SUBCRITICAL GROWTH OF INDENTATION MEDIAN CRACKS IN SODA-LIME-SILICA GLASS

The influence of load and dwell time on the subcritical growth of inde ntation cracks was investigated, The behavior of soda-lime-silica glas s in both active and inert environments was considered, Vickers indent ations were obtained using loads ranging from 1.96 to 39.2 N and the d well time at maximum load was varied between 5 s and 30 min, Indented samples were broken and the fracture surfaces observed using optical m icroscopy, Anomalous crack shapes were obtained in air and water for l ong dwell times, particularly at higher indentation loads, The cracks were markedly circular in shape and substantially deeper than conventi onal median/radial indentation cracks, evidencing a time-dependent gro wth, Comparison of the measured crack depth with a theoretical analysi s demonstrated that the appearance of the ''circular'' cracks was rela ted to the subcritical growth of median cracks.