Frozen Strains in Glass Fibres

JSGT 1955 V39 T134-T144

Two glasses, a borosilicate ("Pyrex") and a soda-lime, have been studied. Series of fibres were prepared, each fibre being drawn under a constant axial load which was retained during cooling. Determinations of dimension changes as a result of heat treatment were made. Changes in length were determined by direct observations, while changes in radius were calculated from observed changes in density. The results show that, superimposed on the uniform compacting usually associated with the heat treatment of chilled glass, there is a relaxation of a "frozen" elastic deformation which is proportional to the stress applied, this stress being calculated from the load and the diameter of the fibre after drawing. Each glass was thus found to exhibit a modulus, analogous to Young's Modulus, being the ratio of stress to "frozen" longitudinal strain, and a ratio: of "frozen" lateral strain to "frozen" longitudinal strain, analogous to Poisson's Ratio. In addition, the fibres as drawn show optical activity proportional to the stress applied in drawing the fibre. It is therefore concluded that glass fibres as drawn cannot be isotropic, since some stress must be applied in drawing, and consequently an elastic deformation "frozen in." A short discussion of the significance of the results in relation to theories of glass structure is given. An elementary mathematical treatment, in which it is assumed that the glass, when being drawn into fibre form, behaves as a number of elastic elements and a number of fluid elements, gives a linear equation connecting the observed changes in length with the applied stress.

J. F. Stirling