Griffith mechanical energy release rate

For equilibrium systems with no contact hysteresis G = W, which is the classical Griffith criterion in fracture mechanics. For such a system, Eqs. 12 and 37 are the same. That is, the strain energy release rate is given by... 

This quantity, which is called the energy release rate, is of central importance in fracture mechanics the minus sign accounts for the fact that Q is a measure of the continuum potential energy reduction per unit crack advance per unit depth. Imposition of the Griffith condition G a) = T, which is a necessary condition for the onset of crack growth, implies that... 

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