Time integration algorithms for finite element analysis of creep problems

Bibliography: leaves 127-132.The fundamental principles involved in the selection and implementation of time integration schemes for finite element analysis of nonlinear creep problems are investigated. The relationship between the nature of the integration algorithms and the mechanical principles of the time-dependent problem is explored. The emphasis is on uniaxial creep and simple constitutive laws are adopted. The essential nature of the problem is presented in different formulations. The creep problem is contained in a system of nonlinear first order ordinary differential equations in the creep strains only. This suggests that the integration scheme should be applied to the creep strains whereas traditional methods approximate the stresses. An internal variable framework is used to demonstrate the links between a consistent mathematical programming formulation and the conventional Newton-Raphson procedures. The incremental creep problem is cast as a nonlinear programming problem and is written as a minimum principle in the incremental displacements and creep strains