Strategies for fracture toughness, strength and reliability optimisation of ceramic-ceramic laminates Dedicated to Prof. F. D. Fischer on the occasion of his 70 th birthday Layered ceramics are, compared to conventional monolithic ceramics, a good choice for highly loaded structural applications having improved fracture toughness, strength and mechanical reliability. The use of tailored residual compressive stresses in the layers is the key parameter to adjust these properties. In this work two types of ceramics are analysed which have external (ECS-laminates) or internal (ICS-laminates) compressive stresses. The most important factors having influence on the strength and toughness of these laminates are discussed. Clear recommendations on the proper selection of a suitable mismatch strain, volume ratio of the layer materials and thickness and distribution of individual layers are given, either to achieve a high toughness and/or a high lower limit (threshold) for strength.

For wedge splitting test specimens, the stress and displacement fields both in the vicinity and also in larger distance from the crack tip are investigated by means of numerical methods. Several variants of boundary conditions were modeled. The stress intensity factor K, T-stress and even higher-order terms of William series were determined and subsequently utilized for analytical approximation of the stress field. A good fit between the analytical and numerical solution in dependence on the distance from the crack tip was shown, compared and discussed. Presented approach is considered as suitable for estimation of the fracture process zone extent in silicate composite materials.

A description of stress and displacement fields by means of the Williams power series using also higher-order terms is the focus of this paper. Coefficients of this series are determined via the over-deterministic method from the results of conventional finite element (FE) analysis. A study is conducted into the selection of the FE node set whose results are processed in this regression technique. Coefficients up to the twelfth term were determined with high precision. The effect of the position of the FE node set on the accuracy of the values of the higher-order term coefficients is reported.

Using higher-order terms of the Williams expansion is necessary for assessment of fracture behavior of quasi-brittle materials. Multi-parameter fracture mechanics enables more accurate determination of the stress/displacement field even in a larger distance from the crack tip, thus the extended zone with non-elastic behavior typical for this kind of material can be well described. The so-called over-deterministic method (ODM) seems to be a suitable tool for the higher-order terms coefficients calculation, but its utilization exhibits some limitations. Therefore, extensive analyses have been performed in order to summarize recommendations regarding the mesh sensitivity, boundary conditions influence, etc. List of pieces of advice and author’s experiences presented in the end of this work should contribute to more accurate and effective utilization of the ODM.