Abstract
In order to elucidate the notch width or notch root radius dependence of the critical stress intensity factor evaluated by SENB (Single Edge Notched Beam) specimens, 4-point bending tests were carried out on hot-pressed SiC (with 2wt% AlN) and sintered Si3N4. The Experimental results showed that the critical stress intensity factor Kc determined with SENB specimens coincides with a valid fracture toughness KIc value only when notch root radius ρ is smaller than about 10μm. For larger root radii, the variation of Kc with ρ was observed, and this can not be attributed only to the stress concentration effect of notch.
In the present study, by introducing a micro-damage concept a method was proposed for evaluating KIc from Kc values given by SENB specimens having rather blunt notch.
On the basis of the published strength data for SiC, the relation between strength and microstructure was examined. It was assumed that a micro-damage existed at the machine finished surface, and its size should be the sum of the machined damage depth hmax and the maximum grain diameter dmax. Here, hmax can be estimated from the average grit size of the diamond disk used. An analysis was made of the dependence of Kc on ρ by considering the micro-damage at the notch root, and a theoretical expression was formulated. The experimentally observed Kc-ρ relation agreed quite well with the theoretical prediction.
