Section-and-Etch Study of Hertzian Fracture Mechanics

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Abstract
A section‐and‐etch technique is used to study the mechanics of Hertzian cone‐crack growth in glass. With a suitable choice of test environment the cone cracks propagate at a convenient rate through successive phases of stability at constant indenter load. Systematic measurements of the etched‐crack lengths as a function of indentation time permit a detailed description of the fracture mechanics. Within a certain range of indenter load, the cone crack is observed to grow as a shallow surface ring to a critical depth prior to full development. This directly confirms a salient feature of the energy balance theory of Hertzian fracture outlined in earlier papers. The current status of the long‐standing Auerbach law, which relates the critical fracture load linearly to the indenter radius, is discussed in the light of the present evidence.