The measurement of surface free energies

Any effect for which the specific surface free energy y appears in the equations describing it can be used for the measurement of y, but only a few such methods give reliable results. Some of these will now be considered. 

For brittle solids an estimate of the specific surface free energy 

The material on each side of the cleavage plane may be treated as a bent beam, a cantilever in fact, and, if only elastic deformation is considered to occur, it can be shown that  

When the crack increases its length from L to L A- SL the new surface produced is 2z8L and, if this is done reversibly and iso-thermally, the energy needed is  

The spreading of the crack results in the storage of additional elastic energy of amount  

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