“How strong is acrylic tube?” usually means one of two things: can it hold water, or can it take a knock. The two answers come from different equations, and a tube sized for one is often undersized for the other.
Holding water: hoop stress
Hoop stress is pressure times diameter divided by twice the wall. At 2 m of water the pressure is 19.6 kPa. On a 400 mm tube at 10 mm wall that is 0.39 MPa — comfortable against a 6 MPa long-term design stress. On a 1,200 mm tube at the same 10 mm wall it is 1.18 MPa, still inside the limit but with only a factor of five, and a factor of five is thin once temperature and creep are added.
Taking a knock: bending and contact
A 40 kg side load applied at mid-height on a 1,500 mm span produces a bending moment of about 147 N·m. On a 400 mm tube at 10 mm wall the section modulus is roughly 1.19 × 10⁻³ m³, giving 0.12 MPa — negligible. The failure in practice is not bending but local contact: a point load on a 3 mm contact area will dent and craze long before the section runs out of capacity, which is why tube in a public space gets a protective sleeve or a bollard, not a thicker wall.
The 6 MPa design stress, and why it is not 70 MPa
Cast acrylic has a short-term tensile strength near 70 MPa. Design stress for a permanently wet, permanently loaded part is about one-tenth of that, because PMMA stress-crazes in the presence of water and the crazes propagate at loads far below the short-term figure. Any calculation quoting 70 MPa for a tank wall is quoting a burst number as a service number.
Three geometry decisions that beat adding wall
- Keep the water line below a joint wherever possible — a bond line at the water line is the classic failure point
- Support the tube at no more than 1.5 times its diameter to stop the sag adding its own bending moment
- Avoid sharp internal corners at the base; a radius of at least one wall thickness spreads the stress
The cylindrical tank build specification applies these limits to a full tank, and underwater viewing window sizing covers the flat-panel equivalent at the same depth.
Temperature is the quiet multiplier on every one of these numbers. PMMA loses stiffness well before it loses strength: at 40 °C the modulus is roughly two-thirds of its 20 °C value, so the same tube deflects half again as much in a warm plant room. A tube specified for a cool gallery and installed beside a heat source is a different calculation.
The practical answer for a buyer is to ask for the design stress and the deflection at the service temperature, not just the burst pressure. A supplier who can only quote a burst figure has not done the calculation the installation actually needs, and the difference shows up as a sagging tube rather than a failed one.
For a strength review on a tube geometry, send the drawing and the load case to oversales@rpplexiglass.com; we will reply with the wall that holds it.
Engineers usually need one of these three numbers clarified.
Frequently Asked Questions
Q: How much pressure can acrylic tube hold?
A: At 2 m of water a 400 mm tube with a 10 mm wall sees 0.39 MPa hoop stress, well inside a 6 MPa long-term design limit. Diameter drives the answer far more than depth.
Q: Should I use acrylic’s 70 MPa tensile strength for design?
A: No. For a permanently wet, loaded part the design stress is about 6 MPa, roughly one-tenth of short-term strength, because acrylic stress-crazes in water.
Q: What usually breaks an acrylic tube in service?
A: A bond line at the water line, or local contact damage from a point load. Both fail before the tube section reaches its bending capacity.