Calculation Methods for Prediction of Plastic Ovalisation of Pipes under Cyclic Bending

Abstract

This Ph.D. thesis deals with research into pipe cross section ovalization due to plastic deformation under cyclic bending. The theoretical part of this thesis begins with reviewing the state of the art of the solved problem, including a summary description of incremental plastic theory and a study of a material model of the influence on ovalization prediction. The kinematic material models include those of Prager, Prager combined with nonlinear isotropic hardening, Chaboche combined with nonlinear isotropic hardening, and the Abdel-Karim–Ohno modified Marquis relation. The mechanism responsible for cross section ovalization was identified as the phenomenon of the accumulation of plastic deformation, the so-called ratcheting. A thorough sensitivity study was done for the material parameters influencing the cross section ovalization. The practical part of this thesis begins with an explanation of the proposed solution model based on the finite element method including a comparison between a simulation with a simplified approach built on layered shell elements and on solid elements. A script enabling a parametric study of a process for pipe-laying using a reel was written. Moreover, a conclusion as to almost everything influencing the ovalization phenomenon could be made. Finally, there will be presented an evaluation of this method on a real full scale test. According to the comparison of predictions with measured values, the results of this approach are evidently conservative.