This thesis is concerned with developing numerical models to predict the mechanical behaviour of closed-cell aluminium (AI) foams under uniaxial loading. Many of the existing models produce responses which are very stiff initially and fall sharply after an initial peak (Type-II response). In contrast, the real Al foams exhibit a reduced stiffness and give a flat-topped curve in the neighbourhood of peak-load (Type-l response). Thus, the weakness of the existing unit-cell models in relation to the two qualitatively different load displacement responses was identified. A link has been established between the stretching dominated mode of deformation to the Type-II response and the bending dominated to the Type-l response respectively. The absence of Type-l response in the current unit-cell models in the literature moved the present research in a direction to identify an improved new 3D unit-cell, which can be used by future researchers to represent closed-cell Al foam in loading scenarios with large deformations. The unit-cell identified has a right polyhedra structure. Furthermore, the deformation for this model is bending dominated thus giving Type-l response in three principal loading directions. Two sets of polyhedra unit-cell based finite element (FE) models for the crushing of closed-cell Al foam were presented. The first set was constructed by stacking up to 6-cells along principal material loading directions. Perpendicular to the loading direction, the mechanical behaviour for an infinite system was simulated by using periodic boundary conditions (PBC). In the second set, a finite domain sample containing up to 64 unit-cells was used to capture the compressive response behaviour. The crush features of this system were obtained using 3D array of many cells (MC). Additionally, the application of the second type of FE models was extended further to study the performance of Al foam containing imperfections under low-velocity impact by a rigid indenter. A simple imperfection in the form of reduced cell-wall thickness was introduced into the unit-cell Al foam and a set of two different populations of imperfections were considered. All these aforementioned FE foam model predictions were compared with experimental results. Finally, the new 3D unit-cell model results were compared with the contemporary stretching dominated (Type-II response giving) models in literature namely, the truncated-cube and cubic-spherical. A parametric study was conducted to quantify the mechanical properties of aforementioned unit-cell models alongside the model developed in this thesis. It was demonstrated that the plateau phase stress-strain response of the developed 3D unit-cell model was more representative of real closed-cell Al foams. Further, it was shown that the crushing resistance and energy absorption features of this new 3D unit-cell model were higher, compared to the truncated-cube and cubic-spherical models.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:573674 |
| Date | January 2011 |
| Creators | Nammi, Sathish Kiran |
| Publisher | University of Bolton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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