Mark Bush

The effects of grain size and porosity on the elastic modulus of nanocrystalline materials

Abstract A phase mixture model was considered in which nanocrystalline materials are treated as a mixture of crystalline phase, intercrystalline phases (grain boundary, triple line junction and quadratic node) and pores. In order to investigate the effects of grain size and porosity on the elastic modulus, Budiansky’s self-consistent method in conjunction with the phase mixture model was employed. The calculated results are compared with the experimental measurements in the literature. The elastic modulus of nanocrystalline materials decreases with a decrease of the grain size and the decrement is relatively large at grain sizes below about 10 nm. The effect of porosity, however, is substantially greater than the grain size effect.

Perceptions of Competency Deficiencies in Engineering Graduates

Summary Engineering education in Australia continues to evolve. This study asks: “Are current changes to engineering education consistent with competence deficiencies in engineering graduates perceived by engineers ? “The method is different from previous international and Australian studies, and the findings are consistent with the results of previous studies. Practical engineering competencies and engineering business competencies featured strongly. The findings support continuation of current trends in the evolution of engineering education: cultural change, broadening of curricula, and introduction of non-traditional pedagogies, assessments and learning environments.

Constitutive modelling of strength and plasticity of nanocrystalline metallic materials

Abstract A phase mixture model is proposed to describe the deformation behaviour of a nanocrystalline metallic material. The deformation behaviour of the crystallite phase is described by constitutive equations based on dislocation density evolution and includes a contribution from diffusion controlled plastic flow. The dislocation glide contribution to the plastic strain rate is considered to vanish below a certain critical grain size. The grain boundary material, which is treated as a separate phase, is considered to deform by a diffusional mechanism, resulting in a viscous Newtonian behaviour. The strain in both phases is assumed to be the same and equal to the imposed macroscopic strain. The stress is calculated using a simple rule of mixtures. The grain size dependence of the stress–strain curves obtained is shown to be in reasonable agreement with experiments, as are the predicted strain rate effects. In particular, observed deviations from the Hall–Petch behaviour are described by the model correctly. The effect of grain size distribution is also considered.

A phase mixture model of a particle reinforced composite with fine microstructure

Abstract A phase mixture model is considered in which a mechanically alloyed, particle reinforced metal matrix composite with ultrafine microstructure is treated as a mixture of a matrix phase, a reinforcing particle phase and a boundary phase. The finite element method is employed in conjunction with a unit cell of the composite to investigate the compressive and tensile behaviour of the system. The reinforcing ceramic particles are taken to be elastic. A unified constitutive model based on dislocation density evolution is used to describe the plastic flow behaviour of the matrix; grain size effects are included. A yield criterion for porous materials, including the evolution of density, is applied to the boundary phase. The boundary phase is assumed to have the mechanical properties of a quasi-amorphous material. The effects of the volume fraction of the reinforcing particles, overall effective density, grain size of the matrix, and the density of the boundary phase on the overall mechanical properties are discussed. The calculated stress–strain curves based on the unit cell model are used to simulate an indentation test and are compared with experimental measurements on Al-alumina composites.

Failure of curved brittle layer systems from radial cracking in concentrated surface loading

A study was made of radial crack evolution in curved brittle layers on compliant support substrates. Three-dimensional boundary element analysis was used to compute the stepwise growth of radial cracks that initiate at the bottom surfaces of glass on polymeric support layers, from initiation to final failure. The algorithm calculates reconstituted displacement fields in the near-tip region of the extending cracks, enabling direct evaluation of stress-intensity factors. Available experimental data on the same material systems with prescribed surface curvatures were used to validate the essential features of the predicted crack evolution, particularly the stability conditions prior to ultimate failure. It was shown that the critical loads to failure diminish with increasing surface curvature. Generalization of the ensuing fracture mechanics to include alternative brittle-layer/polymer-substrate systems enabled an explicit expression for the critical load to failure in terms of material properties and layer thicknesses. Implications concerning practical layer systems, particularly dental crowns, are briefly discussed.

