On fracture mirror formation in glass and polycrystalline ceramics

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Abstract
Specimens of one soda-lime silicate glass and three polycrystalline ceramics were fractured at room temperature and fracture-mirror radii (r) measured at the onset of crack branching or ‘hackle’. Measured radii and fracture stresses (σf) for each material obeyed the relation σf r l/2 = A, where A is a material constant. The ratio A/K ic, where K ic is the critical stress-intensity factor of a material, was, however, independent of material type. A criterion based on energy balance is presented to show that the hackles, which bound the fracture mirror, form at a fracture velocity approximately equal to one-third the elastic wave velocity in a brittle material.

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