Abstract
In order to clarify whether our theoretically derived equation of σm=ΨKicSmf1/2(σm; three-points bending fracture strength, Ψ shape factor, KIC; fracture toughness, Smf; total macroscopic area of fracture surface) can be applied to brittle materials having considerably lower KIC and σm than those of WC-10mass%Co cemented carbide, etc., the relation among σm, KIC and Smf was investigated for Mn-Zn ferrite (KIC; 1.3MPa.m1/2, the mean value of σm; 0.18GPa) as an example. In addition, the applicability of equation of σo=σd{1+2(a/ρ)1/2} or σ0-1=σ0-1+2σ0-1ρ-1/2 a1/2, i.e., the applicability of the estimation method of attainable strength (σa; σd or σm at 2a=mean grain size) to Mn-Zn ferrite was also investigated {σo; ideal bending fracture strength, i.e., the strength which is assumed to be obtained when the length (2a) of longer axis of fracture source becomes imaginarily zero, although the actual attainable minimum value of 2a is the mean grain size of the materials, σd; external stress which operated on the fracture source at the moment of fracture, ρ; effective curvature-radius of the edge of fracture source}.
The results obtained were as follows: (1) The linear correlation was found between σm and Smf1/2, in a similar way to the cemented carbide, etc., which indicates that the equation of σm=ΨKICSmf1/2is also applicable to the ferrite or brittle materials with considerably low KIC and σm. The ratio of the slopes of two regression lines of the ferrite and the cemented carbide coincided well with the ratio of the measured KIC values of both materials. This indicates that the Vvalue in equation ofσm=ΨKICSmf1/2 of this ferrite was nearly equal to that of the cemented carbide; the ψ values of both materials were about 15×103m-3/2. (2)The equation of σd-1=σ0-1+2σ0-1+2σ0-1ρ-1/2a1/2 also held for the ferrite. The σa of this material was estimated to be about 0.26GPa, which was about 44% higher than the measured mean value of σm (0.18GPa).
