抄録
A new device for impact testing suitable for evaluating ceramic strength was developed and was used in the experiments to develope a unified method for estimating both fast fracture and fatigue strengths. The fracture strength was measured by making 3 point and 4 point impact bending tests for three kinds of sintered Si3N4 specimens. Weibull analysis was performed for all the strength data with our original idea by considering the both effects of applied stress pattern and specimen size. With introduction of the effective hold time teff and the effective volume Veff, the impact strength obtained by each test was expressed commonly as the delayed fracture strength by using the following formula.
σf(Nf teff)nf=Dfm√ln(1-Pf)-1/Veff
where nf is the strength decrement index, Df the strength constant and m the Weibull Modulus.
This formula is applicable to the probabilistic estimation of impact strength of ceramics.
On the other hand, the strain rate-dependence of impact strength could be estimated by the following formula derived from the above formula.
σf=ε1/(1+n){(1+n)EDfn}1/(1+n)m√ln(1-Pf)-1/Veff
where n is the crack growth rate index (1/nf) and E Youngs modulus.
