The Growth of Indentation-Induced Microcracks in Glass

Abstract

The slow growth characteristics of indentation-induced microcracks in soda-lime-silica glass were investigated. Vickers indentations were made on glass specimens at loads of 4 and 7 kgf, respectively, in both air and water environments at room temperature, and the extension of median/radial and lateral cracks was monitored by optical microscopy over a long period of time after indentation. Fractographic features of the post-indentation slow crack growth were also examined on specimens subsequently broken from the indentation-induced flaw under bending. Although a theory suggests that a logarithmic plot of the median/radial crack size vs. the time after unloading should give a linear relation, the crack growth data in both air and water environments could not be represented by a single straight line over the entire range of time followed. In air environment, a characteristic slow-down in growth rate of the median/radial crack was observed at about 0.5 to 1 hours after unloading. This may be caused by a relaxation of the effective residual stresses resulting from an appreciable extension of lateral crack. The post-indentation slow crack growth behaviour in moist environments was interpreted in terms of median/radial-lateral crack interaction effects. A possible use of the post-indentation crack growth data for fatigue parameter evaluation of glass was also discussed.