Kateřina Štegnerová

Description of Strengthening Mechanism in Layered Ceramic Composites

The aim of this paper is to describe specific crack behaviour in the layered alumina-zirconia ceramic composite with strong interfaces and its strengthening mechanism. Different coefficients of thermal expansion of individual constituents of ceramic composite cause high residual stresses inside the layers during the sintering process. Compressive residual stresses can significantly influence the crack propagation through the laminate hereby improve the resistance of the material to the crack propagation. Estimation of crack behaviour in laminate was performed assuming the validity of linear elastic fracture mechanics using the criterion based on the strain energy density factor derived by Sih. This paper describes the strengthening mechanism in layered ceramic composites and prediction of their failure which contributes to better understanding of the fracture behaviour of the layered ceramic composites.

Estimation of stepwise crack propagation in ceramic laminates with strong interfaces

During the last years many researchers put so much effort to design layered structures combining different materials in order to improve low fracture toughness and mechanical reliability of the ceramics. It has been proven, that an effective way is to create layered ceramics with strongly bonded interfaces. After the cooling process from the sintering temperature, due to the different coefficients of thermal expansion of individual constituents of the composite, significant internal residual stresses are developed within the layers. These stresses can change the crack behaviour. This results to the higher value of so-called apparent fracture toughness, i.e. higher resistance of the ceramic laminate to the crack propagation. The contribution deals with a description of the specific crack behaviour in the layered alumina-zirconia ceramic laminate. The main aim is to clarify crack behaviour in the compressive layer and provide computational tools for estimation of crack behaviour in the field of strong residual stresses. The crack propagation was investigated on the basis of linear elastic fracture mechanics. Fracture parameters were computed numerically and by author’s routines. Finite element models were developed in order to obtain a stress distribution in the laminate containing a crack and to simulate crack propagation. The sharp change of the crack propagation direction was estimated using Sih’s criterion based on the strain energy density factor. Estimated crack behaviour is qualitatively in a good agreement with experimental observations. Presented approach contributes to the better understanding of the toughening mechanism of ceramic laminates and can be advantageously used for design of new layered ceramic composites and for better prediction of their failure.

Optimization of Design Parameters of Fracture Resistant Piezoelectric Vibration Energy Harvester

This paper is focused on an analysis of a multilayer ceramic-based piezoelectric vibration energy harvester, which could be excited by ambient vibrations or external forces and thus provide a useful source of electricity for modern electronics. The proposed multilayer concept of the energy harvester enables introduction of tensile / compressive residual stresses inside particular layers. These stresses are intended to be used for enhancement of the harvester ́s fracture resistance and simultaneously for the improvement of the energy gain upon its operation. A crack arrest, by means of compressive residual stresses (in the outer “non-piezo” layer), will be utilized to this end. Primarily, the extended classical laminate theory (taking into account the piezoelectric characteristics of selected layers) will be used to define various designs of particular layers with various levels of residual stresses inside them. The weight function method is subsequently employed to select a design, which is most resistant to propagation of preexisting cracks. Selected laminate configurations are verified by means of FE simulations. Such analysis is essential for development of new energy harvesting systems formed of new smart materials and structures, which could be integrated in future development processes.

Lifetime Assessment of Particulate Ceramic Composite with Residual Stresses

A micro-crack propagation in particulate ceramic based composite was studied using finite element method (FEM). Subcritical crack growth (SCG) was numerically simulated under complex load conditions (mechanical loading and loading by internal residual stresses). The effect of residual stresses on the crack propagation was studied. Two-dimensional computational model of particulate ceramic composite with material properties corresponding to low temperature co-fired ceramics (LTCC) was developed. The results indicate that the presence of residual stresses significantly reduces values of stress intensity factor in the vicinity of composite surface and the direction of residual stresses around the particles contributes to the micro-crack deflection from the particles. The time to failure of the composite under mechanical loading was determined. Results obtained contribute to a better understanding of the role of residual stresses during micro-crack propagation in ceramic particulate composites.

