DYNAMIC FRACTURE IN ZONE-TEMPERED GLASSES OBSERVED BY HIGH-SPEED PHOTOELASTIC COLOR-PHOTOGRAPHY

High speed photoelastic color photography was used to observe the details of the propagation procedures of fast cracks in zone tempered glasses. Zone tempered soda lime glass specimen plates were used in the study. The tempered zone had two internal zones, zone I and zone II, which lay alternatively. The degree of temper was defined by compressive stresses on the surface which were determined by a surface refractmeter. Compressive stresses in zone I were slightly larger than those in zone II. Cracks propagating in zone II toward zone I tended to move a neutral line between the two zones toward zone I. The same effect was observed for cracks propagating in zone I. Movement of the neutral line was less in the latter case. Cracks propagated in zone I upward and downward. When cracks met, they were connected smoothly with each other.