Interaction between a crack front and cleavage steps

Observations of the interaction of a crack front with cleavage steps on MgO fracture surfaces are presented and show that (a) steps exert a dragging force proportional to their height and (b) the crack front overlaps itself prior to the formation of the step. Based on these observations and on the hypothesis that a crack front possesses a line tension, a mechanism is proposed to explain the overlapping of one part of a crack front with the other, resulting in the formation of cleavage whiskers, steps undercut with cracks and foils of material that are associated with cleavage fracture. The energy absorbed during the parting of the steps is discussed.