Machining damage Cracks: How to Find and Characterize Them by Fractography

G. D. Quinn, L. K. Ives, and S. Jahanmir, “Machining damage Cracks: How to Find and Characterize Them by Fractography,” Ceramic Eng. and Science Proceeding, 27th International Cocoa Beach Conference on Advanced Ceramics and Composites, 24 (2003) 383-394.

Subsurface machining cracks may control strength and performance of finished ceramics. Previously, machining cracks had been though to be difficult or impossible to detect and characterize, especially in toughened ceramics with interlocking grain microstructures that create rough fracture surfaces. New fractographic examinations have shown that machining damage leaves telltale markings on fracture surfaces that may be easily detected by fractographers using simple fractographic techniques. A remarkable finding is that machining damage may be fractographically identified as a probable cause of fracture even if the fracture origin itself is obscured or lost. The telltale signs of machining damage are presented and simple refinements to ordinary fractographic techniques are reviewed. These techniques were applied to several toughened silicon nitrides and other ceramics. A comprehensive study with over 400 ground rods and rectangular bars was conducted on one commercial ceramic to study the effects of various machining conditions. Machining crack size and shape strongly depend upon the machining conditions, and in particular, on the grinding wheel grit size. In some instances, specimens broke from the material’s inherent flaws and machining damage had no effect on strength.

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