Most materials expand when heated and contract when cooled, because higher temperature makes atoms vibrate more and take up more space. For length changes, the linear thermal expansion formula is:
Coefficients (approx., per °C)
| Material | α (×10⁻₆/°C) |
|---|---|
| Steel | ~12 |
| Stainless steel | ~17 |
| Aluminum | ~23 |
| Copper | ~17 |
| Concrete | ~12 |
Worked example
A 10 m steel beam heated 30°C: ΔL = 12×10⁻₆ × 10 × 30 = 0.0036 m, or 3.6 mm. It sounds small, but across bridges, rails, and pipelines it’s why expansion joints exist — without room to grow, thermal stress can buckle or crack structures.
Frequently asked questions
What’s the thermal expansion formula? ΔL = α × L × ΔT for length change.
Which expands more, aluminum or steel? Aluminum — its coefficient is about double steel’s.
Why do bridges have expansion joints? To give materials room to grow and shrink without cracking.
A worked example
Steel expands by roughly 12 millionths of its length for each degree Celsius, and aluminum by about 23 millionths. A one meter steel bar heated by 50 degrees Celsius therefore grows by about 0.6 millimeter, and an aluminum bar of the same length grows by about 1.2 millimeters. The change looks tiny but it is large compared with the tolerances of precision parts.
Designing for expansion
Leave clearance or use expansion joints where a part is constrained between fixed points, or the growth will turn into stress. Parts made of different materials expand by different amounts, which can loosen or tighten a fit as the temperature changes. Plastics expand several times more than metals. For precise measurement, the standard reference temperature is 20 degrees Celsius, so let parts reach it before checking them.