Fracture toughness testing of brittle materials

Abstract

Research within the past two decades has achieved a dramatic upsurge of improvements in the mechanical properties of engineering ceramics. These improvements have often been made through increased toughness by novel toughening mechanisms such as the stress induced phase transformation, microcracking, fibre/whisker crack bridging, etc. These may occur not only in the frontal process zone ahead of a sharp crack, but also in the following crack wake region. The consequences of these microfracture processes and mechanisms in the wake and the crack bridging regions are significant, for they result in very complex fracture processes and they create many critical issues and difficulties in the experimental determination of the fracture resistance of brittle materials. The lack of a physical basis for a fracture criterion in the present fracture mechanics framework adds further confusion to fracture mechanics studies. This paper is a state of the art review of the application of fracture mechanics to brittle ceramics and ceramic composites for the determination of the fracture resistance. The details of various experimental techniques are addressed. Included are a wide variety of specimen geometries, as well as crack dimensions from large macrocracks or macronotches to indentation induced microflaws. The effects of R-curve behaviour and various toughening processes on the fracture toughness have been carefully considered. It is emphasised that ‘fracture physics considerations’ are very important for understanding the influence of the test conditions on the experimentally determined fracture resistance values of brittle materials with various microscopic toughening processes and mechanisms.