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Cone fractures have been produced in fused silica, by indenters ranging from fraction one-sixtyfourths inch to ¼ inch diameter. In some respects these differ from cone cracks in silicate glass. Those in silica have a significantly smaller semi-angle, 65.5 as opposed to 68.5° in glass. Whereas cracks in glass grow indefinitely under a constant load, and remain visibly open when the load is removed, those in quartz cease to grow about ten minutes after each increment of the load, and cannot be seen when the specimen is unloaded.
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J.J. Benbow (1960) studied this question.
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