Effect of negative T-stress on R-curves for Si3N4-ceramics

The main advantages of DCDC-specimens, completely stable crack extension and very high path stability due to the strongly negative Tstress term call for an application in R-curve determination. In this report, it is studied whether the T-stress affects the R-curve of silicon nitride. For this purpose, we selected three Si3N4-ceramics, which showed very flat crack resistance curves in test with edge-notched 4-point bending specimens. These materials were tested with DCDC-specimens. In contrast to the bending tests, the crack resistance decreased clearly. This effect can be understood as a consequence of the T-stress term.