Pulsed-laser light was used to generate internal flaws in fused quartz
. The size of the flaw produced was proportional to the amount of lase
r pulse energy above a threshold value of 2.5 mJ. Specimens of differe
nt flaw sizes were tested at room temperature under four-point bending
. The bending strength decreased as the flaw size increased, and a Gri
ffith relationship was established between the lateral flaw size and t
he critical stress at the flaw tip. Characteristic demarcation lines w
ere observed on the fracture surfaces of specimens with a flaw size gr
eater than 0.25 mm. It is suggested that the formation of the demarcat
ion line is caused by a discontinuous change of stress intensity in th
e dynamic process of the crack propagation.