Now showing 1 - 10 of 14
  • Publication
    Influence of an Atmospheric Pressure Plasma Surface Treatment on the Interfacial Fracture Toughness on Bonded Composite Joints
    The aim of this work is to investigate the influence of a variety of plasma treatments on the surface properties of an epoxy-based composite material and to establish a relationship between these properties and the subsequent mechanical behaviour of adhesively bonded joints. To this end, specimens were subjected to three different types of plasma treatment: two short treatments (2min) of Helium and Helium plus Oxygen, and one long treatment (15min) of Helium plus Oxygen. The variation in surface energy of the composite specimens was examined in each case over a period of up to 3 days using contact angle measurements. Initial results show that the surface energy was increased from an untreated value of approximately 40 mJ/m2 to a value of 65 mJ/m2 immediately after treatment. The surface energy then fell by approximately 10 mJ/m2 over the course of three days for each treatment. The composite substrates were then bonded together using an epoxy film adhesive and the Mode I fracture toughness of the joint was determined from a series of symmetric and asymmetric double cantilever beam (DCB) tests. It was found that for both test geometries the adhesive failed cohesively. As a result, the values calculated for the mean propagation strain energy release rate, GIC, were those of the cohesive fracture toughness of the adhesive as opposed to the interfacial fracture toughness between the composite surface and adhesive.
  • Publication
    Micromechanical Study of Strength and Toughness of Advanced Ceramics
    Numerical investigations using the finite volume (FV) method were conducted to examine the effect of microstructure and mi- crostructural properties on the fracture strength of advanced ceramics with industrial applications. Statistically representative microstructural volumes were created using a diffuse-interface model using OpenFOAM-1.6-ext. Crack initiation and growth was modeled using a recently developed arbitrary crack propagation model. It was found that by varying the Young's modulus of the second phase material, a significant change in the maximum failure load was observed. It was also shown that there exists an opti- mum Young's modulus for which a maximum failure load will be reached. A number of microstructures with a varying percentage second phase material were investigated in this study. Results indicate that for a given set of material and cohesive parameters the maximum failure load was insensitive to the percentage second phase material. This study highlights the role that microstructure adconstituent properties of brittle ceramics have on influencing the fracture strength of such material. With this in mind, a para- metric study was undertaken to examine the competition between crack deflection and crack penetration at the interface between two materials. It was found that appropriate choice of interface strength and toughness as well as second phase material compliance was required in order to promote an overall strength and toughness increase through crack deflection and bridging. Such numerical modeling is essential in order to gain a greater understanding into the structure-property relationship that exists for such advanced ceramics.
  • Publication
    The influence of microstructure on the fracture statistics of polycrystalline diamond and polycrystalline cubic boron nitride
    Flexural strength data of a number of grades of polycrystalline diamond (PCD), and polycrystalline cubic boron nitride (PCBN) were analyzed using Weibull, normal and lognormal distributions. The role of microstructure in the failure mechanism of such materials was analyzed 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 specimens were tested to investigate possible volume scaling effects for these materials. The different microstructures between the two materials was shown to have an effect on the statistical strength distributions. It was found that for PCD, in general, a lognormal distribution provided a better fit than the other distributions and no specimen size effect was observed. For PCBN a significant specimen size effect was observed and this also corresponded to the data fitting to a Weibulldistribution.
      470Scopus© Citations 14
  • Publication
    Analysis of two-phase ceramic composites using micromechanical models
    Micromechanical models of two-phase ceramic composites are created using a modified Voronoi tessellation approach. These representative Finite Volume (FV) microstructures are used to investigate the role of microstructure on fracture of advanced ceramics. An arbitrary crack propagation model using a cell-centred finite volume based method is implemented. In particular the effect of matrix content is examined. It is shown that the underlying microstructure significantly affects the local stress and strain distributions for a two-phase ceramic containing hard particles in a softer matrix. Simulation results indicate that an increase in the volume fraction of these hard grains leads to an increase in strength of the composite material. Furthermore, it is found that the homogeneity of the microstructure affects the overall strength.
