Subcritical crack growth and threshold in borosilicate glass

The crack-growth behaviour of borosilicate glass was investigated by use of lifetime measurements on Knoop-damaged specimens. The threshold behaviour in particular was observed, and the influence of residual wedging stresses and preloading below the threshold value were studied in detail. A threshold ofK_lth = 0.38 MPam^1/2 was found. The residual stress intensity factor caused by the Knoop indentations was found to beK_lres,0 0.1 MPam^1/2. In a static test performed below the threshold, the threshold value was increased by K_lth 0.15MPam^1/2.