Spiral Cracks in Glass Tubing

Abstract

DR. J. J. HOPFIELD'S description of spiral cracks in glass capillary tubing¹ reminds us that similar cracks were observed some ten years ago with thinwalled tubing, during the development of flashed sodium-resistant glasses. The tubing in question was a conventional soda-glass, about 5 mm. in external diameter and rather less than 1 mm. in wall thickness, flashed internally with a thin layer of rather weak sodium-resistant glass. Specimens of such tubing in which the expansions of the two glasses were not sufficiently closely matched developed, soon after drawing, spiral cracks. The pitch of the crack increased towards the end of the stick of tubing, as in the specimen described by Dr. Hopfield, and tended to become infinite. In addition, the slope of the spiral varied throughout each turn in a regular manner. Cracks of the type described were only found in tubing having very high strain between the flashing and the base glass, the circumferential tension in the flashing glass corresponding to a stress of about 200 kgm. per sq. cm., a figure of the same order as the average breaking stress of the glass.