      377Scopus© Citations 10
  • Publication
    Arbitrary crack propagation in multi-phase materials using the finite volume method
    An arbitrary crack propagation model using cell-centre nite volume based method is presented. Crack growth in an elastic solid, across an interface perpendicular to the initial crack path and into a second elastic solid is analysed. Crack initiation and the subsequent path of propagation are shown to arise naturally out of the selection of appropriate cohesive parameters. It is shown that the allowable crack propagation path is restricted by the underlying mesh. Results are presented for a number of values of interfacial strength and ratios of elastic properties between the two elastic solids. For higher values of interfacial strength, the crack is shown to propagate straight through the interface, while for lower values of interfacial strength, the crack is shown to change direction and propagate along the interface. It is shown that with careful selection of material and interface parameters it is possible to arrest a propagating crack at the interface. The method represents a useful step towards the prediction of crack propagation in complex structures.
      613Scopus© Citations 28
  • Publication
    Micromechanical modelling of ceramic based composites with statistically representative synthetic microstructures
    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. Two dimensional representative finite volume (FV) microstructures were created using Voronoi tessellation to synthetically represent the microstructure of a two phase ceramic composite. It is shown, by comparing with real micrographs, that the method captures the features of real microstructures in terms of grain size distribution, grain aspect ratio and the distribution of second phase agglomerations. Simulation results indicate the computed elastic parameters are within the Hashin–Shtrikman bounds and also agree well with the Eshelby–Mori–Tanaka method. It is found that the underlying microstructure significantly affects the local stress and strain distributions in these advanced ceramics.
      712Scopus© Citations 18
  • Publication
    Effect of notch root radius on fracture toughness of polycrystalline cubic boron nitride
    The fracture toughness of five grades of polycrystalline cubic boron nitride (PCBN) has been determined using Single Edge V-Notched Beam specimens. Both coarse and fine grade materials were considered, containing CBN grain sizes of between 1 μm and 22 μm. The influence of notch root radius on the measured fracture toughness was examined. The notch root radius was found to have a major effect for materials with smaller CBN grain sizes while only a small effect was noted for the material with large CBN grain sizes. A simple analytical model was developed to explain the effect of the notch root radius on the fracture toughness and was found to agree well with experiment for all the materials tested. It was shown that the effect of notch root radius is directly linked to the size of the CBN grain. It is proposed that this effect results from the interaction between the microstructure and the stress field around the notch tip.
      1085Scopus© Citations 30
  • Publication
    Thermal shock resistance of polycrystalline cubic boron nitride
    The effect of thermal shock on the exural strength has been investigated experimentally. It was found that the variation in exural strength with quench temperature was influenced by the CBN grain size. Polycrystalline material containing small CBN grains showed a discontinuous drop in measured exural strength above a material dependent critical quench temperature difference, delta Tc. The sharp decrease in measured strength is accompanied by unstable crack propagation. Material containing a significantly larger CBN grain size, exhibited a gradual decrease in strength above the critical quench conditions. The experimental observations agreed with an established theory developed for thermal shock of alumina. The theoretically calculated critical temperatures agree well with the observed experimental data for each material when a aw size equal to the CBN grain size is employed.
      753Scopus© Citations 16
  • Publication
    Fracture properties of PCBN as a function of loading rate and temperature
    Polycrystalline Cubic Boron Nitride (PCBN) is a superhard material which is used in machining of hardened steels and other abrasive and aerospace grade alloys. In these applications the tools are subjected to high operating temperatures, abrasive and impact loading. Impact loading can lead to the sudden fracture and hence failure of the tool. In this work the static and dynamic fracture toughness of PCBN is determined via a combined experimental-numerical approach. The results show that the fracture toughness of PCBN varies with loading rate
      245Scopus© Citations 4
  • Publication
    Micromechanical Modelling of Advanced Ceramics with Statistically Representative Synthetic Microstructures
    (Trans Tech Publications, 2013-11) ; ; ;
    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 sts 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.