Quasi-dynamic visualization of crack propagation and bridging evolution in Si₃N₄ and SiC ceramics by mechano-luminescence

The propagations of macro-scale cracks were visualized in Si ₃N₄ and SiC ceramics using a mechano-luminescence (ML) of SrAl₂O₄:Eu,Dy. The bridging zones behind the crack tip were also clearly detected in the crack path within realistic time frames. The ML made it possible to detect cracks propagating within a speed range of 200 m/s to 250 m/s, thereby realizing so-called quasi-dynamic R-curves. The magnitudes and shapes of the bridging stress distributions were found to change with the advancing cracks, continuing to change as the applied load changes, even after the cessation of crack propagation. The effective toughening then commenced and the applied stress intensity factors increased dramatically up to ̃120 MPa √m in Si₃N₄ and 150 MPa √m in SiC. The K _Tip values expected on the basis of the instantaneous bridging stress distributions obtained from ML observations deviated greatly from those measured using the conventional crack tip lengths; rather, the values support the results obtained by using bridging tips in quasi-dynamic crack propagations.