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.
      259
  • Publication
    Micromechanical Modelling of Advanced Ceramics Using Statistically Representative Microstructures
    (Trans Tech Publications, 2013-09) ; ; ;
    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 cont, it is possible to specify material improvements.
      415
  • 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.
      494Scopus© Citations 10
  • 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.
      951Scopus© Citations 19
  • 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.
      365
  • 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.
      1280Scopus© Citations 34
  • Publication
    A combined experimental–numerical investigation of fracture of polycrystalline cubic boron nitride
    Numerical modelling of a series of experimental Single Edge V-Notched Beam tests was carried out for a number of grades of polycrystalline cubic boron nitride using the finite volume method (FV) and cohesive zone model approach. The effect of notch root radius observed experimentally was reproduced numerically via a unique CZM for each material examined. It was also found that the shape of the cohesive zone model can be signiffcant, especially when the material has a relatively high fracture energy. It was also demonstrated that the experimentally observed drop in fracture toughness with increase in test rate was not explainable in terms of the system dynamics. It was found that in order to predict the experimental fracture loads for a range of loading rates, it was necessary to modify the CZM in such a way as to preserve the micro-structural length scale information of the material embedded within the CZM.
      573Scopus© Citations 11
  • 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
      305Scopus© Citations 4
  • Publication
    Fracture properties of PCBN as a function of loading rate and temperature
    (Trans Tech Publications, 2010-11) ; ; ;
    Polycrystalline Cubic Boron Nitride (PCBN) is a super-hard material used in some of the most demanding material removal operations today. These include turning of hardened steels, as well as the machining of highly abrasive alloys. In these applications the tools are subjected to high operating temperatures, abrasive and impact loading. This can lead to the brittle fracture of the tool. Accurate determination of the fracture toughness and mechanical properties of PCBN under a wide range of operating conditions is therefore essential in order to evaluate the performance of the tool under these highly demanding conditions. For this study, a laboratory scale three point bend test rig has been used for the fracture tests. The fracture toughness of two different grades of PCBN are measured at a range of loading rates and temperatures corresponding to the actual in-service conditions. The results show the measured properties of these materials vary with both loading rate and temperature. The fracture surfaces of the specimens are examined using scanning electron microscopy to determine dominant fracture mechanisms
      410Scopus© Citations 4
  • Publication
    The Role of Microstructure on the Fracture Behaviour and Statistics of Advanced Ceramics
    (Trans Tech Publications, 2013-09) ; ; ;
    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.
      727Scopus© Citations 1