Cyclic plasticity and creep of power plant materials

Saad, Abdullah Aziz (2012) Cyclic plasticity and creep of power plant materials. PhD thesis, University of Nottingham.

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The thermo-mechanical fatigue (TMF) of power plant components is caused by the cyclic operation of power plant due to startup and shutdown processes and due to the fluctuation of demand in daily operation. Thus, a time-dependent plasticity model is required in order to simulate the component response under cyclic thermo-mechanical loading. The overall aim behind this study is to develop a material constitutive model, which can predict the creep and cyclic loading behaviour at high temperature environment, based on the cyclic loading test data of the P91 and the P92 steels.

The tests on all specimens in the study were performed using the Instron 8862 TMF machine system with a temperature uniformity of less than ±10°C within the gauge section of the specimen. For the isothermal tests on the P91 steel, fully-reversed, strain-controlled tests were conducted on a parent material of the steel at 400, 500 and 600˚C. For the P92 steel, the same loading parameters in the isothermal tests were performed on a parent material and a weld metal of the steels at 500, 600 and 675°C. Strain-controlled thermo-mechanical fatigue tests were conducted on the parent materials of the P91 and the P92 steels under temperature ranges of 400-600°C and 500-675°C, respectively, with in-phase (IP) and out-of-phase (OP) loading. In general, the steels exhibit cyclic softening behaviour throughout the cyclic test duration under both isothermal and anisothermal conditions.

The cyclic softening behaviour of the P91 steel was further studied by analyzing stress-strain data at 600°C and by performing microstructural investigations. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images were used to investigate microstructural evolution and the crack initiation of the steel at different life fractions of the tests. The TEM images of the interrupted test specimens revealed subgrain coarsening during the cyclic tests. On the other hand, the SEM images showed the initiation of microcracks at the end of the stabilisation period and the cracks were propagated in the third stage of cyclic softening.

A unified, Chaboche, viscoplasticity model, which includes combined isotropic softening and kinematic hardening with a viscoplastic flow rule for time-dependent effects, was used to model the TMF behaviour of the steels The constants in the viscoplasticity model were initially determined from the first cycle stress-strain data, the maximum stress evolution during tests and the stress relaxation data. Then, the initial constants were optimized using a least-squares optimization algorithm in order to improve the general fit of the model to experimental data. The prediction of the model was further improved by including the linear nonlinear isotropic hardening in order to obtain better stress-strain behaviour in the stabilisation period.

The developed viscoplasticity model was subsequently used in the finite element simulations using the ABAQUS software. The focus of the simulation is to validate the performance of the model under various types of loading. Simulation results have been compared with the isothermal test data with different strain ranges and also the anisothermal cyclic testing data, for both in-phase and out-of-phase loadings. The model’s performance under 3-dimensional stress conditions was investigated by testing and simulating the P91 steel using a notched specimen under stress-controlled conditions. The simulation results show a good comparison to the experimental data.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Hyde, T.H.
Sun, W.
Keywords: Steel, creep, fatigue, electric power-plants
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 12538
Depositing User: EP, Services
Date Deposited: 12 Sep 2012 08:23
Last Modified: 15 Dec 2017 10:31

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