The Breaking Strength of Glass. The Effect of Flaws and Scratches
JSGT 1936 V20 T279-T302
The influence of various factors on the bending strength of strips of flat drawn sheet glass, 0·26 to 0·285 cm thick, 10 cm long and 0·8 cm wide was investigated. A new diamond produced specimens which, when tested with the cut edges in tension, had a breaking strength of 570 kg/cm2; for the worn diamond the value was 673 kg/cm2. The nature of the cementing material when preparing, in blocks, specimens with sides ground and polished, was quite marked, values of the breaking strength 580, 877, and 938 kg/cm2 being obtained when Canada balsam, shellac and paraffin wax were used in the different tests made. Smoothing the edges with emery and then polishing with either pumice on wood or rouge on felt, reduced in general the breaking strength. When the edges of specimens were stoned and subsequently pumice-polished, however, a value of the breaking strength 1056 kg.fcm.2, similar to that of specimens having their sides and edges fire-finished was obtained. The depth of penetration of the residual flaws in the edges of specimens was correlated with the values of the breaking strength. Two straight line graphs were obtained, the one corresponding to those specimens with sides polished only, the other to those in which both sides and edges were polished. The straight lines converged to a value 1050 kg/cm2 similar to that for fire-finished specimens free from flaws. The effect of scratches in reducing the strength of glass was investigated, using as scratching tools a ground and polished conical diamond 90°, and standard type HMV half-tone gramophone needles. A diamond scratch of width 0·005 mm. reduced the bending strength from 877 to 505 kg/cm2, and one of 0·0007 cm width, which was invisible until etched, from 877 to 855 kg/cm2. Consistent results were only obtained when using gramophone needles with loads less than 100 grams, the scratches being then invisible until etched with dilute HF. A scratch of width 0·0007 mm was not effective in reducing the strength. The evidence suggested that a scratch of width 0·001 mm was needed to cause an appreciable reduction in the breaking strength.
A. J. Holland & W. E. S. Turner
