Indentation Strength of Glass

The tensile strength of plate and sheet glasses was investigated by means of indentation experiments with small spherical indenters. In agreement with earlier experiments by Powell and Preston, tensile strengths were observed about 10 to 100 times higher than those found in tensile or bending tests, the highest values being about one‐half the greatest strength of thin fibers or freshly drawn rods. It did not seem that any further reduction of the diameter of the indenting sphere below 1 mm. would lead to a significant increase of the highest strength values. The highest values of the indentation strength of freshly cleaved surfaces were not above those of ordinary surfaces, although the average was considerably higher. This indicates that, unless the use of the Hertz stress distribution involves a significant inaccuracy, the upper limit of about 300,000 lb. per sq. in. of the tensile stress may represent the order of magnitude of the tensile strength of bulk glass in the absence of surface defects. This value is within a factor of two to five of the probable molecular tensile cohesion of a glass of maximum homogeneity. Fracture stress histograms and maximum endured stress histograms are given for a variety of flat glasses.