TY - BOOK

T1 - Creep behaviour of refractories under various loading conditions

AU - Schachner, Stefan

N1 - no embargo

PY - 2020

Y1 - 2020

N2 - In service, refractory linings are exposed to thermo-mechanical loads which cause internal stresses, e.g., uniaxial tensile and compressive stresses, shear stresses, and multiaxial stresses. These stresses lead to creep of refractories. At elevated temperatures refractories experience primary creep and might further proceed to the secondary and tertiary creep stages. Usually, the creep behaviour of refractories is mainly investigated in the primary stage under uniaxial compressive loads. Nevertheless, for a comprehensive understanding of their thermo-mechanical material behaviour and the development of material models, it is indispensable to investigate creep under various stress states. The aim of this study is the experimental investigation of the creep behaviour of coarse ceramic refractories under various uniaxial and multiaxial stress states and to consider their material behaviour in finite element (FE) simulations. The uniaxial creep behaviour was determined at different stress levels with two advanced compressive- and tensile creep testing devices, respectively. With these devices creep curves comprising all three stages were obtained. A new developed procedure for the determination of the creep stage onset in the creep curves was established and the Norton-Bailey creep parameters were inversely evaluated for each stage. Subsequently, a criterion was developed to predict the transitions between the creep stages, based on the applied stresses and the critical strains at the beginning of the respective stage. This criterion was implemented together with the Norton-Bailey creep parameters in a FE model. The simulation results showed a satisfactory accordance with the experimental results. Furthermore, the severe asymmetry between tensile and compressive creep was quantified. The shear creep behaviour was investigated using prismatic specimen geometries with inclined notches on the surface. These notches define the shear plane in the specimens when they are loaded with uniaxial compressive loads. The simulation of the experiments with the uniaxial compressive or uniaxial tensile creep parameters showed an unsatisfactory accordance with the test results. Therefore, an asymmetrical creep model was further developed to determine the shear creep parameters, in combination with the uniaxial tensile and compressive creep parameters and weighting functions for divergent stress states. The simulation results showed a good accordance with the experimental results. The shear creep parameters showed significant differences to the uniaxial tensile and compressive creep parameters and thus an asymmetry between the different stress states. Furthermore, a setup was developed which offers the possibility to generate multiaxial compressive stresses in a cylindrically shaped refractory sample when the mechanical load is applied in axial direction. In the setup a stiff ceramic tube was used which restricts the radial deformation during the creep test. The investigated coarse ceramic refractory showed significant volume decrease during the experiment. FE simulations with the von Mises stress-based creep model and the Drucker-Prager creep model showed an unsatisfactory accordance to the experiments. Therefore, a new volumetric creep model was developed, which considers the volume change due to creep in FE simulations and the experimental tests were satisfactorily depicted with this model.

AB - In service, refractory linings are exposed to thermo-mechanical loads which cause internal stresses, e.g., uniaxial tensile and compressive stresses, shear stresses, and multiaxial stresses. These stresses lead to creep of refractories. At elevated temperatures refractories experience primary creep and might further proceed to the secondary and tertiary creep stages. Usually, the creep behaviour of refractories is mainly investigated in the primary stage under uniaxial compressive loads. Nevertheless, for a comprehensive understanding of their thermo-mechanical material behaviour and the development of material models, it is indispensable to investigate creep under various stress states. The aim of this study is the experimental investigation of the creep behaviour of coarse ceramic refractories under various uniaxial and multiaxial stress states and to consider their material behaviour in finite element (FE) simulations. The uniaxial creep behaviour was determined at different stress levels with two advanced compressive- and tensile creep testing devices, respectively. With these devices creep curves comprising all three stages were obtained. A new developed procedure for the determination of the creep stage onset in the creep curves was established and the Norton-Bailey creep parameters were inversely evaluated for each stage. Subsequently, a criterion was developed to predict the transitions between the creep stages, based on the applied stresses and the critical strains at the beginning of the respective stage. This criterion was implemented together with the Norton-Bailey creep parameters in a FE model. The simulation results showed a satisfactory accordance with the experimental results. Furthermore, the severe asymmetry between tensile and compressive creep was quantified. The shear creep behaviour was investigated using prismatic specimen geometries with inclined notches on the surface. These notches define the shear plane in the specimens when they are loaded with uniaxial compressive loads. The simulation of the experiments with the uniaxial compressive or uniaxial tensile creep parameters showed an unsatisfactory accordance with the test results. Therefore, an asymmetrical creep model was further developed to determine the shear creep parameters, in combination with the uniaxial tensile and compressive creep parameters and weighting functions for divergent stress states. The simulation results showed a good accordance with the experimental results. The shear creep parameters showed significant differences to the uniaxial tensile and compressive creep parameters and thus an asymmetry between the different stress states. Furthermore, a setup was developed which offers the possibility to generate multiaxial compressive stresses in a cylindrically shaped refractory sample when the mechanical load is applied in axial direction. In the setup a stiff ceramic tube was used which restricts the radial deformation during the creep test. The investigated coarse ceramic refractory showed significant volume decrease during the experiment. FE simulations with the von Mises stress-based creep model and the Drucker-Prager creep model showed an unsatisfactory accordance to the experiments. Therefore, a new volumetric creep model was developed, which considers the volume change due to creep in FE simulations and the experimental tests were satisfactorily depicted with this model.

KW - Kriechen

KW - asymmetrisch

KW - feuerfest

KW - multiaxial

KW - uniaxial

KW - volumetrisches Verhalten

KW - Norton-Bailey

KW - creep

KW - asymmetric

KW - refractory

KW - multiaxial

KW - uniaxial

KW - volumetric behaviour

KW - Norton-Bailey

M3 - Doctoral Thesis

ER -