Fracture behavior of fused quartz with laser-induced internal flaws
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Pulsed-laser light was used to generate internal flaws in fused quartz. The size of the flaw produced was proportional to the amount of laser pulse energy above a threshold value of 2.5 mJ. Specimens of different flaw sizes were tested at room temperature under four-point bending. The bending strength decreased as the flaw size increased, and a Griffith relationship was established between the lateral flaw size and the critical stress at the flaw tip. Characteristic demarcation lines were observed on the fracture surfaces of specimens with a flaw size greater than 0.25 mm. It is suggested that the formation of the demarcation line is caused by a discontinuous change of stress intensity in the dynamic process of the crack propagation.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
In brittle ceramics, flaws resulting from material processing, surface finishing, or handling determine a material's strength. At the present time, the indentation method proposed by Lawn and Wilshaw1 is widely used for evaluating the fracture properties of brittle solids. The well-defined indentation crack system and its associated theoretical formulations permit highly reproducible measurements of toughness with simplicity and economy, which have accelerated the recent progress in hightoughness ceramics.2 However, it is also recognized that environmentally assisted crack growth is a very serious problem for ceramics. In fact, for a given material, the value of the critical stress intensity factor is a function of the environment, loading rate, and test geometry.3 For example, water can increase the rate of crack growth in glass by several orders of magnitude,4 and studies based on surface cracks inevitably involve the effect of the environment. The environmental influence can be eliminated by studying the internal cracks produced by laser irradiation,5'6 as illustrated in a previous study on thermal healing of internal cracks in LiF.7'8
Optical-grade fused quartz (OZ), supplied by the Nippon Silica Glass USA, Inc., was used in this study. The material contains approximately 150 ppm hydroxyl (OH), 50 ppm aluminum, and other impurities of less than 13 ppm. The quartz plate was cut to rectangular and trapezoidal shapes. The rectangular specimens had a size of either 76.2 by 12.7 by 6.3 mm (3 by 0.5 by 0.25 in.) or 44.5 by 12.7 by 6.3 mm (1.75 by 0.5 by 0.25 in.). The trapezoidal specimens were cut from the longer rectangular specimens perpendicular to the 76.2 X 6.3 mm surface and at an angle of 22° from the longer edge at the corner. The four side surfaces were mechanically polished using 6 /mm diamond paste followed by 1 fim A12O3. The as-received specimens were not annealed because of possible devitrification.
In the present paper, we are exploring a new approach to fracture study based on laser-induced internal flaws for the system of fused quartz. The first phase in this investigation is to establish a quantitative relationship between the laser energy and the flaw size so that the latter is controllable. Once the flaw size is under c
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