AbstractsBiology & Animal Science

Experimental and numerical evaluations of viscoplastic material behaviour and multiaxial ratchetting for austenitic and ferritic materials

by Yu Wang




Institution: University of Stuttgart
Department: Fakultät Energie-, Verfahrens- und Biotechnik
Degree: PhD
Year: 2014
Record ID: 1106939
Full text PDF: http://elib.uni-stuttgart.de/opus/volltexte/2014/9417/


Abstract

Components in power plants are subjected under cyclic loading, which can yield in-elastic deformation. When the materials are loaded under uniaxial cyclic loading with mean-stress or under multiaxial combined constant (primary) and cyclic loading (sec-ondary), a progressive plastic deformation can gradually accumulate. This progressive plastic deformation, so-called ratchetting, is related to low cycle fatigue, in which high loading amplitudes are existent, therefore plays an important role in service safety of power plant facilities. For the accurate determination on life-time of highly loaded components in the frame of strength and fatigue analyses, material models, which are able to describe complex inelastic deformation processes under cyclic loading, should be applied. A material model, also called constitutive model, represents the mathematical relationship be-tween stress and strain tensors, and thereby describes the nonlinear time dependent cyclic material behaviour in multiaxial stress-state. The objective of this work is to de-velop and verify a material model, in order to numerically simulate the cyclic inelastic material behaviour of the austenitic steel X6CrNiNb18-10 and the ferritic steel 20MnMoNi5-5, especially the ratchetting effect. The tensorial kinematic hardening variable, so-called back-stress, is used to describe the direction dependent hardening (strain-hardening). In this work, different nonlinear kinematic hardening models are investigated, that include the Armstrong-Frederick-model as fundamental nonlinear kinematic hardening model, the Ohno-Wang-model, which is particular suitable to simulate the ratchetting deformation, and the Krämer-Krolop-model for taking into account the nonproportional effect under multiaxial non-proportional cyclic loading. By applying four back-stress variables in the material mod-el, the cyclic strain hardening in a large strain-range can be accurately described. The direction independent hardening and softening, so-called cyclic hardening and softening, is included in the material model by means of scalar isotropic hardening variable. In the frame of this work, different isotropic hardening models are developed and investigated. By using these models, various mechanisms, such as cyclic harden-ing with and without saturation, cyclic softening, or combined cyclic hardening and sof-tening, can be represented. In addition, the evolution equation for so-called strain-memory-effect is implemented in the viscoplastic Chaboche model, in order to take into account the memory-effect ob-served in experiment. The extended viscoplastic Chaboche model is implemented in different versions as subroutine UMAT of commercial finite element program ABAQUS and can be used for the simulation of real components. Regarding formulation of the kinematic hardening variable, the different versions are denoted as Armstrong-Frederick-model, Ohno-Wang-model and Krämer-Krolop-model subsequently. To determine the parameters of the material models with the numerical optimization program…