A Study of the Breaking Strength of Glass
JSGT 1934 V18 T225-T251
Determinations of the modulus of rupture have been made on 2300 specimens cut with a diamond under constant load, from 26 oz drawn sheet glass of composition: SiO2 72·22, Al2O3 + Fe2O3 1·92, CaO 8·84, MgO 2·89, Na2O 13·96%. The dimensions of the specimens were: length 10 cm (effective length between knife edges 7·6 cm) width 0·6–1·5 mm, and thickness 0·25–0·31. The modulus of rupture varied according to a variety of conditions, in particular with (I) the position of the diamond-cut edge according as it was in tension or in compression, (2) the state of annealing or re-annealing, (3) the interval of time after re-annealing, a steady state being attained after several days. (For practical purposes three day interval was adopted.) (4) the width of the specimen, (5) the condition of the edges, (a) as cut with the diamond, (b) sealed by edge-melting. (c) ground and mechanically polished, (d) ground and mechanically polished, followed by fire-finishing. For specimens tested 3 days after re-annealing, with the diamond-cut edge in tension, the modulus of rupture varied from 613 kg/cm2 for a width 0·6 cm to 486 for width 1·5 cm. Specimens with mechanically ground and polished edges had a modulus of 572 kg/cm2 for widths 0·89–0·93 cm. Other values were: with diamond-cut edge in compression, 903 kg/cm2 for widths 0·89–1·14 cm; diamond-cut edge fire-finished, 901 kg/cm2 for widths 0·83–1·09 cm and 1023 kg/cm2 for widths 0·72–0·93 cm; cut edges mechanically ground and polished and then fire-finished, 940 kg/cm2 for width 1·02 cm and 1123 kg/cm2 for width 0·8 cm. Specimens tested with the diamond-cut edge in compression gave values of the same order as those with fire-finished edges, and much greater than with cut edges in tension or edges mechanically ground and polished. On the other hand the two last-named types always gave the more consistent results. Three types of fracture, straight, V, and 1/2V shape, were found, dependent on width and on edge condition. Straight fractures were regularly obtained with specimens less than 0·9 cm wide, with cut edges or ground and polished edges. Specimens with fire-finished edges always gave a large proportion of V and 1/2V fractures whatever the width tested.
A. J. Holland & W. E. S. Turner
