Fracture of fused silica with 351 nm laser-generated surface cracks
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Fracture of fused silica with 351 nm laser-generated surface cracks F. Dahmani and J. C. Lambropoulos Laboratory for Laser Energetics, and Department of Mechanical Engineering, University of Rochester, 250 East River Road, Rochester, New York 14623-1299
A. W. Schmid and S. Papernov Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623-1299
S. J. Burns Department of Mechanical Engineering, University of Rochester, 250 East River Road, Rochester, New York 14623-1299 (Received 16 January 1998; accepted 5 June 1998)
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 Fexit/th 10 Jycm2 were irradiated at a constant fluence of FL 1.8 3 Fexit/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.
I. INTRODUCTION
The rapid improvement in the development of reliable, high-intensity, high-repetition-rate ultraviolet (uv) lasers has been accelerated in recent years by applications in inertial confinement fusion (ICF) and materials processing.1 Beam-transport optics used in these lasers and in the uv-wavelength range in general are predominantly fused-silica lenses and flats.2,3 The response of optical components to laser irradiation provides information of central interest to some important phenomena such as machining damage and (surface-controlled) failure strength. This association is particularly strong in fused silica for which subsequent multipulse laser-irradiation fracture dictates the material degradation process. Laser-induced cracks are now used to investigate fracture processes and fatigue life in fused silica.4,5 The initial damage results in catastrophic crack growth under cyclic laser pulses once the crack size exceeds a critical value. Thus, the rate of crack growth with the number of laser pulses is an important property that can be measured. Under fatigue conditions, i.e., where strength tends to diminish as the duration of loading (here, the number J. Mater. Res., Vol. 14, No. 2, Feb 1999
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of laser pulses) is increased, the essential a
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