Konstitutive Kriech-und Kriechermüdungsbeschreibung

FKM 2006
Issue number 290
Project no. 251

Abstract:

The aim of this project was to develop a robust, thermodynamically consistent description of the inelastic behaviour using the example of a hot forged steel of type 28CrMoNiV4-9 in the form of a constitutive elasto-viscoplastic material model for the service life calculation and optimization of high-temperature components in power plant and plant engineering under practical creep and creep fatigue loading. The material model is able to capture creep stress and creep fatigue stress and to describe deformation and service life. The theoretical work focused on the adaptation of a material model to the existing complex stress and questions of the development and testing of a suitable method for parameter identification as well as the development of a UMAT for finite element calculations. The constitutive material model takes into account kinematic and isotropic hardening as well as isotropic damage and is designed for 3D simulation. It is based on the concept of effective stress combined with the principle of generalized energy equivalence. The method of neural networks with subsequent optimization by the Nelder-Mead method was used to identify the material parameters. A specially developed program package supports the user in the simultaneous consideration of different test types such as creep, fatigue and creep fatigue tests. The material parameters were identified on the basis of existing 1 D test data. The focus of the experimental work was on carrying out verification tests on round notch specimens and cross specimens. The stress durations achieved in the verification experiments ranged from 2000 to 3000 h with variation of the holding time influencing the creep damage and the stress level. In particular, the strain-controlled mapping of a biaxial creep fatigue stress with the aid of cross specimens proved to be advantageous with regard to the proximity to the heated surface of solid high-temperature components. In summary, the work carried out here demonstrated the potential of this advanced material model. Overall, the material model was able to achieve satisfactory prediction results for multiaxial creep and creep fatigue loading with one set of parameters. Future extensions concern the damage approach in interaction with long-term multiaxial creep fatigue experiments. The advantage for industrial application lies in the comparatively small number of experiments to determine the material parameters and in the greater flexibility of this material model for a wide range of stress parameters. The objective of the research project has been achieved. Scope of report:
150 p., 84 ill., 9 tab., 89 lit. Start of work:
01.07.2001 End of work:
31.12.2001 Funding body:
AVIF-No. A166 Research unit:
Institute for Materials Science Technical University Darmstadt Prof. Dr.-lng. C. Berger Dr.-lng. A. Scholz Processor and author:
Dipl.-lng. A. Samir Chairman of the working group:
Dr.-lng. C. Richter, Siemens, Power Generation