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Carolan, Declan
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Carolan, Declan
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Carolan, Declan
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Now showing 1 - 10 of 13
- PublicationInfluence of an Atmospheric Pressure Plasma Surface Treatment on the Interfacial Fracture Toughness on Bonded Composite JointsThe 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.
242 - PublicationMicromechanical Study of Strength and Toughness of Advanced CeramicsNumerical 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.
354 - PublicationThermal shock resistance of polycrystalline cubic boron nitrideThe 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.
844Scopus© Citations 16 - PublicationFracture properties of PCBN as a function of loading rate and temperaturePolycrystalline 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
392Scopus© Citations 4 - PublicationA combined experimental–numerical investigation of fracture of polycrystalline cubic boron nitrideNumerical 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.
563Scopus© Citations 11 - PublicationEffect of notch root radius on fracture toughness of polycrystalline cubic boron nitrideThe 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.
1248Scopus© Citations 34 - PublicationAnalysis of two-phase ceramic composites using micromechanical modelsMicromechanical 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.
456Scopus© Citations 10 - PublicationMicromechanical Modelling of Advanced Ceramics with Statistically Representative Synthetic MicrostructuresAdvanced 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.
358 - PublicationThe Role of Microstructure on the Fracture Behaviour and Statistics of Advanced CeramicsStrength 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.
567Scopus© Citations 1 - PublicationThe influence of microstructure on the fracture statistics of polycrystalline diamond and polycrystalline cubic boron nitrideFlexural 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.
555Scopus© Citations 15