On the bifurcation of cone cracks in glass plates

The cone cracks which propagate from Hertzian ring cracks when glass plates are impacted with steel spheres show zones of mist and hackle and then bifurcate. Experiments show that the crack length or cone depth prior to bifurcation is inversely related to the velocity of impact. These findings are considered in terms of the impact stresses, bending stresses and internally reflected stress waves. It is concluded that the bifurcation phenomena is confined to thin plates because the contact time is sufficiently large. The 'mirror equation' describes the observations provided the stress value is corrected for depth below the contact surface.