Efectos de la estructura celular sobre el comportamiento mecánico de espumas de aluminio de poro cerrado obtenidas por fusión. Aplicación en absorbedores de energía
[EN] This PhD Thesis is aimed at widening and improving the existing knowledge on the relationships between structure and uni-axial compression mechanical properties of aluminium foams obtained by the melting route. Aluminium foams, like other cellular materials, show high specific stiffness and mec...
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| Tipo de recurso: | tesis doctoral |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | Universitat Politècnica de València (UPV) |
| Repositorio: | RiuNet. Repositorio Institucional de la Universitat Politécnica de Valéncia |
| Idioma: | español |
| OAI Identifier: | oai:riunet.upv.es:10251/61298 |
| Acceso en línea: | https://riunet.upv.es/handle/10251/61298 |
| Access Level: | acceso abierto |
| Palabra clave: | Espumas de aluminio Absorción de energía Espumas de poro cerrado Espumas metálicas Modelos de comportamiento mecánico Simulación MEF Estructura celular CIENCIA DE LOS MATERIALES E INGENIERIA METALURGICA |
| Sumario: | [EN] This PhD Thesis is aimed at widening and improving the existing knowledge on the relationships between structure and uni-axial compression mechanical properties of aluminium foams obtained by the melting route. Aluminium foams, like other cellular materials, show high specific stiffness and mechanical strength, and are able to absorb energy under compression loads by plastic collapse. They are fire resistant and more isotropic than honeycomb structures. These characteristics justify recent industrial and scientific interest on manufacture and applications of metal foams in crash protection systems for vehicles or in lightweight modular structures. Despite their potential advantages, existing foams still show limitations compared to other cellular materials due to high manufacturing costs, the difficulty to obtain higher and more reproducible mechanical properties and the limited knowledge on the stabilization mechanisms and on the control of the structure. Experimental work has been carried out on aluminium foams obtained by the melting route, stabilized with calcium and foamed by addition of TiH2. In contrast with other studies, the extent of the structural characterization has spread from microstructure to the global macro-structure of the foam, providing a more complete picture of the phenomena that control the structure and properties. Cellular structure has been analysed by original image analysis techniques, determining the main parameters of the foam: thicknesses, material distribution, cell sizes and preferential orientations. Macrostructural analysis allowed determining the existence of a systematic and reproducible density gradient in the vertical direction, associated with the increase of cell edge thickness due to gravitational drainage. The foam Al-Ca-Ti alloy has been analysed and characterized, identifying the intermetallics that play the role of stabilizing agents during foaming. As original contribution, tensile mechanical testing of the solid material has been performed. Mechanical strength and energy absorption have been evaluated by compression testing at room and high temperature. Structural anisotropy and density gradients allowed quantifying and justifying deviations in mechanical properties for samples with similar density, allocated in previous studies to random effects. Density gradients in foam panels influence as well the slope of the stress-strain curve in the plastic zone and as a consequence, the efficiency in energy absorption, generating a predictable and measurable scale effect. Strength values measured experimentally have been checked against Gibson and Ashby classic dimensional models, concluding that these cannot predict correctly the mechanical behavior of closed cell aluminium foams. As an alternative, optimized models incorporating the effect of material ratio in cell edges as a function of relative density have been proposed, allowing the correct prediction of strength and the potential effects of the foam relative density. As an original contribution for the analysis of the energy absorption properties, the use of "diagrams of energy absorption per unit of stress" is suggested, allowing the determination of the optimum design strength and strain values for impact protection in a quicker and easier way than previously existing procedures. Finally, FEM simulation has been carried out for the analysis of the effects of wall thickness dispersion and structural anisotropy on collapse stresses. |
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