FRACTURE-TOUGHNESS TESTING OF BRITTLE MATERIALS

Research within the past two decades has achieved a dramatic upsurge o f improvements in the mechanical properties of engineering ceramics. T hese improvements have often been made through increased toughness by novel toughening mechanisms such as the stress induced phase transform ation, microcracking, fibre/whisker crack bridging, etc. These may occ ur not only in the frontal process zone ahead of a sharp crack, but al so in the following crack wake region. The consequences of these micro fracture processes and mechanisms in the wake and the crack bridging r egions are significant, for they result in very complex fracture proce sses and they create many critical issues and difficulties in the expe rimental 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 mecha nics studies. This paper is a state of the art review of the applicati on of fracture mechanics to brittle ceramics and ceramic composites fo r 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 macrocrac ks or macronotches to indentation induced microflaws. The effects of R -curve behaviour and various toughening processes on the fracture toug hness have been carefully considered. It is emphasised that 'fracture physics considerations' are very important for understanding the influ ence of the test conditions on the experimentally determined fracture resistance values of brittle materials with various microscopic toughe ning processes and mechanisms.