Optimisation of low pressure processing for honeycomb sandwich structures

Vacuum-bag processing of honeycomb sandwich structures faces particular challenges compared to autoclave sandwich fabrication for aeronautics, a consequence of using, at the most, atmospheric pressure for their manufacturing. In particular, the adhesion of the skin to the honeycomb core is reduced and the skins tend to have higher values of porosity. On the other hand, autoclave processing is expensive, in particular for large structures, and a growing interest has been observed for vacuum-bag only processing of sandwich structures. During honeycomb sandwich cure, the skin represents a barrier to air flow between the honeycomb and the vacuum-bag so the air transport through the prepreg, adhesive and consumables or other additional materials may be a crucial factor to increase skin debulking, reduce void content and extract air from the honeycomb cells. The pressure inside the honeycomb is, therefore, a result of the through thickness permeability to air of the skins and vacuum pump work. The goal of this thesis is to develop methods for improving the low pressure processing of honeycomb sandwich structures by quantifying the through thickness air permeability of fibre reinforced composites during cure and analysing related processing issues and governing phenomena. An experimental method based on a falling pressure measurement was proposed to evaluate the through thickness air permeability of prepregs during cure, with two set-up variants. One concerns the measurement of the inherent air permeability and the other applies to processing conditions, implying the use of both consumables and vacuum-bag. The methods were successfully tested and fulfil the criterions used in soil science for the evaluation of the air permeability. Prepreg and adhesive permeabilities were determined separately and in skin combination. Prepreg permeability to air during cure varies between 10-19 m2 and 10-17 m2 and was found to depend on the evolution of the resin viscosity, which shifts from an initially immobile phase to a mobile one and then back to an immobile phase. The adhesive film was found to have very low initial permeability, at least two orders of magnitude lower than that of the prepreg. A range of initial skin through thickness air permeability could be achieved by perforating the prepreg plies, the adhesive layer, or a combination of both. An adhesive deposition method which results in the centre of honeycomb cells being free of adhesive was also tested. A corresponding range of achievable pressure was measured inside the honeycomb. The evolution of the through thickness permeability with the curing cycle was determined for each case. The adhesive layer was identified as the element that reduces the initial through thickness air permeability the most in skin manufacturing. This suggests that one of the ways of increasing the initial through thickness air permeability of the skin is to modify the permeability of the adhesive layer. The role of the resulting pressure inside the honeycomb on skin-core adhesion and skin quality was then evaluated. Skin permeability was found to control skin-core adhesion through the pressure drop in the honeycomb cells and potential outgassing of the adhesive layer. An optimal range of pressure inside the honeycomb was found to be between 40 kPa and 70 kPa. A process window is proposed for achieving the optimal pressure inside the honeycomb, as a function of the skin permeability and time of vacuum application. A semipreg alternating dry and resin impregnated areas along the fibre bed surface was also investigated as an alternative to prepreg in skin manufacturing. The semipreg through thickness air permeability before cure is approximately three orders of magnitude higher than that of a unidirectional prepreg impregnated with the same resin, thus allowing the reduction of air pressure in the honeycomb. A model was proposed for the air permeability change during cure, as dry areas get infiltrated. Due to resin pouring inside the honeycomb cells, this type of semipreg is viable as a skin only if combined with a material that has low permeability to resin, e.g., a prepreg. The resulting initial pressure in the honeycomb cells might then be tailored to specific needs. Finally, it was possible to produce sandwich samples combining a consolidation system, such as a prepreg, with an infiltration system, as wet lay-up or with a semipreg. The combination of a prepreg with a semipreg revealed to be a very practical and clean system, where it is possible to have the same resin in both materials. Moreover, it allows to tailor the initial honeycomb pressure as well as during cure, and to have the initial permeability assessed.