Machining damage cracks: How to find and characterize them by fractography

Machining cracks may control strength and performance of finished ceramics. Machining cracks previously had been thought 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 comprehensive study with over 400 ground rods and rectangular bars was conducted on one commercial ceramic to study the effects of various machining conditions. Crack size and shape strongly depended 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.