NOTCH EFFECT OF SURFACE COMPRESSION AND THE TOUGHENING OF GRADED Al_2O_3/TiC/Ni MATERIALS

摘要

The fracture behavior of graded Al_2O_3/TiC/Ni materials with a symmetric structure was investi- gated using single-edge notch-bend (SENB) specimens with surface compression. The fracture toughness of the graded materials was determined according to ASTM Standard E399. The results show that the effec- tive fracture toughness increases with an increase in notch depth in the compressive stress zone, and reaches the maximum of 39.2 MPa m~1/2 at the interface of compressive/tensile stress zones. Finite element analysis reveals that the surface compression will be intensified at the notch root once the specimen is edge-notched because of the stress concentration, and the degree of the compressive stress intensification increases with an increase in notch depth. The dependence of the effective fracture toughness of the graded materials on the notch depth shows a behavior similar to the R-curve that is usually associated with micro- structural toughening mechanisms. This toughening behavior is caused by the intensification of the com- pressive stress concentration with the increase of the notch depth. A theoretical analysis based on fracture mechanics verifies that the mechanical reliability of brittle ceramics can be improved effectively by tailoring and controlling the internal stresses.