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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.

Keywords: Autofrettage, Residual Stress, Stress Intensity Factor, energy release rate, external surface crack

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