Comparison of fracture toughness testing by the single edge v-notch beam and the surface crack in flexure method on silicon nitride

Abstract

This work is intended to compare the SEVNB and the SCF methods for fracture toughness testing. Both of the test methods are applied to a silicon nitride. Both methods are standardized and both have some minor shortcomings with respect to practicability and match of the theoretical description of the test situation with the actual test situation. SEVNB is easy to perform, but uses a notch of known depth (+ small micro-cracks of unknown size in front of a notch) instead of a crack, SCF uses a presumably residual stress-free indentation crack which is sometimes difficult to measure, not always semi-elliptical, as assumed to apply the evaluation formula, and may be accompanied by lateral cracks. Both methods will be analysed with respect to possible experimental errors. Variations of the SCF method are applied in order to examine how pre-cracks can be introduced which can be detected on the fracture surface. The geometry of the SCF-pre-crack is varied by applying different loads and by removing different amounts of material. The geometry of the Knoop pre-cracks system is investigated. The results help to identify conditions under which the SCF method can be used successfully to measure the fracture toughness of silicon nitride ceramics.