An initially stress-free massless elastic beam of length $L$ and circular cross-section with diameter $d$ $(d << L)$ is held fixed between two walls as shown. The beam material has Young's modulus $E$ and coefficient of thermal expansion $a$.
If the beam is slowly and uniformly heated, the temperature rise required to cause the beam to buckle is proportional to
- $d$
- $d^{2}$
- $d^{3}$
- $d^{4}$