Origin of mist and hackle patterns in brittle fracture

A velocity growth rule for secondary cracks in brittle materials is suggest ed. This rule agrees with the assumption that the mist-heckle transition or iginates from the inability of the primary crack to overtake secondary crac ks. The geometrical shapes of the secondary cracks in the mist and hackle z ones change gradually with distance from the fracture origin. The velocity rule is used to calculate these shapes across the mist and at the beginning of the hackle zone. These calculations are shown to be in agreement with e xperimental results.