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In this paper, the variations of the stress intensity factor and energy release rate have been investigated based on the displacement correlation and modified crack closure integral methods for external surface cracks in the autofrettaged functionally graded cylinder (FGC). Mechanical properties vary in the radial direction according to the desired function. Isotropic material behavior and bilinear elastoplastic stress-strain relationship are considered for the FGC. Autofrettage process induces the tensile residual stresses in the outer parts of the cylinder wall, which causing the undesirable effects on the external surface cracks. Many variables affect the distribution of tensile residual stresses. Effects of autofrettage ratio, volume fraction of material and cylinder thickness on the residual stress changes and addition, changes in the size and direction of surface cracks on the stress intensity factor and energy release rate are studied. The results show that the volume fraction has the greatest effects on both crack parameters. The axial cracks are critical in compared with circumferential and angled cracks. The principle of superposition can be used to determine the combinational effects of the residual stresses and applied loads on the behavior of cracks in the graded materials.

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In this study, fatigue growth of external surface cracks on the autofrettaged cylinders under bending is investigated. Autofrettage is a process in which a thick-walled cylinder subjected to internal pressure with known amount, causing some portions on the inner zone of the cylinder deformed plastically. In this case, removing the pressure causes compressive residual stresses on the inner layers and tensile stresses on the outer wall. The goal is increasing the fatigue durability of the product by inducing residual compressive stresses into materials, but along with this, there are adverse tensile stresses which can decrease the life due to the outer defects. In this paper, the external cracks are in the forms of half-elliptical, semi-elliptical and semi-circle. Samples made by aluminum 2024 alloy. The cylinders were autofrettaged up to 40 and 60 percent. Cracks were located in circumferential direction and normal to cylinder axis. The numerical simulations were performed by finite element method. Experimental data and numerical results were compared. Results show that the number of load cycles to fracture, in the 60% autofrettaged cylinders are smaller than those for 40% and also smaller than the state without autofrettage. Distribution of stress intensity factor along the crack front is symmetric and crack grows in its initial plane which indicating the dominance of the first mode of failure during the crack growth. In all samples, after some steps of the growth, crack front transforms to the semi-elliptical shape until complete fracture.

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Thick-walled cylindrical vessels are specially used in oil, chemical, nuclear and military industries in order to withstand internal pressure. The presence of the compressive residual stress in the walls increases the bursting pressure and fatigue life. Autofrettage processes and radial interference in multilayer cylinders are among the conventional methods of creating residual stresses in the pressure vessels. In order to achieve higher strength and fatigue life, the combination of these processes is also considered. J integral method is a suitable criterion for evaluating the crack parameters in elastic and elastoplastic strain fields. In this research, distribution of the J integral along the semi-elliptical crack front on the inner surface of the interferenced two-layered cylinder with closed end has been studied. Inner layer was autofrettaged. Burst pressure was determined based on the fracture toughness criterion (J_ΙC). Also, the effects of the autofrettage percent, radial interference; depth, angle and aspect ratio of the crack on the J integral and burst pressure variations have been investigated. The inner and outer layers of the cylinder were made of 7075-T6 aluminum alloy. The periodic nonlinear hardening behavior of this alloy has been predicted using Chabooche model. The validity of the results and their accuracy were examined