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.... 

Aluminium cylinders with a constant ratio of outer to inner radii, k=2.2, with different diametral interferences and various shrinkage radii were subjected to bursting and autofrettage pressures. Numerical simulations of the compound cylinders were also performed using the finite element code, NISA. The results can predict the optimum shrinkage radius to a reasonable accuracy with the use of finite element analysis. This radius corresponds to the situation when the maximum von-Mises stress at the internal radii of both the inner and outer cylinders become equal. It was shown that the maximum von-Mises stress across the wall of the cylinder is at the minimum at this shrinkage radius. The... 

The effect of the autofrettage process on high pressure cylinders has been investigated. Both numerical and experimental techniques have been used for the investigation. It was observed that the best autofrettage pressure for raising the pressure capacity of the cylinder was the pressure Py2 which is just sufficient to bring the outer surface of the cylinder to yielding. Pressures higher than Py2 will have the inverse effect. It was found that the number of autofrettage stages has no effect on pressure capacity. It was also shown that to reduce the flow stress within the wall of the cylinder, the autofrettage pressure must be greater than the working pressure. For pressures lower than... 

Swage autofrettage process is used for internal forming and sizing of thick-walled tubes designed to withstand high internal pressures. In this process, a mandrel with a diameter slightly larger than that of the tube is pressed inside the tube and in so doing, creates the internal profile. This paper presents an experimental and numerical study of this process. Two dimensional axi-symmetric and three dimensional finite element analyses are used to investigate the effects of workpiece and mandrel geometry on the forming load and part quality. Experiments are carried out to validate the numerical results. © 2005 Elsevier Ltd. All rights reserved  

Autofrettage is a technique for introducing beneficial residual stresses into cylinders. Both analytical and numerical methods are used for the 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 the special cases of this general model. This quadratic material model is convenient for the... 

For characterizing Bauschinger effect factor (BEF) and Bauschinger modulus reduction of an A5083 aluminum alloy experimentally, several uniaxial tension- compression tests carried out in different pre-strain levels using INSTRON testing machine. BEF was investigated using both Welter and Milligan's definitions for various offset values. It was observed that Milligan's definition predicts BEF less than Welter's definition for all offset values. In addition, real loading-unloading behavior of such alloy was recorded to predict residual stresses resulting from autofrettage and shrink fit processes. Variable material properties (VMP) method, which is capable of incorporating real unloading... 

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  

In this paper the effect of the re-autofrettage process on the residual stress distribution at the wall of a thick-walled tube is considered. For accurate material behavior modeling, it is assumed that the yield surface is a function of all the stress invariants. Also for estimating the behavior of the material under loading-unloading process, a modified Chaboche's hardening model is applied. For evaluation of this unloading behavior model a series of loading-unloading tests are conducted on specimens that are made of the high strength steel, DIN1.6959. In addition the finite element simulations are implemented to simulate the re-autofrettage process and to estimate the residual stresses.... 

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... 

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... 

Recent research indicates that accurate material behavior modeling plays an important role in the estimation of residual stresses in the bore of autofrettaged tubes. In this paper, the material behavior under plastic deformation is considered to be a function of the first stress invariant in addition to the second and the third invariants of the deviatoric stress tensor. The yield surface is assumed to depend on the first stress invariant and the Lode angle parameter which is defined as a function of the second and the third invariants of the deviatoric stress tensor. Furthermore for estimating the unloading behavior, the Chaboche's hardening evolution equation is modified. These... 

An analytical method for reconstructing residual stresses within axisymmetric cylinders is presented. The method uses the stress equilibrium and boundary conditions together with an Airy stress function that satisfies these conditions. The method allows the reconstruction of residual stresses from limited measurements. The analysis is also coupled to a least squares approximation and a regularization analysis to provide stability of the inverse problem. The application of the method is demonstrated for four cases; two correspond to analytical solutions for residual stresses created during autofrettage and two sets of experimental results for through wall measurements of residual stresses. In... 

In this paper a general variable material property (VMP) formulation for the solution of thick-walled tubes with constant axial strains was developed and compared with the alternative VMP method that is called the Hencky program The VMP method was initially developed for the analysis of plane stress and plane strain states. However, the actual autofrettage process is under constant axial strain, i.e., open-end and closed-end conditions. Results indicate very good agreement with the Hencky program. Our method is simple, accurate, and very efficient, so that the number of iterations for convergence reduces approximately to one-tenth of Hencky program iterations. The solution algorithm for... 

We used an extension of the Variable Material Property method for materials with varying elastic and plastic properties to evaluate the residual stresses in an autofrettaged thick vessel made of functionally graded metal-ceramic composite. It is shown that the reinforcement of the metal vessel by ceramic particles, with an increasing ceramic volume fraction from inner to outer radius, increases the magnitude of compressive residual stresses at the inner section of an autofrettaged vessel and thus, could lead to better fatigue life and load-carrying capacity of the vessel. A parametric study is carried out to highlight the role of ceramic particle strength and spatial distribution, as well as...