ANALYSIS OF ASME CODES FOR FATIGUE OF PRESSURE VESSELS

In industries, pressure vessels or in general thick-wall cylinders under internal pressure are important parts and analysis of their applications in various conditions is essential. Therefore, for design of pressure vessels usage of standard Codes like ASME is necessary. Most of cracked or damaged pressure vessels are exposed to cyclic loading. This failure process is fatigue. ASME codes have some codes for analyzing this process. These codes show the conditions and formulas for fatigue analysis. In this paper, a thick wall pressure vessel is analyzed with three cyclic loading regimes. The maximum stress intensity, fatigue life and damage factor are calculated by ASME codes. Then by usage of finite element method, ASME results are compared for fatigue life analysis. Previous investigations show that nozzle connection area of pressure vessels have high stress concentration, and usually crack is propagated from this zone. Thus fatigue analysis is accomplished for nozzle connection of pressure vessel by ASME codes and finite element method. Then nine shape of crack with same crack front size are modeled on the maximum stress zone of the nozzle connection. Then stresses of crack fronts and stress intensity factors of cracks are computed by finite element method with ABAQUS software which is powerful for fracture mechanic analysis. The critical crack which is elliptical prismatic crack virtually is grown. step by step and for each step, stress intensity factor is computed by ABAQUS software. With relation between stress intensity factor and crack size also using Paris formula, fatigue life is computed. This operation is done for two type of crack growth. In first type length and depth of crack are grown and in second type only crack length is grown. Finally, the fatigue life obtained from Paris formula and ASME codes are compared.