Carbon Fibre Internal Structures
Design, interfacing, analysis, optimisation, and manufacturing of carbon sandwich 'I beam' bulkheads for Solar Car 4's internal structures
Photo of exposed car internals at the Motor Vehicle Inspection Facility in Darwin
Overview
Our 2025 Solar car uses unique 'I beam' style bulkheads to form its internal structure and integrate geometry and loads among most of the subsystems in the car. It is the direct mounting point for suspension, steering, latching, array hinging and battery box systems, as well as allowing for routing paths for brake, electrical, and latching lines.
I was responsible for seeing through this subsystem for substantial sectors from beginning to end, from geometric integration, to load path optimisation, to analysis, to manufacturing
Structural bulkheads highlighted in purple
Design & Analysis
Design and analysis involved a lot of geometric and load integration, interfacing and packaging key subsystems on the vehicle while ensuring load paths are optimised and weight minimised.
Geometric Design
The fun part of designing the geometric interface for almost 10 subsystems is that they all have their own ideal positioning requirements and contraints. This required a lot of liaising to optimise system compatibility, sharing mounting constraints, and trimming the number of bulkheads.
As it was our team's first time designing an asymmetric catamaran, we prioritised a modular design that allowed for fast and (relatively) easy geometric changes for our novice subsystems. This led us to 'I beam' bulkheads rather than the 'torsion box' structure of other similar cars. Below are some initial iterations of geometry, and the final integrated geometry.
1: Early geometric option exploration 2: Final bulkheads with subsystems integrated
Analysis
The bulkheads saw immense loads from our suspensions due to their tall nature with slim mounting points at the top - essentially acting as a large moment arm and exacerbating bump and cornering loads. To err on the safe side for our first in-house engineered composites structure, we brought onboard the EuroComp design code - a composites structural design code that provided the standards and the appropriate safety factors we needed to design a safe structure.
Some initial analysis was conducted using the Rule of Mixtures approach in an excel spreadsheet to provide some initial design outputs, getting an idea of the stresses and deflections at play for the loads experienced.
Excel RoM analysis
Suspension beam element representation
Detailed analysis was then conducted using Ansys mechanical with ACP compostes layup modelling. Suspension loads were modelled through a representative beam element model.
A shell model was used to model the bulkheads, including the unidirectional capping to form the I beam, shear patches, and foam and solid core replacements. A range of load cases were analysed, including various suspension and internal mounting loads.
One challenge I worked through was in model constraint for suspension loads. Because of ever changing subsystem parameters and our inexperience, we chose not to conduct a inertia relief simluation, as we cannot be confident of its representative accuracy, thus relying on modelling representative supports. I thus modelled worst case scenarios with static analysis, including cases of fixed constraints on the occupant cell to analyse representative loads through transverse bulkheads, as well as triangulated displacement constraints to analyse loads through diagonal bulkheads.
Boinding and overlaminate joints between bulkheads were verified with hand calculations due to the limitations of the shell mesh model.
Bump load composite failure analysis on FE model
Manufacturing
Manufacturing the bulkheads was a job split between our in-house workshop and our composites manufacturing partner Sydney Composites.
We manufactured the pre-preg transverse and diagonal bulkheads - the ones taking suspension and most subsystem loads - while Sydney Composites resin-infused the 4m long longitudinals and the side bulkheads.
Our in-house prepreg bulkheads under a vacuum debulk on a flanged glass table
Snippet of bulkhead cross section from engineering drawing
After a lot of process research and analysis, I decided on a tooled flange manufacturing process
This involved building flanges onto a glass table, laying up an outer face sheet that covers the height and curves around the top, laying in unidirectional capping and the core, as well as any core inserts, placing in a foam (note change from drawing) former to round the corner, then closing off with an inner face sheet that ties together with the top.
Fitment was completed with our partner Sydney Composites. Bulkheads were bonded together with Spabond-445, a thickened structural epoxy, with structural epoxy fillets throughout. High load areas had epoxy bonding reinforced with 400gsm biaxial overlaminates.
Bulkhead fitment at Sydney Composites
Results
Despite taking and interfacing loads from most systems in the car, the final bulkheads assembly amounted to less than 20kg of the car's mass
Is there room for improvement? Most certainly. A torsion box structure, as used by most other teams running catamaran style cars, would see much nicer load paths and will allow for more weight to be trimmed. However, I believe it was the right decision for us this time to run I beam bulkhead structures due to the easy servicibility and numerous geometric integration changes it allowed for during design.
Final assembled bulkheads pre subsystem integration
The car's internals catching some fresh air in Coober Pedy
She handles cattle grids beautifully!