Edge chip testing of ceramics

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(Credit: G.D. Quinn.) Figure 1. Edge chipping. An indenter applies force P at a distance d away from the edge until a flake pops off. The photo shows chips in glass. Eof fracture for ceramics, glasses,dge chipping is a common mode and lithic materials. Edge chipping is a use- ful technique for shaping lithic (“of the nature of or relating to stone,” New Oxford American Dictionary) materials into cutting tools and, indeed, was a key manufacturing Edge chip testing spear tips. However, it is a nuisance andinnovation for prehistoric cultures making of ceramics glasses. For instance, one report mentionsoften a problem for technical ceramics and an incident wherein 49 silicon nitride cam roller follower parts fell from a measurement By George D. Quinn bench and 40—more than 80 percent—sus- tained chip damage.1 McCormick and Almond2–4 started quantitatively assessing Prehistoric engineers chipped stone edges to make spear edge-chipping resistance of technical ceramics in 1986 at the tips, but modern engineers need to avoid edge chipping. National Physical Laboratory outside London. They initially evaluated carbide cutting-tool materials, but also looked at New test methods provide quantifiable measurement of polycrystalline alumina, sapphire, zirconia, ceramics, and edge chipping of ceramics. crown glass for comparison and to investigate the general applicability of their method. Other groups adopted their methodology and applied it to other materials. The edge-chipping test involves applying an increasing force near the edge of a specimen until a chip (or flake) forms, as illustrated in Figure 1. Usually, specimens are rect- angular blocks with 90° edges. However, the test provides a means for designers to experiment with various edge geom- etries during the design process. For example, McCormick,3 for example, experimented with edges other than 90°. Testing can be done with a dedicated edge-chipping machine that 24 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 1


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