.stress rel ief zone
G is the fracture energy, which has the dimension of J / m 2 (or N / m ) and represents a basic material property. For G < Gf , the crack cannot propagate, and for the case G > Gf equilibrium is impossible. If G = Gf and d G / d a > 0 , which is normally the case, the crack is unstable under load control (i.e., the structure fails). There exist, however, some cases in which G = Gf and d G / d a < 0 , and then the crack can grow under load control in a stable manner.