Buckon

Meet Buckon, a steel rail who wants to grow in the heat but has nowhere to go.

Most materials expand when they warm up. A free bar simply gets longer. If its ends are held fixed, it cannot grow, so it pushes against whatever holds it. That push spread over its cross-section is a thermal stress.

The stress is σ = EαΔT. Steel has a Young's modulus of E ≈ 200 GPa and expands by α ≈ 12 × 10⁻⁶ per kelvin. A rail clamped tight and warmed by 30 K would feel about 72 MPa, a big fraction of what mild steel can take before it bends for good.

If the force is large enough, a long rail can bow sideways. Engineers call this sun kink or buckling.

Rails are laid under tension in cool weather or with small gaps, and bridges have expansion joints, so that everyday temperature swings do not build up dangerous stress.