Effect of environment and polyaxial stress states on subcritical crack growth in glass ceramics.

Abstract

In order to examine the effect of environment and polyaxial stress on the subcritical crack growth parameters of glass ceramics, constant stressing rate tests were conducted by four-point bending of plates (uniaxial stressing) and by Concentric ring loading of disks (equibiaxial stressing) under different environments of vacuum, air, and ion-exchanged water. The crack growth parameters were determined from a straight line on a logarithmic plot of fracture stress as a function of the stressing rate. In the air environment the parameter n was obviously lower for the equibiaxial tension than for the uniaxial tension, whereas in the water environment no difference was observed between the two modes of stressing. In addition, the n values in water were higher than those in air for both modes of stressing (especially for equibiaxial stressing mode). The higher values of n in water can be attributed to the fact that the resistance to fracture was increased by a rise in the extent of microcracking in the microstructure at the crack tip as the crack grew longer at the low stressing rate. The predictions of lifetime were made for this glass ceramic using the crack-growth parameters obtained by the stressing rate technique.