switzerland. In spite of similarity between chemical compositions of this steel with that of A723 steel, due to different material processing, these two steels have an unlikely behavior. In this paper the material behavior of DIN1.6959 was accurately modeled by uniaxial tension-compression test results. Both 6 mm... 

In the present study, in the first part for a spherical vessel with known dimensions and working pressure, two methods of hoop and equivalent stress optimization across the wall thickness are employed to determine the best autofrettage pressure. In the next part for a predefined working pressure the minimum wall thickness of the vessel is calculated using two other design criteria i.e. (A) optimizing the hoop stress, and (B) assuming a suitable percent for the penetration of yielding within the wall thickness. Finally, the optimum thickness and the necessary strengthening pressure are extracted and different plots are introduced for different types of structural materials under different... 

A study of thick-walled spherical vessels under steady-state radial temperature gradients using elasto-plastic analysis is reported. By considering a maximum plastic radius and using the thermal autofrettage method for the strengthening mechanism, the optimum wall thickness of the vessel for a given temperature gradient across the vessel is obtained. Finally, in the case of thermal loading on a vessel, the effect of convective heat transfer on the optimum thickness is considered, and a general formula for the optimum thickness and design graphs for several different cases are presented. © 2004 Elsevier Ltd. All rights reserved  

Stress analysis of thick walled functionally graded (FG) cylindrical pressure vessels subjected to uniform axisymmetric thermo-mechanical loads is presented using Bernstein polynomials. All thermal and mechanical properties except Poisson's ratio of the FG vessels vary through the thickness with arbitrary functions of the radial coordinate. Based on the thermo-elasticity theory, the first law of thermodynamics and axisymmetric assumption, the governing equations of the semi-coupled thermo-elasticity problem reduce to a set of second order boundary value problem. Galerkin method together with Bernstein polynomials is used to obtain solution for the governing equations. The presented method is... 

α-Fe with low carbon content is a base material which is commonly used in manufacturing of Reactor Pressure Vessel (RPV) of commercial nuclear power plants. Carbon is generally diffused to α-Fe matrix to improve some of its mechanical properties. The presence of carbon may alter the irradiation response of the steel. In the current study, using molecular dynamics simulations, we have investigated the influence of carbon (∼in either dispersed form or carbon-rich region as chain) in the primary damage states of α-Fe low carbon steels. It is found that carbons in dispersed form have no significant effect on the self-interstitial atoms (SIAs) in α-Fe. While, carbon-rich (C-rich as... 

Autofrettage is a technique for introducing beneficial residual stresses into cylinders. Both analytical and numerical methods are used for analysis of the autofrettage process. Analytical methods have been presented only for special cases of autofrettage. In this work, an analytical framework for the solution of autofrettaged tubes with constant axial strain conditions is developed. Material behavior is assumed to be incompressible and two different quadratic polynomials are used for strain hardening in loading and unloading. Clearly, elastic-perfectly plastic and linear hardening materials are special cases of this general model. This material model is convenient for description of the... 

Material modeling of the high strength steels plays an important role in accurate analysis of autofrettaged tubes. Although, the loading behavior of such materials is nearly elastic-perfectly plastic, their unloading behavior due to Bauschinger effect is very complicated. DIN1.6959 steel is frequently used for construction of autofrettaged tubes in some countries such as Germany and Switzerland. In spite of similarity between chemical compositions of this steel with A723 steel, due to different material processing, two steels have unlikely behavior. In this paper material behavior of DIN1.6959 has been accurately modeled by uniaxial tension-compression test results. Both 6 mm and 12.5 mm... 

In this paper, the numerical studies on the semi-elliptical crack behavior in different locations of welded zones in the supporting structure of a spherical pressure vessel under an earthquake are presented. The cracks in the welded zones of supporting structures under earthquake effects may jeopardize the safety of spherical pressure vessels and result in catastrophic failure. A detailed finite element sub-modeling technique is carried out to compute the mixed-mode stress intensity factors along the crack front. Furthermore, crack behavior with different aspect ratios a/c: 0.25, 0.5, and 0.75 at the weld and the heat-affected zone of the supporting structure is evaluated. The... 

In this paper, decomposition of the total strain into elastic and plastic parts is investigated for extension of elastic-type constitutive models to finite deformation elastoplasticity. In order to model the elastic behavior, a Hookean-type constitutive equation based on the logarithmic strain is considered. Based on this constitutive equation and assuming the deformation theory of Hencky as well as the yield criteria of von Mises, the elastic-plastic behavior of materials at finite deformation is modeled in the case of the proportional loading. Moreover, this elastoplastic model is applied in order to determine the stress distribution in thick-walled cylindrical pressure vessels at finite... 

This paper continues previous research performed by the authors on the modelling of dissimilar welded joints with varying clad thicknesses. This study aims to validate the use of a clad plate model as a replacement to the previous clad pipe model. To fulfill the hypothesis of the study, possible deformation or angular shrinkages occurring at weld joints have been simulated using a commercial finite element software. In parallel, angular shrinkages have been validated using the experimental data with the underlying concept of Gaussian transformation of plates into pipes. The welding of the two dissimilar materials has been carried out in-house with the aid of a Tungsten Arc weld with dynamic...