Tube Collapse Pressure Calculator

This calculator estimates collapse under uniform external pressure using two bounding checks: an elastic buckling estimate and a yielding estimate. The governing pressure is the smaller value.

• Elastic buckling (with knockdown): p_cr = γ · E / (4(1 − ν²)) · (t/r)³
• Yielding (circumferential stress): σ_θ = p·r/t so p_y = σ_y·t/r
• Allowable pressure: p_allow = min(p_cr, p_y) / SF

The mean radius r is approximated from the provided diameter and thickness. For thin-walled tubes, this offers a practical estimate for screening designs.

1. Choose whether your diameter input is outer or inner.
2. Enter diameter, thickness, and length in the same length unit.
3. Enter material properties: elastic modulus, Poisson’s ratio, and yield strength.
4. Set a knockdown factor to represent imperfections and fabrication.
5. Set a safety factor to convert governing pressure to an allowable value.
6. Click Calculate. Review notes for thin-wall and slenderness guidance.
7. Use Download CSV to save outputs, or Download PDF to print.

• Imperfections matter. External pressure buckling is highly sensitive to ovality and dents, so γ is often below 1.0.
• Boundary conditions matter. End restraint and stiff rings can increase collapse pressure compared with free ends.
• Use codes for final design. For critical service, follow applicable standards and validated methods.

1) What collapse pressure represents

Tube collapse pressure is the external pressure level where a round cross‑section becomes unstable and begins to ovalize. After ovalization, the tube can rapidly lose stiffness and strength. This calculator reports a governing estimate by comparing elastic instability and yielding behavior for thin‑walled tubes.

2) Geometry drives sensitivity

The thickness ratio t/r is the dominant geometric factor. Because the elastic estimate scales with (t/r)³, a small thickness change can produce a large change in predicted collapse pressure. Use consistent units for diameter, thickness, and length to avoid hidden errors.

3) Material stiffness contribution

Stiffer materials resist ovalization. Typical elastic modulus values include ~200 GPa for carbon steel, ~70 GPa for aluminum alloys, and ~110 GPa for some titanium alloys. For most metals, Poisson’s ratio is commonly 0.25–0.35, which influences the 1 − ν² term in the elastic expression.

4) Yielding limit as a second check

Even if a tube is stable elastically, hoop stress can reach yield as pressure increases. A practical estimate uses σθ = p·r/t, giving p = σy·t/r. Typical yield strength ranges are roughly 200–450 MPa for common structural steels and 150–350 MPa for many aluminum alloys, depending on temper and specification.

5) Why knockdown factors are needed

External pressure collapse is highly imperfection‑sensitive. Small ovality, dents, welding mismatch, or residual stresses can lower the measured collapse pressure below the ideal elastic prediction. The knockdown factor γ (often 0.6–1.0) lets you model that reduction in a transparent, adjustable way.

6) Interpreting the controlling mode

When the elastic result is lower than the yielding result, the tube is predicted to buckle before yielding. When yielding is lower, plasticity is expected first. The controlling mode matters for mitigation: stiffeners, improved roundness, and tighter fabrication tolerances mainly help buckling‑controlled cases.

7) Safety factor and allowable pressure

Design typically uses an allowable pressure, not the raw governing estimate. This tool divides the governing pressure by your selected safety factor. For screening studies, values from 1.5 to 3.0 are common, but final choices should match your risk level, inspection capability, and governing design practice.

8) Data checks for credible inputs

Before accepting a result, confirm that t/r < 0.1 for thin‑wall behavior and review the slenderness note L/r. Extremely short or very long tubes can deviate from simplified assumptions due to end restraint, local ovalization, or manufacturing variability. Use the CSV/PDF export for records.

1) Is this result a code-compliant collapse pressure?

No. It is an engineering estimate for quick checks. For critical equipment, use the applicable external pressure rules, verified material data, and qualified fabrication tolerances.

2) Which diameter should I enter?

Enter whichever you know best and select the matching diameter type. The calculator estimates mean radius using diameter and thickness, which is suitable for thin‑walled screening.

3) What does the knockdown factor represent?

It reduces the ideal elastic prediction to reflect imperfections such as ovality, dents, weld mismatch, and residual stress. Lower values mean greater imperfection sensitivity.

4) Why can thickness changes affect results so strongly?

The elastic collapse estimate scales with (t/r)³, so a small change in thickness can create a large change in predicted collapse pressure, especially for thin walls.

5) What output unit should I choose?

Select the unit used by your design documents and instruments. The calculator converts internally in Pascals and reports results in your chosen unit for clarity and exporting.

6) How should I pick a safety factor?

Use a factor consistent with your service severity, inspection plan, and organizational standards. Higher factors are common when loads are uncertain or consequences are high.

7) What if my tube is short and has stiff end caps?

End restraints can increase collapse capacity versus a free tube. Treat the result as conservative screening, then evaluate boundary conditions with more detailed methods if needed.