Determination of the crack-tip toughness in silicon nitride ceramics

Excerpt

Fracture in most ceramic starts from natural cracks with sizes a₀ in the order of a few 10 μm introduced during the processing of these materials. A crack in a component starts to propagate when the externally applied stress intensity factor K_appl exceeds the so-called crack-tip toughness K_I0. In the absence of an increasing crack growth resistance, the strength σ_c is exclusively governed by K_I0 according to Many ceramic materials exhibit the effect of an increasing crack growth resistance during crack extension Δa, i.e. a rising R-curve with K_R = f(Δa) (Fig. 1a). In cases of a moderately rising R-curve, Eq. 1 remains valid. Only in cases of strongly rising R-curves, the strength is affected. For materials with a sufficiently steep R-curve, stable crack extension follows under increasing load. Failure of the component then occurs when the so-called tangent condition is fulfilled, i.e. when the slope of the applied stress intensity factor K_appl(a) and the R-curve K_IR(a) are identical The stress intensity factor at which (2) is fulfilled is called “fracture toughness” K_Ic, which is introduced in Fig. 1a as a triangle.

…