Effect of crosshead speed on the fracture toughness of soda-lime glass, Al2O3 and Si3N4 ceramics determined by the surface crack in flexure (SCF) method

Abstract

The fracture toughness of soda-lime glass, Al2O3 and Si3N4 specimens was measured by the surface crack in flexure method. For the soda-lime glass specimens, the fracture toughness was calculated from the initial crack size and flexure strength, and the value increased with increasing crosshead speed. This trend seems to be related to the difficulty in determining the critical crack size at fracture, since slow crack growth occurs during bending test. For the Al2O3 specimens, a halo region (stable crack growth region) was formed around the initial precrack during bending test. The halo size increased and the resultant flexure strength decreased with decreasing in the crosshead speed. The halo region, however, could not be observed in the Si3N4 specimens. Despite of the difference in the appearance of halo region, the fracture toughness of the Al2O3 and Si3N4 specimens was constant irrespective of the crosshead speed when the values were calculated with the critical crack sizes at fracture (halo incorporated crack sizes). The constant fracture toughness with the crosshead speed could be explained by the relation between the changes of halo size (thus critical crack size at fracture) and resultant flexure strength.