Strength/crack-size relationship for Knoop-indented bending specimens and its application to silicon nitride ceramics

With a detailed theoretical analysis, a linear relationship is predicted to exist between the fracture strength, sigma(f), and the inverse square roof of flaw depth, (1/a)(1/2), of Knoop-indented bending specimens. 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 slope term. These predictions are then verified by a variety of tests employed with a pressureless sintered silicon nitride. Variations of the residual stresses in Knoop-indented specimens with annealing temperature are also explored. It is shown that annealing may not be an effective method for eliminating the residual stresses in Knoop-indented bending specimens. (C) 1998 Elsevier Science Limited. All rights reserved.