STRENGTH CRACK-SIZE RELATIONSHIP FOR KNOOP-INDENTED BENDING SPECIMENSAND ITS APPLICATION TO SILICON-NITRIDE CERAMICS/

With a detailed theoretical analysis, a linear relationship is predict ed to exist between the fracture strength, sigma(f), and the inverse s quare roof of flaw depth, (1/a)(1/2), of Knoop-indented bending specim ens. The effects of residual stresses can be described with a strength revisionary term of the fitted sigma(f)similar to(1/a)(1/2) equation, while the fracture toughness can be determined directly from the slop e term. These predictions are then verified by a variety of tests empl oyed with a pressureless sintered silicon nitride. Variations of the r esidual stresses in Knoop-indented specimens with annealing temperatur e are also explored. It is shown that annealing may not be an effectiv e method for eliminating the residual stresses in Knoop-indented bendi ng specimens. (C) 1998 Elsevier Science Limited. All rights reserved.