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-tempera ture flexure strength (sigma) of various ceramic materials are reviewe d from the standpoint of their grain-size (G) dependence and the corre lation of this dependence with sigma-G-1/2 behavior. The correlation s hows that the reduction in strength from grinding specimens perpendicu lar versus parallel to their tensile axes is greatest at fine G, about zero at intermediate G, and then progressively increases again for la rger G and for single-crystal specimens (G = infinity). The lower sigm a due to machining perpendicular to versus parallel to the tensile axi s is due primarily to crack shape, not depth, i.e., elongated and appr oximately half-penny cracks. Thus, the reduction in sigma difference i mplies 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.