CRACK FRONT STABILITY AND HACKLE FORMATION IN HIGH-VELOCITY GLASS FRACTURE

Optical and scanning electron microscopy of ''hackle streaks'' and interference microscopy on fracture surfaces showed that major velocity hackle features can be produced at substantially lower K values than that associated with the hackle boundary in glass fracture. A novel method was used to determine crack propagation velocity along the streaks. Hackle generation was associated with distortion along the crack front from prior fracture events or from stress pulses generated at other hackle features. Explanations are presented for changes in velocity hackle at mist and hackle boundaries and for the low experimental values of the terminal velocity for fracture in glass.