| Popis: |
The thermal shock resistance of a material is a measure of three energies associated with the material, i.e., the elastic strain energy, the energy associated with the formation of new surfaces (surface energy), and the kinetic energy associated with the formation and growth of cracks. Within limits, the work done per unit volume in stressing a material is converted to strain energy where, if Hook’s Law applies: $$SE = \sigma \in /2$$ (1) or, $$SE = E{ \in ^2}/2$$ (2) The limits for a truly elastic material are established when the material is stressed in tension to the level necessary to overcome the energy barrier for creation of a new surface by breaking bonds of atoms lying in a plane of fracture. This barrier is the surface energy, γ, of the material. Griffith1 established a relationship between the surface free energy and the amount of stress, σg (the Griffith stress), necessary to start growth of a crack of half width c: $$\sigma \begin{array}{*{20}{c}} 2 \\ g \end{array} = \frac{{2\gamma E}}{{\pi c}}$$ (3) |
