Browsing by Author "Wang, Yu"

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    Experimental and numerical evaluations of viscoplastic material behaviour and multiaxial ratchetting for austenitic and ferritic materials
    (2014) Wang, Yu; Roos, Eberhard (Prof. Dr.-Ing. habil.)
    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 MINUIT, uniaxial tests in conventional bench-scale are performed at first. In parallel, the material model in uniaxial formulation is integrated in the optimization pro-gram. By comparing measured and calculated results of the selected materials, the parameters are optimized, until the minimal deviations between measurement and cal-culation are reached. For the verification of the material model under different alternating loading conditions, a comprehensive test program with the selected materials is implemented. This in-cludes axial tensile tests, axial strain-controlled cyclic tension-compression tests and axial stress-controlled stepwise tension-compression tests with mean-stress at differ-ent test temperatures. With the determined parameters, very good and partially even complete agreements between the performed uniaxial cyclic tests and numerical simu-lations are achieved. For the further verification of the material models, component tests with straight pipe section are performed at room temperature and 300 °C. The test program incorporates multiaxial cyclic torsion tests with and without axial ratchetting deformation as well as tests, in which the torsional loading and the axial tension-compression loading are re-spectively in-phase and out-of-phase cyclic applied. The following conclusions can be drawn based on the component tests and corre-sponding numerical simulations with the extended Chaboche models: For the simulation of strain hardening, both the Armstrong-Frederick-model and the Ohno-Wang-model show good agreements between calculation and measurement. Via the combination with isotropic hardening models, the material models can very well simulate the strain-range dependent cyclic hardening and softening. With the existence of multiaxial ratchetting, the Armstrong-Frederick-model overesti-mates the ratchetting strain in all researched cases with respect to temperature and loading range, while the Ohno-Wang-model provides acceptable results: The multiax-ial ratchetting with a high primary stress is accurately simulated by using the Ohno-Wang-model. However, under low or pronounced alternating primary stress, the ratch-etting strain is overestimated in the beginning phase, the first several cycles. This de-viation between simulation and experimental observation results from the difference between uniaxial and multiaxial ratchetting behaviour. The material parameters identi-fied by using uniaxial ratchetting tests cannot absolutely describe the multiaxial ratch-etting behaviour. Effects of nonproportional loading with additional hardening under cyclic nonpropor-tional multiaxial loading can be in principle simulated by using the Krämer-Krolop-model. The magnitude of the additional hardening is dependent upon the non-proportionality of the cyclic loading, for instance the form of load path.