Advances in Mechanics of High-Temperature Materials

Power plant components are subjected to high temperatures up to \(903\,\mathrm {K}\), which induce creep deformations. Furthermore, power plants are frequently started and shut-down, thus resulting in cyclic loads on the components. Since they provide adequate mechanical and thermal properties, tempered martensitic steels are ideal candidates to withstand these conditions. The contribution at hand presents a phase mixture model for simulating the mechanical behavior of tempered martensitic steels at high temperatures. To provide a unified description of the rate-dependent deformation including hardening and softening, the model makes use of an iso-strain approach including a hard and a soft constituent. The model is implemented into the finite element method, using the implicit Euler method for time integration of the evolution equations. In addition, the consistent tangent operator is derived. As a final step, the behavior of an idealized steam turbine rotor during a cold start and a subsequent hot start is simulated by means of a thermo-mechanical finite element analysis. First, the heat transfer analysis is conducted, while prescribing the instationary steam temperature and the heat transfer coefficients. The resulting temperature fields serve as input for the subsequent structural analysis, which yields the stress and strain fields in the rotor.

In this paper, the influence of the microstructure of a cast aluminium alloy used for pistons in combustion engines on the local and global deformation behavior is investigated by means of microstructure-based cell models and the finite element method. Therefore, a representative microstructure is digitized using nano computer tomography. In the digitized and segmented data, the aluminium matrix, silicon particles, pores and two intermetallic phases are distinguished. Microstructure-based cell models are created and linear-elastic, thermal and viscoplastic material properties are assigned for the finite element simulation in ABAQUS/Standard. The elastic, macroscopic nearly isotropic material behavior is shown for 64 different microstructure-based cell models with 200 \(\times \) 200 \(\times \) 200 elements with microstructure-dependent material properties at room temperature. A microstructure cell is subjected to a thermal cycle with zero macroscopic loading in order to examine the influence of the thermal mismatch between the individual microstructure phases on the resulting stresses and strains on the micro level. High stresses at interfaces of silicon particles and the aluminium matrix occur in the linear-elastic simulation, whereas an elastic-viscoplastic material behavior of the aluminium matrix leads to a overall stress relief in the microstructure cell.

The present work is an extensive overview of several aspects of thick FGM structures subjected to thermomechanical loadings. The concept of reducing the full 3D thermo-mechanical FGM problem to the plane stress one is demonstrated in case of both classical and quasi- polar continua. Theorem on the stress free deformation accompanying linear gradation of thermomechanical properties of the material staying in constant temperature condition is presented. The influence of several approximations of FGM thermal barrier coatings on the temperature and stress distributions in thick-walled cylinder including FGM Thermal Barrier Coating is considered. A special graded finite element to discretize FGM properties is introduced.

This study focuses on the mechanical analysis of high temperature gas turbine blades based on finite element method. The aim is to evaluate the feasibility of a Mo-based alloy Mo-12Si-8.5B as a possible candidate as a new type of turbine blade material. For that purpose, the numerical analysis of typical turbine blades under common stationary load was carried out based on the finite element method. The alloy Mo-12Si-8.5B was compared to the state-of-the-art Ni-based superalloy CMSX-4. The creep deformation is taken into account, so that the stationary load under high temperatures could be represented.

In this study the microstructure evolution and the mechanical properties of different cast Mo-XZr (X = 5, 10, 15, 20 at.%) alloys were investigated. It focuses on the effect of Zr concentrations on the second phase formation in binary Mo-Zr alloys. All alloys exhibit polycrystalline Mo₂Zr precipitations as well as a (Mo, Zr) solid solution phase, which in most cases forms the matrix phase. Microhardness measurements were carried out by the Vickers indentation method. The Mo₂Zr precipitations show a strengthening effect in addition to the solid solution hardening. Additionally, constant displacement tests in the compressive mode at room temperature confirm these findings. However, the homogeneously distributed Mo₂Zr phases offer an extraordinary potential to improve the strength of Mo-based alloys without decreasing the ductility.

Most industrial structures are affected by material mismatch effects, due to the design necessity that leads to the use of dissimilar materials like welding of different parts. In other circumstances, this mismatch is introduced by material transformation like radiation embrittlement, hydrogen attack or carburisation, which can drastically change the material response of a restricted area of the component. Such an outcome can have an unpredicted effect on the behaviour and endurance of the component. Carburisation has been identified within the thin stainless steel pipes of UK Advanced Gas-cooled Reactors (AGR). This carburisation is known to affect crack initiation and creep-fatigue properties, ultimately impacting on service life. The current assessment procedure for UK AGRs has several limitations when addressing carburisation and is believed in some circumstances to be overly conservative and in other conditions non-conservative. The work of this study aims to aid in clarifying the effect of creep properties mismatch due to carburisation in thin pipes within such high-temperature facilities. A numerical study is undertaken to investigate the effect of creep properties mismatch in a thin pipe subjected to a combination of primary (load controlled) and secondary (displacement controlled) cyclic loading. In order to perform an extensive parametric study, a special numerical procedure based on the finite element commercial code Abaqus is used to predict the cyclic behaviour of the structure. The effect of creep properties mismatch on global shakedown and creep ratcheting will be investigated providing new insight in the field of structural integrity of pressurised components.

This treatise deals with Cohesive Zone Models which were developed around 1960 through Barenblatt and Dugdale. At first, we present an overview about these models and the numerical treatment of these models in the sense of the Finite Element Method. Further on, a rate-dependent Cohesive Zone Model is presented and tested through a simulation of a Four-Point-Bend-Test with a metal compound. The required material parameters are determined through numerical optimisation by using a neural network which is explained, as well.

We analyze different strategies used for the identification of material parameters, which appear in a certain model of finite strain viscoplasticity. The main focus is set on the sensitivity of the parameters with respect to measurement errors. In different strategies we combine experimental data obtained from various torsion tests with a heterogeneous stress state. A direct problem is solved using the nonlinear FEM. To estimate the stability of a certain identification strategy we perform Monte Carlo simulations for a series of noisy experimental data. A distance between two sets of material parameters is measured using a special mechanics-based metric. Both the identification of material parameters and the estimation of their stability are illustrated by an example. In this example we employ a set of synthetic experimental data obtained for the steel 42CrMo4. As a material model, we choose a model of finite strain plasticity with a combined isotropic-kinematic hardening.

The statement and solution method of the two-dimensional problem of orthotropic creep under static and periodic loadings are presented. Experimental investigations of short-term deformation under uni-axial and plane stress states are carried out. Short-term creep curves were obtained for uni-axial specimens and plates with holes. Constitutive equations are developed for steel characterized by the first stage of unsteady creep with orthotropic properties under static and periodic loading. The calculation method in general was verified by comparing numerical and experimental results under the uni-axial and plane stress states.