CORRELATION OF MACHINING-GRAIN-SIZE EFFECTS ON TENSILE-STRENGTH WITH TENSILE STRENGTH-GRAIN-SIZE BEHAVIOR

Existing data on the effect of machining direction on the room-temperature flexure strength (sigma) of various ceramic materials are reviewed from the standpoint of their grain-size (G) dependence and the correlation of this dependence with sigma-G-1/2 behavior. The correlation shows that the reduction in strength from grinding specimens perpendicular versus parallel to their tensile axes is greatest at fine G, about zero at intermediate G, and then progressively increases again for larger G and for single-crystal specimens (G = infinity). The lower sigma due to machining perpendicular to versus parallel to the tensile axis is due primarily to crack shape, not depth, i.e., elongated and approximately half-penny cracks. Thus, the reduction in sigma difference implies a reduction in flaw elongation at intermediate G. This is shown to correspond with the sigma-G-1/2 behavior with larger G and finer G branches meeting when the flaw size is about equal to G. This result is supported by fractographic observations.