The path-independent J-integral and the crack-extension energy-release rate

The equilibrium, path-independent J-integral is commonly interpreted as the rate of decrease in the potential energy of an elastic body, containing a crack, per unit area of the new surface created by crack extension. Such an interpretation encounters difficulties. The correct, contradiction-free description of the energy-release rate is the approach based on the dynamic energy flow into the crack tip. As a corollary, it leads to the correct quasistatic criteria for brittle-fracture propagation.