3D Model of Crack Propagation in Particulate Ceramic Composite Containing Residual Stresses

A crack propagation and fracture behaviour of particulate ceramic composite were investigated. Influence of 3D shape of particles on the crack propagation was studied together with influence of the presence of residual stresses, which are developed inside the composite during manufacturing process. Finite element (FE) method was used for numerical simulation of propagating crack in the composite. Basic numerical models of low-temperature co-fired ceramics (LTCC) with alumina particles homogenously dispersed in the glass matrix were developed. Volume fraction of alumina phase was 20vol.%, which is typical amount for LTCC. The results show that existence of residual stresses retards the crack propagating under conditions of sub-critical crack growth (SCG). Presented results contribute to a better understanding of the role of residual stresses in particulate ceramic composites.

Influence of Particle Attributes on Residual Stresses in Particulate Ceramic Composites

The contribution deals with the issue of residual stresses in particulate ceramic composites used in microelectronics. Residual stresses are developed in the composite due to cooling during manufacturing process. Different coefficients of thermal expansions of particles and matrix cause important residual stresses in the composite influencing their mechanical behaviour. The main aim of the paper is to determine influence of particle properties on magnitude and distribution of residual stresses in the composite matrix. Three dimensional numerical model was developed and finite element method (FEM) was used for numerical simulations. Results obtained contribute to a better understanding of residual stresses distribution and fracture processes in the studied type of composite.

Study of Influence of Residual Stresses on Crack Propagation in Particulate Ceramic Composites

The aim of the present work is to analyze the influence of residual stresses in the particulate ceramic composite on the crack propagation. The crack propagation direction was estimated using Sih’s criterion based on the strain energy density factor. A two-dimensional finite element model was developed for determination of crack path. The residual stresses resulting from the mismatch of coefficients of thermal expansion during the fabrication process of the composite were implemented to the computational model. The effect of the particles shape on the crack propagation was investigated. Conclusions of this paper can contribute to a better understanding of the propagation of micro-cracks in particulate composites in the field of residual stresses.

Residual Lifetime Determination of Low Temperature Co-Fired Ceramics

The effect of subcritical crack growth is nowadays intensively studied mainly in relation to the strength of ceramic materials. The main aim of the contribution is to describe behavior of micro-crack propagating in the Low Temperature Co-fired Ceramics (LTCC) under subcritical crack growth (SCCG) conditions. The micro-crack behavior is significantly influenced by residual stresses developed in the LTCC due to different coefficients of thermal expansion of individual components. Two-dimensional numerical model was developed to simulate micro-crack propagation through the composite. The micro-crack propagation direction was determined using Sih’s criterion based on the strain energy density factor and the micro-crack path was obtained. The residual lifetime of the specific ceramic particulate composite (LTCC) was estimated on the basis of experimental data. The paper contributes to a better understanding of micro-crack propagation in particulate ceramic composites in the field of residual stresses.

Influence of Residual Stresses and Particle Properties on Mechanical Response of the Material in Particulate Ceramic Composites

The contribution deals with the issue of mechanical response of the particulate ceramic composites used in microelectronic. Mechanical properties and behaviour of composites are highly influenced by residual stresses which are developed in material during cooling in manufacturing process due to the different coefficients of thermal expansions of individual constituents. The main aim of this paper is to estimate the elastic constants and strength of the selected particulate ceramic composites with considering the residual stresses. Three dimensional models and finite element method are used for numerical simulations. Results contribute to determination and better understanding of mechanical behaviour of the particulate ceramic composites.

Influence of the V-Notch Opening Angle on Critical Applied Force Values for the Crack Initiation from the Sharp V-Notch

The aim of this paper is to estimate a value of the critical applied force for a crack initiation from the sharp V-notch tip. The classical approach of the linear elastic fracture mechanics (LELM) was generalized, because the stress singularity exponent differs from 0.5 in studied case. The value of the stress singularity exponent depends on the V-notch opening angle. The finite element method was used for a determination of stress distribution in the vicinity of the sharp V-notch tip and for the estimation of the generalized stress intensity factor depending on the V-notch opening angle. Critical value of generalized stress intensity factor was obtained by using stability criterion based on the tangential stress component averaged over a critical distance d from the V-notch tip. Calculated values of the critical applied force were compared with experimental data taken from the literature.