Home | Impressum | Datenschutz | Sitemap | KIT
Ansprechpartner

Dr.-Ing. Mario Walter

mario walter does-not-exist.kit edu

Telefon +49 721 608 23753

Characterization and Modeling of the Ratcheting Behavior of Ferritic-Martensitic Steels

Progressive deformation
Fig.1: Progressive deformation under stress-controlled fatigue test (simulation result)
Fatigue Test
Fig.2: Hysteresis loop of strain-controlled fatigue test (Points: experimental result, Curves: simulation result)
Fatigue Test
Fig.3: Shakedown under stress-controlled fatigue test (simulation result)
Simulation test
Fig.4: Mean value of ratchet strain rate [%/cycle] versus stress ratio (simulation result)

This presented project is within the European project MATTER (MATerials TEsting and Rules), which is a material research project for the generation IV reactors.
The target of the presented project is to study the ratcheting behavior of 9%-Cr-1%-Mo ferritic-martensitic steel (P91). For the application of ferritic-martensitic steel in construction of nuclear power plant, not only a wide database of its mechanical characteristics is required, but also new rules are to be built, with which a reliable construction planning can be made according to the characteristics of the steel. One of the characteristics of this steel is obvious cyclic softening, which leads to the decrease of initial strength and increase of inelastic deformation. The current criteria about the increase of inelastic deformation due to cyclic loading, so called ratcheting (Fig. 1), are limited. These criteria, if without further improvement, should not be applied on cyclic softening ferritic-martensitic steels.
In this project the isothermal ratcheting behavior of 9Cr-1Mo steel P91 will be studied and corresponding constitutive model will be built. The influencing factors, such as middle stress, stress amplitude and temperature will be focused. In modeling process a viscoplastic model will be applied and will be further modified if necessary. Furthermore, suggestions should be provided to build new reliable rules for the application of ferritic-martensitic steel.

This project will be separated into two stages:

      1. Determination of parameter
      2. Research of ratcheting

In the first stage strain controlled low cycle fatigue (LCF) tests are to be performed. These strain controlled LCF tests are with different strain amplitudes and different hold times under tension and compression. And these tests will be performed isothermally both at room temperature and at 550°C. These experimental results will be used to determine the parameters of P91 steel, which are parameters of a previous viscoplastic constitutive model. This model has good simulation capability for the mechanical properties of reduced activation ferritic martensitic (RAFM) steels such as EUROFER97 (Fig. 2). The simulation capability of this model for P91 steel should be checked and modified if necessary. The results acquired in the first stage will be the base for the next research stage.

 

In the second stage stress controlled LCF tests will be performed on P91 steel. These stress controlled LCF tests are with different stress amplitudes, different stress ratios, different stress rates and different hold times under tension and compression. As in the first stage, the tests will be performed isothermally both at room temperature and at 550°C. There are two possibilities of the material’s behavior under stress controlled cyclic loading: One is that it will be continuously elongated until fracture (Fig. 1). Another possible behavior is shakedown (Fig. 3), which means the elongation will stop after a certain number of cycles. The previous constitutive model for RAFM steels shall be modified if necessary to fit both the results of the stress controlled and strain controlled LCF tests. Influencing factors such as stress amplitude, stress ratio, stress rate and hold time will be analyzed. Such as in Fig. 4 the relation between ratchet rate and stress ratio is to be studied.