Fracture of fused silica with 351 nm laser-generated surface cracks

Laser-induced-surface-flaw experiments on fused silica at 351 nm and 500 ps pulse duration are reported here. Specimens with surface flaws produced at a measured exit-surface damage threshold fluence of F-zxit/th = 10 J/cm(2) were irradiated at a constant fluence of F-L = 1.8 X F-exit/th by different numbers of laser pulses, N = 110 to 520, Micrograph observations show that (i) the produced cracks have a semielliptical shape and (ii) the material strength predictions based on the radial crack depth (normal to the surface) instead of the crack surface length (parallel to the surface) are in good agreement with measured strengths obtained using a four-point bending fixture. The underlying basis of conventional crack analysis is first examined critically and is argued to be deficient in the way the failure strength for the cracks is related to the characteristic parameters of crack geometry. In general, it is necessary to incorporate a residual term into the failure strength formulation. The crack depth and the failure strength are found to increase and decrease with the number of laser pulses, respectively.