It is well-known that under uniform heating, a thin plate develops a quadratic temperature profile with the thermal gradient normal to the plate surface (Figure 1). The central part of the plate is warmer than the surfaces, and also undergoes thermal expansion. The thermal expansion normal to the surface produces no stress, but the thermal expansion parallel to the plate surface puts the surface in tension and the central part in compression. A simple but useful model of the plate treats it as a set of three isothermal plates arranged in the order cold-warm-cold (Figure 2). The warm part is AT warmer than the cold surfaces, and left to itself it would expand by aAT. Being constrained by the surface, it is in compression. If the warm part had infinite modulus, the resulting strain in the surface would be aAT. Because it relaxes, the actual surface strain is reduced by 1/3. By Hooke's law the stress is proportional to the strain, so that the surface stress is simply 2/3.EaAT, where E is Young's modulus.