Evaluation of manufacturing methods for pultruded rod based hierarchical composite structural members with minimal porosity

Bio-inspired, hierarchical structures following the example of natural composites such as bone, wood or bamboo promise a new approach to advanced composites. This work focuses on hierarchical composites based around pultruded carbon fibre-epoxy rods, rather than layered plies of material. A structural member, or strut, of circular cross section, consisting of cured pultruded rods and epoxy resin, demonstrates this hierarchical concept. This paper focuses on manufacturing of struts by vacuum infusion and by pressurized resin transfer moulding, with the aim of minimizing porosity while retaining the desired cross section. Rod alignment and packing are also considered. Vacuum infusion is carried out with stiff and flexible tooling, and pressurized resin transfer moulding using rigid cylindrical copper tools with and without a flexible liner. Porosity is measured via X-ray computed tomography. The results indicate a way forward for manufacturing low porosity hierarchical composites based on pultruded rods, either via vacuum infusion with a flexible tool, requiring machining to reach a circular cross section, or pressurised resin transfer moulding using a combination of rigid tool and flexible liner at 3 x105 Pa or higher, where porosity is below the limit of detection in a Nikon XTH-320 CT scanner.