Tesis doctoral de Luca Ceseracciu
The use of ceramic materials in many industrial fields is spread and everincreasing, for their excellent properties, eight mechanical, thermal, tribological or biological however, their brittleness and lack of reliability are obstacles to further spreading these materials in applications where structural resistance is required. To build multilayered composite structures is a promising way which aims to increase the reliability of ceramics. As it is common in composite materials, layered materials allow the mechanical properties to be superior to those of the constituent materials, in the studied case due to the presence of compressive residual stress in the surface. the best applications for such materials are those related to the surface properties for this reason the response to contact loading is especially important to characterize the mechanical properties and to assist in the design of advanced ceramic composites hertzian indentation techniques provide a powerful tool to study such type of loading which is otherwise difficult to characterize with the traditional mechanical testing methodologies. Contact damage in brittle materials appears mainly as surface ring-cracks, which can develop in a characteristic cone crack. Such fissuration is detrimental to the functionality of the material and can lead to the failure of the component. Tough ceramics often present another type of damage, the so-called quasi-plasticity, generated as subsurface microcracking and which is cause of inelastic deformation. In this thesis, alumina-based ceramic laminates were characterized in their resistance to contact damage in all its aspects, starting from the appearance of surface fissures, to the propagation of brittle cracks in the layer and its influence on the material strength, to the contact loading induced failure. Experimental measurements were coupled with finite element analysis of the involved parameters, which assisted in formulating comprehensive guidelines for the correct characterization and the design of advanced multilayered ceramic. The presence of residual stress in ceramic laminates proved to be effective in improving the material resistance to the ring cracking generated by monotonic, cyclic and longlasting tests. The better resistance to these latter revealed the existence of grain bridging hindering the crack formation, unexpected in fine-grained alumina and which was related to the small crack character of the ring crack. Longer lasting cyclic tests showed that more severe damage appears in the multilayered materials than in the monolithic one, suggesting a modification of the predominant damage mode to queasiplastic-derived surface degradation. propagation of long cone cracks is affected by residual stress in both the length and angle an automatic finite element model of crack propagation allowed to predict crack growth as a function of both the extrinsic residual stresses and of microstructural parameters which helped address the long-open question of the cone crack angle on polycrystalline materials. The response to remote loading of indented materials in other words the strength degradation, is conditioned by the cone crack geometry, as well as by other factors deriving from the laminated structure, such as the presence of residual tress it self and the load redistribution due to the elastic mismatch between layers similarly, the contact strength, i.E. The resistance to local blunt compression, is improved in the composite materials as a consequence of the residual stresses nevertheless, the risk of high stress in the lower tensile layers was highlighted for both types of loading and general consideration on the design of laminated materials were proposed. In the overall, a comprehensive characterization of the contact properties of the studied materials was achieved and the understanding of crack propagation on brittle polycrystalline materials was broadened and improved.
Datos académicos de la tesis doctoral «Contact damage on ceramic laminates«
- Título de la tesis: Contact damage on ceramic laminates
- Autor: Luca Ceseracciu
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 22/05/2008
Dirección y tribunal
- Director de la tesis
- Marcos Juan Anglada Gomila
- Tribunal
- Presidente del tribunal: Luis Llanes pitarch
- José ygnacio Pastor caño (vocal)
- robert Danzer (vocal)
- gofredo De portu (vocal)