Conceptual design study of a modular fiber composite AUV

Abstract: The conceptual design of a modular AUV hull in fiber composite material has been determined. The goal has been to minimize the hull weight. Matrix and fiber materials have been investigated to find a composite combination that reduces the hull weight whilst being resistant to changes in the mechanical properties caused by submersion in water and the operational temperatures. A composite of a PEEK thermoplastic matrix with high strength carbon fibers is picked as the most suitable material option and used for hull calculations. Different composite part manufacturing processes are investigated to find a suitable method for the geometry and material that can produce high quality hull modules. Part quality factors such as fiber waviness and out of roundness in shape affects the collapse pressure of the hull. Filament winding adapted for a thermoplastic composite with in situ consolidation is picked as the most suitable manufacturing option.The structural designs considered for the hull are a fiber composite single skin construction (ring-stiffeners possible) and a sandwich construction, these would be manufactured as shorter modules that can be joined together to form the hull. The minimum hull thickness required for a single skin hull operating at 300 meters depth, considering material compression failure and buckling failure of the structure, is calculated for the PEEK/carbon fiber composite material. Buckling is the dimensioning failure mode of a thin walled cylinder with the AUV hull dimensions at the intended operational depth. The lay-up of the composite affects the thickness required so the lay-up is optimized to minimize the hull weight. For the cylindrical modules under hydrostatic pressure a [90/0/90] lay-up minimizes the thickness required and is the recommended lay-up. For comparison of hull weight with the existing AUV the minimum thickness required for a single skin hull in aluminium 7075 considering material compression failure and buckling failure of the structure is also calculated.From the analytical buckling analysis of a simple cylinder hull without joints the minimum thickness is determined as 11.82 mm for the composite hull and 15.23 mm for the aluminium hull, both values with a safety factor of 1.3 for the collapse pressure equating to 3.9 MPa. The single skin composite hull weight becomes 153 kg and the aluminium hull weight becomes 343 kg for these thicknesses. If the added stiffness of the structure from the joints would be taken into consideration it is expected that the thickness could be decreased further, but the relative weight difference between the composite and aluminium hull is expected to remain similar. From the finite element linear buckling analysis of the composite hull with thickness 11.82 mm the buckling pressure is determined as 3.39 MPa and for the aluminium hull with thickness 15.23 mm it is determined as 4.42 MPa.For a sandwich hull the minimum core thickness (using a weak core approximation and quasi-isotropic faces) is calculated as 19.96 mm, with safety factor 1.3 for the collapse depth and factor 1.1 for material failure of the faces. The weight for this sandwich hull with a carbon foam core becomes 72 kg. Based on a heat generation of 3 kW maximum during AUV operation heat transfer calculations through the thickness of the single skin composite hull give the maximum hull thickness as 50 mm before the AUV will overheat. The maximum 3 thickness of a regular PVC foam core sandwich hull is 4 mm and for a carbon foam core it is 21 mm before the AUV will overheat, making a sandwich with a carbon foam core a possible structural design choice but with some complicating factors compared to the single skin composite hull.