Lucie Šestáková

Strategies for fracture toughness, strength and reliability optimisation of ceramic-ceramic laminates

Abstract 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.

Fracture Mechanisms of Structural and Functional Multilayer Ceramic Structures

The fracture of mechanically loaded ceramics is a consequence of material critical defects located either within the bulk or at the surface, resulting from the processing and/or machining and handling procedures. The size and type of these defects determine the mechanical strength of the specimens, yielding a statistically variable strength and brittle fracture which limits their use for load-bearing applications. In recent years the attempt to design bio-inspired multilayer ceramics has been proposed as an alternative choice for the design of structural components with improved fracture toughness (e.g. through energy release mechanisms such as crack branching or crack deflection) and mechanical reliability (i.e. flaw tolerant materials). This approach could be extended to complex multilayer engineering components such as piezoelectric actuators or LTCCs (consisting of an interdigitated layered structure of ceramic layers and thin metal electrodes) in order to enhance their performance functionality as well as ensuring mechanical reliability. In this work the fracture mechanisms in several structural and functional multilayer components are investigated in order to understand the role of the microstructure and layered architecture (e.g. metal-ceramic or ceramic-ceramic) on their mechanical behaviour. Design guidelines based on experiments and theoretical approaches are given aiming to enhance the reliability of multilayer components.

Estimation of the Crack Propagation Direction of a Crack Touching the Interface between Two Elastic Materials

The objective of the paper is to investigate the direction of a further crack propagation from the interface between two elastic materials. The angle of crack propagation changes when the crack passes the interface. The suggested procedure makes it possible to estimate an angle of propagation under which the crack will propagate into the second material. The assumptions of linear elastic fracture mechanics and elastic behavior of the body with interfaces are considered. The finite element method was used for numerical calculations. The results obtained might contribute to a better understanding of the failure of materials with interfaces (e.g. layered composites, materials with protective coatings) and to a more reliable estimation of the service life of such structures.

Crack Behaviour in Laminar Ceramics with Strong Interfaces

The paper deals with crack propagation in ceramic laminates. Assumptions of linear elastic fracture mechanics and small scale yielding are considered. Crack behaviour in a ceramic laminate body under external loading is investigated. Strong residual stresses due to different coefficients of thermal expansion of individual material layers are taken into account in finite element calculations. The change of crack propagation direction at the material interface is estimated on the base of the strain energy density factor and maximum tangential stress criteria. The influence of thickness of laminate layers on crack propagation direction is estimated. The stepwise crack propagation through the Al2O3-ZrO2 ceramic laminate is numerically estimated. It can be concluded that good agreement between the estimated crack path and experimental data was found.

Numerical Investigation of Stress Singularities in Cracked Bimaterial Body

Composite materials or generally materials with interfaces are nowadays used in many varied engineering applications. In comparison with classical engineering materials the existence of material interface causes locally different stress distribution, which can strongly influence behaviour of whole structure and can have an important influence on failure mechanisms of such materials. The paper presented is devoted to the investigation of stress singularity exponents of a crack growing in a bimaterial body perpendicularly to the interface and touching the material interface. Discrepancies between value of stress singularity exponent in the centre of bimaterial body and on the free surface were found. The assumptions of linear elastic fracture mechanics (LEFM) and small scale yielding (SSY) are considered. For numerical calculations finite element analysis was used. Results obtained can contribute to a better understanding of failure of materials with interfaces.

Generalized Linear Elastic Fracture Mechanics: An Application to a Crack Touching the Bimaterial Interface

In the contribution the limits of the validity of classical linear elastic fracture mechanics are extended to problems connected with failure of composite structures. The work is focused mainly on the case of a crack touching the interface between two different materials, two different constituents. The approach suggested in the paper facilitates the answer to the question what is the influence of particle (in particulate composite) or layer (in laminates) on crack propagation through bimaterial interface. Different composite (bimaterial) structures are considered: layered composites and composites reinforced by particles. The presented approach follows the basic idea of linear elastic fracture mechanics, i.e. the validity of small scale yielding conditions is assumed, and has a phenomenological character.

Crack Propagation in the Vicinity of the Interface Between Two Elastic Materials

The objective of the paper is to investigate the direction of a further crack propagation from the interface between two elastic materials. The angle of crack propagation changes when the crack passes the interface. The suggested procedure based on a generalized form of Sihs strain energy density factor makes it possible to estimate an angle of propagation under which the crack will propagate into the second material. The assumptions of linear elastic fracture mechanics and the elastic behavior of the body with interfaces are considered. The finite element method was used for numerical 2D and 3D calculations. The results obtained might contribute to a better understanding of the failure of materials with interfaces (e.g. layered composites, materials with protective coatings, etc.).