“Fatigue and Creep Rupture Damage of Perforated Plates Subjected to Cyclic Plastic Straining in Creep Regime” M. L. Badlani, T. Tanaka, J. S. Porowski, and W. J. O’Donnell, Welding Research Council Bulletin No, 307, August 1985.
Keywords: elastic analysis; design of structures; plastic strains; creep strains; design analysis; perforated plates; inelastic analysis; plastically deformed; tubesheets; creep-fatigue evaluations; plastic cycling
The use of elastic analysis results for the design of structures undergoing plastic or creep strains are permitted by the ASME Boiler & Pressure Vessel Code. However, a drastic penalty is imposed in order to ensure conservatism of the design analysis. Inelastic analyses provide more effective evaluations. The primary loads in perforated plates are limited by the Code, and the overall deflection of the plate is not essentially affected by creep flow.
The Code restricts also primary load-induced elastic strains. However the situation is different in localized regions of ligaments. Severe local strain concentrations occur in ligaments of plastically deformed perforated tubesheets operating under creep conditions. Such local strain concentrations must be considered when performing creep-fatigue evaluations.
Code Case N-47 of the ASME Boiler &Pressure Vessel Code provides a simplified formula for approximating the concentration of inelastic strains. The formula uses the results of elastic analysis and includes separate terms related to local plastic strain and local creep strain. The local plastic strain is based on Neuber’s Theory extended for alternating loading and uses the square of the elastic strain concentration multiplier while the local creep strain is evaluated using only the elastic strain concentration factor. Extensive research has been performed under PVRC sponsorship on the inelastic behavior of ligaments.
The mechanism of plastic flow in ligaments was analyzed in Refs. 3 and 4. Plastic strain concentrations for perforated plates with triangular and square penetration patterns subjected to monotonic loading were determined. Steady creep solutions for bounding stresses and strains in ligaments are given in Ref. 6. The present work is aimed at extending this research to investigate the more general case of perforated sheets subjected alternating plastic cycling in the creep regime. The finite element method is used for performing the analyses and both equibiaxial and shear loading are considered.