Effect of size on strength in ceramic materials containing high densities of flaws

Lise Meitner Position M 587 Effect of size on strength in ceramic materials Chunsheng LU 09.10.2000 The tensile strength of ceramic materials is limited by flaws which are distributed in the bulk or at it s surface. It is assumed that fracture originates at the most severe ("the largest") flaw. Since in two geometrically identical specimens, the size of the "largest" flaw is, in general, different, the strengths of the two specimens also differ. Therefore, the tensile strength of ceramic materials is not given by a simple number, but by a distribution function. In a large specimen, the probability of finding a large flaw is much higher than in a small specimen, and consequently, the mean strength of large specimens is smaller than the mean strength of small specimens. In general these two features can be observed in ceramics and many other brittle materials, and this behaviour is commonly described by the Weibull distribution function. New results from the "Institute für Struktur- und Funktionskramik" at the Montanuniversität Leoben, however, showed a different behaviour: the strength of two electroceramic materials is still found to scatter (and the distribution of strength can be described using a Weibull distribution), but the size of the specimen has no influence on the (mean) strength. The latter cannot be explained on the basis of common Weibull statistics. In this project a model should be found to explain the observed behaviour. Up to know, two ideas exist: i) the behaviour is a consequence of a pronounced R-curve behaviour of the material causing some stable crack extension before fracture and, as a consequence, leading to a homogenisation of critical crack sizea, ii) it is a consequence of an interaction between the flaws, which causes a homogenisation of critical crack sizes, and an anti-shielding of small cracks and which also causes a homogenisation of critical crack sizes. For both types of models a critical assessment of the statistics of flaws and of the way how a flaw turns into a crack has to be made.