Origin of mist and hackle patterns in brittle fracture

A velocity growth rule for secondary cracks in brittle materials is suggested. This rule agrees with the assumption that the mist-hackle transition originates from the inability of the primary crack to overtake secondary cracks. The geometrical shapes of the secondary cracks in the mist and hackle zones 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 experimental results.