TABLE OF CONTENTS Glasses are brittle materials. As a result, their fracture behavior is usually determined by environmental factors and not by the inherent strength of the bonds forming the vitreous network. The fracture strength of glasses varies with prior surface treatment, chemical environment, and the method used to measure the strength. As brittle materials, glasses are also quite susceptible to failure due to thermal shock.
Other mechanical properties of glasses are inherent to the material. The elastic modulus, E, is determined by the individual bonds in the material and by the structure of the network. The hardness of glasses is a function of the strength of individual bonds and the density of packing of the atoms in the structure.
As classic brittle materials, glasses exhibit nearly perfect Hookian behavior on application of a stress. The ratio of the strain, ?, resulting from application of a stress, ?, is a constant which is known as the elastic modulus, or Young's modulus, E, which is defined by the expression:
| (9.1) |  |
If a tensile stress is applied to a specimen in the direction of the x-axis, the specimen will elongate in that direction. This elongation will be accompanied by contraction in the y and z directions. The ratio of the transverse strain to the axial strain is called Poisson's ratio. Poisson's ratio for oxide glasses generally lies between 0.2 and 0.3, although the value for vitreous silica is only 0.17. The shear modulus, G,...
