CFRP/PVC Sandwich AUV Pressure Hull Optimization

A CFRP/PVC sandwich pressure hull for a subsea autonomous vehicle, optimised zone-by-zone rather than as a single uniform structure. Wet compartment, dry compartment, bulkhead, frames, and stringers were each assigned their own pressure load, core density, and laminate stacking, then iterated through a sequence of FEM configurations until every deflection and safety factor target was met at minimum mass.

The scope also included a dedicated structural study of the bulkhead separating the wet and dry compartments, where the design was benchmarked against a validated baseline and checked for a fundamental structural principle: whether its curvature worked with or against the applied pressure.

The hull was partitioned into nine structural zones — wet compartment top, bottom, front, and hatch; dry compartment top, bottom, back, hatch, and hatch lip — plus the bulkhead, frames, and stringers, each carrying its own pressure load (from 90 kPa on the wet top to 400 kPa on the wet front) and its own PVC core density: 130 kg/m³ in the wet compartment, 150 kg/m³ in the dry compartment and bulkhead, and 200 kg/m³ at frames and stringers where local stiffness mattered most. This zoning strategy let the design carry weight only where the pressure loads demanded it, rather than sizing the whole hull to the worst-case zone.

The design was matured through a documented sequence of FEM configurations. Early iterations showed hatch under-reinforcement and an over-deflecting wet compartment bottom; adding a frame proved ineffective, and switching to a stringer resolved the stiffness deficit more efficiently — a direct engineering finding that shaped the rest of the optimisation. Core thickness in the wet compartment was then increased in stages (60 mm, then 80 mm, then 90 mm at the hatch) to bring deflection within the 6.5 mm target, with each density and thickness change re-checked for its knock-on effect on neighbouring zones such as the bulkhead.

The final configuration met every deflection target — dry compartment bottom under 7.0 mm, wet compartment under 7.5 mm, bulkhead under 7.3 mm — at a total mass of 137 kg, a 14% reduction from the initial 160 kg target. Final stress analysis showed a maximum tensile stress of approximately 200 MPa and maximum compressive stress of approximately 250 MPa, giving safety factors of 2.9 in tension and 2.3 in compression against the material's 580 MPa ultimate strength, both clearing the SF > 2.0 requirement.

The bulkhead separating the wet and dry compartments received its own dedicated structural study, comparing the validated baseline design against an alternative bulkhead geometry. The baseline deflected 4.5 mm under load; the alternative, curved concave against the applied pressure, deflected 21 mm — 367% more — because that curvature direction generates bending moments and reduces effective inertia, the opposite of proper arch action working in pure compression. Reversing the curvature to work with the load, arch-bridge style, was identified as the corrective fix, alongside increasing core thickness or density and considering a hybrid CFRP/aramid layup to address a marginal compression safety factor found in the alternative design's aramid-reinforced sections.