Finite element modelling of deformation in particulate reinforced metal matrix composites with random local microstructure variation

Abstract Deformation in particulate reinforced metal matrix composites (PR MMCs) with locally varying reinforcement volume fraction has been modelled using a two-scale finite element approach. The responses of axisymmetric unit cell models were used to define the constitutive response of mesoscale regions possessing varying volume fractions. Macroscale response was then investigated using two- and three-dimensional “random arrays” of finite elements, in which element properties were randomly assigned in line with a Gaussian distribution. Two-dimensional random arrays developed non-uniform strain fields, severe strain localization ensuing as straining proceeded. Two-dimensional random arrays are, however, inappropriate for modelling the three-dimensional microstructure of PR MMCs. Three-dimensional random arrays also developed non-uniform strain fields, but severe strain localization did not arise. Reinforcement clustering was simulated by varying the standard deviation in element volume fraction. Yield stress, strain hardening and elastic modulus were all found to increase as the severity of clustering increased.

Contact damage in porcelain/Pd-alloy bilayers

An analysis is made of contact damage in brittle coatings on metal substrates, using a case study of a dental porcelain coating of thickness between 0.1 and 1 mm fused onto a Pd alloy base, with spherical indenter of radii 2.38 and 3.98 mm. At large coating thicknesses (>300 μm), the first damage takes the form of surface-initiated transverse cone cracks outside the contact. At small coating thicknesses (

Analysis of mechanical contrast in optical coherence elastography

Abstract. Optical coherence elastography (OCE) maps the mechanical properties of tissue microstructure and has potential applications in both fundamental investigations of biomechanics and clinical medicine. We report the first analysis of contrast in OCE, including evaluation of the accuracy with which OCE images (elastograms) represent mechanical properties and the sensitivity of OCE to mechanical contrast within a sample. Using phase-sensitive compression OCE, we generate elastograms of tissue-mimicking phantoms with known mechanical properties and identify limitations on contrast imposed by sample mechanics and the imaging system, including signal-processing parameters. We also generate simulated elastograms using finite element models to perform mechanical analysis in the absence of imaging system noise. In both experiments and simulations, we illustrate artifacts that degrade elastogram accuracy, depending on sample geometry, elasticity contrast between features, and surface conditions. We experimentally demonstrate sensitivity to features with elasticity contrast as small as 1.1∶1 and calculate, based on our imaging system parameters, a theoretical maximum sensitivity to elasticity contrast of 1.002∶1. The results highlight the microstrain sensitivity of compression OCE, at a spatial resolution of tens of micrometers, suggesting its potential for the detection of minute changes in elasticity within heterogeneous tissue.

On the flow through the human female urethra

The flow characteristic for a human female urethra is determined by direct measurement of flow rate and pressure difference data. The measurements are made on a full-scale physical model of a urethra in its open state, which was created using dimensional information taken from video cystograms. The measured data therefore include viscous dissipation effects associated with developing flow, changes in cross-sectional area and changes in flow direction. These effects are often ignored in mathematical models of this system. The data may therefore assist in the development and testing of more realistic models for urine flow. The measured characteristic is compared with a mathematical model of the flow based on a straight tube of uniform diameter carrying fully developed turbulent flow. When the diameter of the model tube is chosen to be equal to the distal diameter of the urethra, it is observed that the predicted flow characteristic provides a good first approximation to the measured characteristic, despite the substantial differences in geometry and flow regime between the mathematical model and the actual system.

Cracking of Porcelain Coatings Bonded to Metal Substrates of Different Modulus and Hardness

A preceding study of contact damage in a bilayer system consisting of a porcelaincoating on a stiff Pd-alloy substrate is here expanded to investigate the role ofsubstrate modulus and hardness. Bilayers are made by fusing the same dental porcelainonto Co-, Pd-, and Au-alloy metal bases. Indentations are made on the porcelainsurfaces using spheres of radii 2.38 and 3.98 mm. Critical loads to initiate conefracture at the top surface of the porcelain and yield in the substrate below the contactare measured as a function of porcelain thickness. Radial cracks form at the lowersurface of the coating once the substrate yield is well developed. By virtue of itscontrolling role in the metal yield process, substrate hardness is revealed to be a keymaterial parameter—substrate modulus plays a secondary role. A simple elasticity-based analysis for predetermining critical loads for a given brittle/plastic bilayer systemis presented.I. INTRODUCTIONCeramic coatings on metal substrates are of techno-logical importance in engineering and biomechanical ap-plications requiring high-wear resistance and mechan-ical, thermal, or chemical insulation. Dentistry, whereceramic coatings on metal underlayers form the basis oftraditional crown and bridge design,