Patricia Alveen

The Role of Microstructure on the Fracture Behaviour and Statistics of Advanced Ceramics

Strength data of three advanced ceramics were fitted to the Weibull, normal and lognormal distributions. The three ceramics had similar grain size and varied in binder content. The role of microstructure in the failure mechanism of such ceramics was analysed in terms of the chosen strength distributions. The best-fit distributions were determined using the maximum log-likelihood criteria and a comparison between the best and worst fit was conducted using the Akaike Information Criteria (AIC). Both large and small samples were tested to investigate possible scaling effects for these ceramics. It was found that for two of the three ceramics tested that a lognormal distribution rather then the conventionally used Weibull distribution was preferable in characterising the strength data. A small drop in strength was noticed between large and small samples but this trend was not thought to be a result of scaling rather due to the decrease in binder content.

Micromechanical Modelling of Advanced Ceramics Using Statistically Representative Microstructures

Advanced ceramics are a class of materials used as cutting tools in some of the most demanding material removal operations. Their high hardness makes them extremely suited for use at these extreme conditions. However they have a relatively low fracture toughness when compared to other conventional tool materials. A combined experimental-numerical method was used to investigate the role of microstructure on the fracture of advanced ceramics. In particular, the effect of grain size and matrix content were examined. Representative finite volume (FV) microstructures were created using Voronoi tessellation. It is shown, by comparing with real micrographs, that the method captures the features of real microstructures in terms of grain size distribution and grain aspect ratio. It was found that the underlying microstructure significantly affects the failure of this class of materials. Furthermore, it was found that by altering the microstructural parameters in the numerical model, such as grain size and matrix content, it is possible to specify material improvements.

Micromechanical Modelling of Advanced Ceramics with Statistically Representative Synthetic Microstructures

Advanced ceramics are a class of material used in extreme conditions, such as high speed turning of aerospace alloys and rock drilling. Their high hardness makes them suitable for these uses, however their lower toughness means that failure due to fracture and chipping is a problem. They are composed of micron-sized particles of a primary hard phase together with either a ceramic or metallic matrix material. A combined experimental-numerical method was used to investigate the role of microstructure on the fracture of advanced ceramics. Two dimensional, statistically representative microstructures of the advanced ceramics are created using Voronoi tessellation. The synthetic microstructures are compared to real microstructures in terms of particle size distribution and particle aspect ratio. Simulation results indicate that the computed elastic parameters are within the Hashin-Shtrikman bounds and agree closely with analytical predictions made with the Eshelby-Mori-Tanaka method. It is found that the local stress and strain distribution within the model is significantly affected by the underlying microstructure, which in turn affects fracture properties. Hence, tailoring the microstructure can optimise the bulk strength parameters of the material.