The heart of crystalline materials

The term thermal stress is yet another misuse of language, because all stresses are mechanical in nature. These are merely stresses of thermal origin linked to the presence of a temperature gradient within the part under investigation. This type of stress is found in parts, for example the fixed blades of turbojet engines used in aeronautics, that contain variations in thickness between leading edge, blade body and trailing edge. At a given instant, the temperature in the body is greater than the temperature in the trailing edge. The contracting tendency at the trailing edge is therefore greater and consequently the thickest part exerts tensile stresses on the thinner part. The purely elastic deformation that results is expressed as a function of the material's temperature gradient \(\Delta T\) and the coefficient of thermal expansion \(\alpha\).

\(\epsilon = \alpha \cdot \Delta T\)

The stress is expressed by

\(\sigma _\textrm{thermal}= E \cdot \alpha \cdot \Delta T\)

Generally, these stresses provoke premature cracking in the trailing edge. Stresses of thermal origin are not residual stresses in that they are not permanent. Once there is no longer temperature gradient, where the engine is cold in the example we have chosen, these temporary stresses disappear.