The validation of a ball-on-ring, equibiaxial flexural strength method to obtain the transverse rupture strength (TRS) of right cylindrical ceramic specimens was performed in this study. Validation of the test method was achieved using commercially available engineered high purity alumina disks and finite element (FE) model analysis. The validated fixture was then used to obtain the TRS and Weibull statistical analysis of MgOpartially stabilized zirconia (MSZ) and Y 2 O 3-partially stabilized zirconia (YSZ) ceramic disks. TRS data for alumina, MSZ, and YSZ agreed with the TRS values reported in the literature. A statistically relevant number of samples (N>30) for each material were tested to allow for a Weibull statistical analysis. Weibull parameters for these materials were within the expected values for engineered ceramics. The characteristic strength for alumina, MSZ, and YSZ were determined to be 289 MPa, 786 MPa, and 814 MPa, respectively. The Weibull modulus was determined between 10-25 for each material, which is typical of engineered ceramics. In addition, Accepted Article This article is protected by copyright. All rights reserved FE model results were in close agreement with experimental fracture values for the three ceramic materials tested in this study.
Shape memory ceramics (SMCs) are promising candidates for actuators in extreme environments such as high temperature and corrosive applications. Despite outstanding energy dissipation, compared to metallic shape memory materials, SMCs suffer from sudden brittle fracture. While the interaction of crack propagation and phase transformation in SMCs have been subject of several experimental and theoretical studies, mainly at macroscale, the fundamental understanding of the interaction of crack propagation dynamics with evolving martensitic transformation is poorly understood. In this work we use the phase field technique to fully couple the martensitic transformation to the variational formulation of brittle fracture. The model is parameterized for zirconia which experiences tetragonal to monoclinic transformation during crack propagation. For the mode I of fracture, opening mode, crack shows an unusual propagation path which indicates the effect of phase transformation on crack path. The model is efficiently capable of predicting the crack initiation as well as propagation. The results show the dramatic effect of phase transformation on fracture toughening and crack propagation path.
The aim of this study is to develop a semi-analytical method to investigate fluid-structure coupling of concentric double shells with different lengths and elastic behaviours. Co-axial shells constitute a cylindrical circular container and a baffle submerged inside the stored fluid. The container shell is made of functionally graded materials with mechanical properties changing through its thickness continuously. The baffle made of steel is fixed along its top edge and submerged inside fluid such that its lower edge freely moves. The developed approach is verified using a commercial finite element computer code. Although the model is presented for a specific case in the present work, it can be generalized to investigate coupling of shellplate structures via fluid. It is shown that the coupling between concentric shells occurs only when they vibrate in a same circumferential mode number, n. It is also revealed that the normalized vibration amplitude of the inner shell is about the same as that of the outer shell, for narrower radial gaps. Moreover, the natural frequencies of the fluid-coupled system gradually decrease and converge to the certain values as the gradient index increases.
Shape memory ceramics (SMCs) are promising candidates for actuators in extreme environments such as high temperature and corrosive applications. Despite outstanding energy dissipation, compared to metallic shape memory materials, SMCs suffer from a sudden brittle fracture. While the interaction of crack propagation and phase transformation in SMCs has been the subject of several experimental and theoretical studies, mainly at the macroscale, the fundamental understanding of the dynamic interaction of crack propagation and martensitic transformation is poorly understood. This dissertation attempts to provide a mathematical model for crack propagation in transformable zirconia to address the shortage of classical methods. This dissertation uses the phase field framework to fully couple the martensitic transformation to the variational formulation of brittle fracture.
Firstly, the model is parameterized for single crystal zirconia, which experiences tetragonal to monoclinic transformation during crack propagation. For mode I of fracture, the opening mode, crack shows an unusual propagation path that is in good agreement with the experiments and indicates the significant role of phase transformation on the crack propagation path. The investigation on the effect of lattice orientation on crack propagation shows that the lattice orientation has a significant influence not only on the crack propagation path but also on the magnitude of the transformation toughening.
Secondly, the model is parameterized for tetragonal polycrystalline zirconia, and the experimental data from literature were used to validate the model. The model predicts the three dominant crack propagation patterns which were observed experimentally, including the secondary crack initiation, crack branching, and grain bridging. The model shows the critical role of texture engineering in toughening enhancement. Polycrystalline zirconia samples with grains that make low angles between the a-axis in the tetragonal phase and the crack plane, show higher transformation toughening, due to maximum hydrostatic strain release perpendicular to the crack tip. The model also shows the grain boundary engineering as a way to enhance the transformation toughening. The maximum fracture toughness occurs at a specific grain size, and further coarsening or refinement reduces the fracture toughness. This optimum grain size is the consequence of the competition between the toughening enhancement and MT suppression with grain refinement.
Finally, we parameterized the model for the 3D single crystal zirconia, which experienced stress- and thermal-induced tetragonal to monoclinic transformation. The developed 3D model considers all 12 monoclinic variants, making it possible to acquire realistic microstructures. Surface uplifting, self-accommodated martensite pairs formation, and transformed zone fragmentation were observed by the model, which agrees with the experimental observations. The influence of the crystal lattice orientation is investigated in this study, which reveals its profound effects on the transformation toughening and crack propagation path.
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