Fenomén cyklického tečení a jeho predikce pomocí MKP

Abstract

The dissertation thesis complexly focuses on the prediction of cyclic creep, or ratcheting. Firstly, an overview of the state-of-the-art is described, which is divided into two sections. The first section deals with a description of experimental findings and the second section deals with a description of cyclic plasticity modelling focused on the research of ratcheting. Both sections are interconnected as the prediction quality using modern computing methods is closely related with a correct description of stress-strain behaviour of the given construction material. A series of experimental tests is necessary for those purposes. It is important to acquire reliable experimental data whose stress condition is as close as possible to the final application. In the first part, the dissertation thesis text itself is focused on a demonstration of prediction abilities and characteristics of selected plasticity models available in commercial FEM software ANSYS, which is available at the workplace of the Department of Applied Mechanics. This section presents the possibilities of identifying material parameters using an analytical, numerical method as well as using an inverse algorithm by means of experimental data acquired from an indentation test, which is quite fast and low-cost. The second part presents new results of experiments primarily focused on the research of stress-strain behaviour under load with a gradually changing amplitude. Research on effects of cyclic hardening, additional hardening due to nonlinear hardening, and ratcheting at single-axial and multi-axial strain have been carried out. The simulations used the AbdelKarim-Ohno plasticity model with a kinematic hardening rule, additionally modified with the Marquis relation. The third part is focused on a new approach to the evaluation of low-cycle fatigue tests using the method of digital correlation of image, which enables to measure the deformation on a curved sample surface. It brings more information on the behaviour of the tested material exposed to cyclic strain from one test, which primarily makes sense for the research of strain-stress behaviour. The contribution for the research of ratcheting is presented on a case of traction-pressure and a pure torsion with a non-zero static component.