Mechanism of Fracture of Glass and Similar Brittle Solids

Abstract

A theory is proposed which connects the stress, f, required to break a brittle material in simple tension, with its duration of application, t. The slow process preceding fracture is shown to be the orientation of the atomic network contained in an elementary prism of length r=λ0E/f, where E is Young's modulus and λ0 is the critical elongation required for fracture. The rate-controlling factor is the activation energy, Eα/f, for the orientation or rearrangement of the atomic network under the stress, f. Moisture on glass, and moisture plus oxygen on certain metals, are important catalytic or fatigue-promoting factors because they reduce the unit activation energy, α. The theory leads to the equations t=(1/k0)eEα/fkTand logt=−logk0+(Eα/2.3kT)/f,where t is the time for fracture (duration of the stress), k is the Boltzmann constant, T the absolute temperature, and α and k0 are experimentally determined constants. The logarithmic expression has the same form as the Glathart-Preston [J. App. Phys. 17, 189 (1946)] empirical relation logt= −a/m+1/fm, which, in the case of glass, appears to be valid over a time factor of 107. The theory shows why a solid object does not have a single characteristic breaking strength, and how it adjusts its fracture mechanism to whatever stress is applied. Quantitative tests of the theory are made, using fracture data on various glasses and on one glass at various temperatures. Applicability of the theory to certain aspects of fatigue of metals under stress-corrosion conditions, as well as to failure by fracture of the more rigid organic plastics, is indicated. An equation, Eλ0=2γ, is proposed which connects Young's modulus and the critical fracture distance with the surface tension, γ, of the solid. Some examples are given.