The Combined Effect of a Finite Axial Erosion with Bauschinger Modified Autofrettage on the 3D SIFs of Pressurized Cylinders

Autofrettage is the process to introduce favorable residual compressive hoop stresses on the bore of a pressurized cylinder to enhance its strength and durability. For certain type of pressurized vessels under certain severe operating conditions, erosions and cracks often occur causing significant reduction in pressure vessel fatigue life. Those erosions and cracks are in general 3D geometrical configurations. The Stress Intensity Factors (SIFs) of the cracks are often the key to estimate the fatigue life. However the SIFs are largely dependent on not only the crack geometrical configurations but also the geometrical configurations of the erosions introduced during its operation. The Bauschinger effect on the SIFs further complicates the analysis. In this study, a closer look is taken at how a finite axial erosion in combination with the Bauschinger modified autofrettage properties affect the 3D SIFs. The problem is solved via the finite element method (FEM). The SIFs are evaluated for a variety of relative crack depth, different crack configurations and erosion geometrical configurations including arc erosion, semi-circular and semi-elliptical erosions. We show that the effective SIFs can be increased significantly by any of these factors but the combined effect often worsens the situation. NOMENCLATURE a- crack length d- erosion depth Le- half length of the erosion groove L- half length of the cylinder E- Young’s modulus h- half-width of erosion KI- mode I SIF KIeff- mode I effective SIF Kmax maximum value of KIeff KIA- mode I SIF due to autofrettage KIP- mode I SIF due to inner pressure KIO- normalizing SIF for effective SIF p- internal pressure in cylinder r ′- arc erosion radius Ri- inner radius of cylinder Ro- outer radius of cylinder t- cylinder wall thickness GREEK SYMBOLS ε- level of autofrettage ν- Poisson’s ratio σy- yield stres