Finite Element Modelling of Fatigue Damage Using a Continuum Damage Mechanics Approach

In this paper, a fatigue model formulated in the framework of the Continuum Damage Mechanics (CDM) is presented. The model is based on an explicit definition of fatigue damage and introduces a kinematic damage differential equation formulated, directly, as a function of the number of cycles and the stress cycle parameters. The model is initially presented for uniaxial problems, which facilitates the identification of its constants. An extension of the fatigue model to multiaxial problems is also proposed. This model was implemented in a non-linear finite element code in conjunction with a constitutive model for cyclic plasticity. The cyclic plasticity model considered is based on a J2-plasticity theory with non-linear isotropic and kinematic hardenings. In order to enhance the description of the cyclic elastoplastic behaviour, is suggested the superposition of several non-linear kinematic hardening variables. Both fatigue and plasticity models are identified for the P355NL1 (TStE355) steel. Finally, the numerical model is used to predict the fatigue crack initiation for a welded nozzle-to-plate connection, made of P355NL1 steel, and results are compared with experimental fatigue data.