OBSERVATION OF FRACTURE FEATURES IN FUSED QUARTZ WITH LASER-INDUCED INTERNAL FLAWS

Pulsed-laser light was used to generate internal flaws in fused quartz . The size of the flaw was proportional to the laser pulse energy abov e a threshold value of 2.5 mJ. Specimens which fractured under four-po int bending, either from surface origins or from the internal flaws wi th a size less than 0.25 mm, exhibited the commonly observed fracture features of sequential mirror, mist, hackle and branching, and the tra nsition from the mirror to other features was gradual. However, in spe cimens with internal flaws of a size greater than 0.25 mm, a new ''dem arcation'' line (a smoothly curved boundary line between the mirror an d other fracture features) was observed. Its characteristic varied wit h the flaw size and the relative position along the line. It is sugges ted that the formation of the demarcation line is caused by the combin ation of the dynamic process of crack propagation and environmental ef fects.