Energy release rate and the Griffith criterion

To give the Griffith concept of fracture a concrete structure, which can 

The corresponding elastic energy per unit depth in each arm is 

The specific fracture energy is denoted by F, measured as energy expended per unit crack advance per unit distance along the crack edge. For this configuration, what is the value of 6 necessary to bring the system to a state of incipient fracture under displacement control  

For the case of imposed displacement 6 = 6, there is no exchange of energy between the solid and its surroundings during virtual crack advance. Hence, the total potential energy per unit depth V a) is the elastic 

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 

---