圧子押込み時のぜい性材料の延性/ぜい性遷移

抄録

The ductile/brittle transition condition in an indentation of brittle materials is derived to utilize the prediction of the ductile mode abrading conditions. By modeling the action of an abrasive as a semi? spherical indentation, the ductile/brittle transition condition is represented by a critical radius (a_c) of a spherical indenter: at a larger radius than that, a penny-shaped pre-existing crack extends as a median crack in the material.
The stress intensity factors (K_IC's) of the penny-shaped pre-existing cracks which are located at various depths with radii (c's) under the indentation are analyzed by using the stress fields derived by Chiang et al. The crack length-dependent critical stress intensity factor is applied to the evaluation of crack extension.
The critical radii (a_c's) of typical structural ceramics such as silicon nitride, silicon carbide, and alumina are deduced to be from 0.5-2.5μm. Although these materials have wide ranged characteristic constants such as Young's modulus, hardness (Hv), fracture toughness (K_IC), Poisson's ratio, and mean grain diameter (d_g). a_c is given by the following rather simple equation under the assumption that the size of the pre-existing crack is the same as that of the mean crystalline grain.
a_c/d_g=0.075·10^{3.7K_IC/(Hv√d_g)}
The critical radius a_c, which is experimentally evaluated in the view of preventing the bending strength deterioration of indented Si₃N₄ specimens, is fairly coincident with the analytical one.