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Fracture surfaces of bars of glass and ceramics that failed after time under constant load were compared with those generated in rapid failure experiments. For each material, the relationship between the fracture stress, σ, and the fracture mirror radii, rj (σrj1/2 = a constant), was the same for rapid and delayed failure. The ratio of the mirror radius to the critical flaw size for delayed failure is a constant equal to that found previously for rapid failure. However, the ratio of the fractured mirror radius to the initial flaw size increased with increasing time under load due to the growth of the fracture-initiating flaw from the original size to the “critical” size before catastrophic failure. Slow crack growth in several polycrystalline ceramics occurred primarily by intergranular fracture, whereas primarily transgranular fracture occurred during rapid crack propagation. Thus, the boundary between subcritical and catastrophic flaw growth can be quite distinct. The fracture mechanics analysis presented in this paper relates the time under load to the ratio of the mirror and the initial flaw dimensions. This analysis gives good agreement with most experimental observations.
fractography, quantitative fractography, delayed failure, fracture (materials), fracture mechanics, ceramics, brittle materials, mechanical properties, materials, materials science
Member of technical staff, Sandia National Laboratories, Albuquerque, N. Mex.
Ceramic Engineer, National Bureau of Standards, Washington, D.C.,