| Summary: | This dissertation presents the experimental, numerical and analytical investigations on the impact behaviour of composite sandwich panels. The objectives of this research are to predict the low-velocity impact response and damage in composite sandwich panels, and to characterize the energy absorbed by the structure.
Two types of composite sandwich panels were fabricated for this research: 1) traditional composite sandwich panel composed of Nomex honeycomb core with carbon/epoxy laminated facesheets and 2) advanced composite sandwich or fibre-metal laminates composed of glass fibre-reinforced laminates sandwiched between thin metals.
The impact response was divided into three stages, that is, initial elastic stage in which both the core and facesheets are in elastic, indentation stage and the final one, either rebounding or penetration of top facesheet. The principle of minimum potential energy method was applied to solve the impact response with the impactor size taken into consideration. For spherical-ended impactor, the effect of impactor radius is insignificant when the impactor radius is small as compared to a characteristic length of the sandwich panel. When the impactor radius is sufficiently large, the contact stiffness between the impactor and the sandwich panel increases with increasing impactor radius. The effect of impactor radius on failure of composite sandwich panel is more pronounced than that of response. Larger impactor radius leads to much higher maximum load capacity and this in turn affects the damage initiation mode and failure modes.
Numerical work using three-dimensional finite element models was carried out to simulate the indentation tests and impact events. A progressive damage constitutive model was specified to predict damage initiation and progression in the laminated facesheets. Impact behaviour, including the impact response, damage initiation and evolution were investigated. The comparison of the numerical results with the experimental results shows good agreement. The thickness of the facesheets was found to distinctly increase the impact resistance and maximum load capacity. The effect of the thickness and density of the core are, however, interdependent on the properties of facesheets and of the core. If the facesheet is stiff and thick, impact resistance of sandwich panels with higher core density improves. While sandwich panels with thin facesheet and the core of high density may encounter core shear fracture and thus yields lower impact resistance.
After extensive review of potential damage modes and failure modes of composite sandwich panels, competing damage initiation mode for the lowest initial threshold force were explored and damage/failure mode map were sketched. From the failure modes map, investigation on preferable failure modes for energy absorption was followed.
All results from experimental investigation, analytical solution and numerical simulation were presented and compared to give a full understanding of the impact response of composite sandwich panels subjected to quasi-static load and low-velocity impact.
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