The Relaxation of Stress in Fused Silica. Part I. Experimental
JSGT 1955 V39 T061-T082
It is shown that the relaxation of stress in fused silica can be expressed by the equation: S/S0=f(t). This equation is a mathematical expression of the experimental fact that if the material is uniform in composition and previous heat treatment, and after a given time interval the stress at a point has fallen to half its value at that point at the beginning of that time-interval, the stress at any other point will also have halved its value in the same time-interval. For a Maxwellian body S/S0=exp(–Gt/η) where G=rigidity modulus, η=coefficient of viscosity. It is shown that vitreous silica does not behave as a Maxwellian body in this respect for two reasons: 1. If the material is not in configurational equilibrium, η changes with time. 2. Even when in configurational equilibrium, the exponential decay of stress is not found, and this is attributed to the effect of "delayed elasticity" or " visco-elasticity."
J. O. Isard & R. W. Douglas
