Tube is chosen for bending, compression and torsion. Three properties decide the material: stiffness for the weight (E/ρ), corrosion and environment, and electrical or thermal behaviour.
Specific stiffness (calculated)
Stiffness per unit mass is E divided by density. With typical values:
| Material | E | Density | E/ρ (GPa per g/cm³) | Thermal expansion (×10−6/°C, approx.) |
|---|---|---|---|---|
| Steel | 200 GPa | 7.85 g/cm³ | 25.5 | 12 |
| Aluminium | 70 GPa | 2.70 g/cm³ | 25.9 | 23 |
| Pultruded FRP | 20 GPa | 1.90 g/cm³ | 10.5 | 10–15 |
Steel and aluminium are about equal and FRP is roughly half of either: for a given deflection a light FRP tube is not lighter than aluminium. FRP’s advantage is elsewhere.
| Need | Steel | Aluminium | FRP |
|---|---|---|---|
| Stiffest member for the weight | Good | Good | Moderate |
| Corrosion in chemicals / salt | Needs coating | Good in air, attacked by strong alkali/chlorides in some alloys | Good; set by the resin |
| Electrical insulation | None | None | Yes |
| Thermal break (cold bridge) | Poor | Very poor | Good |
| Magnetic / RF neutral | No | Non-magnetic but conductive | Yes |
| Fire | Does not burn | Melts at lower temperature | Needs flame-retardant resin |
| Welding and field modification | Easy | Possible | Cut and bolt/bond only |
Quick rules
- Same job but near live electrical equipment: FRP.
- Washdown, chlorine or acid environment: FRP (vinyl ester if the chemicals are strong).
- Long span, tight deflection, weight critical and dry: aluminium or steel.
- Thermal bridging at a building edge: FRP.
Section sizes and a deflection example are in FRP square tube and FRP round tube.
Example: choosing for a handrail post
A handrail post at a chemical wash-down area needs corrosion resistance and some flexibility under impact: FRP in vinyl ester is a good fit. A stiff, long, lightly loaded boom outdoors in a dry climate suits aluminium. A post next to live equipment suits FRP for insulation. Compare the three on the property that matters most and then check cost.
Send us your requirement
Guangdong Haikuo Composite Materials Co., Ltd. (Huizhou, Guangdong) designs pultrusion moulds in-house and produces custom FRP profiles. Send a drawing, a sample or just the application and the environment, and we will tell you what is feasible.
Email: helena@frphk.cn | Phone: +86 189 5198 8522