Carbon Fiber Tube or Plate for a Structural Member

Tube Wins on Efficiency

A tube puts material as far from the neutral axis as the geometry allows, in every direction at once. For a member that sees bending from more than one plane, or any torsion at all, tube is the efficient answer.

Stock tube runs from CT-5.0-7.0 at 5.0 mm nominal inside diameter up to CT-236.0-242.0 at 236.0 mm, with wall thicknesses from 1.0 mm to 4.0 mm and a standard length of 1000 mm.

Plate Wins on Connections

The problem with tube is always the ends. Attaching to a round section means bonded fittings, clamps or moulded-in inserts, and each of those is a design problem of its own.

Plate is flat, so it can be drilled, bolted and stacked. For a drone centre plate carrying a dozen fixings in two planes, plate is not a compromise - it is the correct shape.

Grades Change the Calculation

Tube is available in standard modulus, high modulus and ultra high modulus grades. Moving up a grade raises stiffness without adding wall thickness, which is the cheapest weight saving available on a long span.

Exterior material is either 3K plain fabric or 3K twill fabric, in matte or gloss, so appearance is a separate decision from structure.

Fibre Angle Is the Real Variable

Unlike metal tubing, carbon fiber tubing is designed with layers of fibres oriented at different angles to provide properties designed for a specific application. Axial fibre resists bending; angled fibre resists torsion. A tube optimised for one is not optimised for the other.

State the load case when ordering. A tube specified only by diameter and wall leaves the most important variable undefined.

A Rule of Thumb

Long unsupported span with bending or torsion, few attachment points: use tube. Short span, many fixings, flat mating faces: use plate. Both in the same assembly is common, and stock exists for both.