Browsing by Author "Nurick, Gerald"

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    Open Access
    A numerical assessment of architectural parameters for anisotropic behavior in idealised trabecular structures
    (2018) Moore, Keelan; Cloete, Trevor; Nurick, Gerald
    Bones macroscopically consist of two major constituents; namely cortical and trabecular (also known as cancellous) bone. Cortical bone is the hard and dense outer layer of bone, which holds majority of the load bearing capacity. Trabecular bone is the porous internal bone, which distributes loads at joints by allowing for a larger bearing surface and acts as an energy absorber. Trabecular bone has a complex, heterogeneous, anisotropic open cell lattice structure with a large variation in mechanical properties across anatomic site, species, sex, age, normal loading direction and disease state. A common attempt to account for this variation is to correlate the structure of the trabecular bone sample to the mechanical response, which requires a means of quantifying the structure. Microstructural indices such as bone volume vs. total volume (BV/TV), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), structural modal index (SMI) and mean intercept length (MIL) have been widely used to find correlations between structure and properties. Early studies only considered densitometric indices, which accounted for much of the variation however cross study correlations did not agree, leading to an interest in capturing non-scalar valued indices to account for features such as the anisotropy of the bone. The structural anisotropy varies from fully equiaxed to highly directional based on where the trabecular bone is located and what the function would be. The mean intercept length has been proposed as a measure of the structural anisotropy, specifically the interfacial anisotropy of the sample, which is commonly used to account for the mechanical anisotropy. This research falls within a longer term goal of investigating and understanding the mechanical anisotropy of trabecular bone. To that end, the anisotropy of regular lattice structures was investigated, with the particular goal that the investigated lattices were simple analogues for the more complex structures seen in trabecular bone. A framework for assessing the structure-property relations of trabecular bone is created, with focus on anisotropy. The mechanical anisotropy of idealised trabecular structures is quantified using well known microstructural indices, which are compared to the numerically determined mechanical response. The modelling methodology initially investigated 2D lattices that have very well known responses, such that the modelled approach could be verified. Three 2D lattices were used to do this, with the aim that the 3D lattices would be their analogues. Specifically a 2D square, hexagonal and triangular lattice were investigated. The square lattice is highly anisotropic as is the cubic lattice. The hexagonal lattice is isotropic with a large constraint effect as is the Kelvin cell, and the triangular lattice is isotropic with a small constraint effect. The octet-truss was the closest analogue to the triangular lattice, having a small constraint effect and being less anisotropic than the cubic lattice. The three 3D lattices were chosen to represent highly directional trabecular bone (using a cubic lattice) and more equiaxed trabecular bone, with the fully isotropic Kelvin cell lattice (also known as a tetrakaidecahedron) and the octet-truss lattice which has a lower degree of anisotropy than the cubic. Two confinement arrangements were also investigated as analogues for the trabecular bone at the free surface and at the cortical surface. To assess the mean intercept length analysis as a measure of mechanical anisotropy, this research performed the analysis on three 3D periodic lattice structures and compared the results to mechanical properties which were numerically determined using finite element analysis. The mean intercept analysis was performed by generating 3D images for the lattices, similar to the output of (mu)CT images, using a combination of open-source software and custom code, and performing the analysis in BoneJ, an open-source software package. The mechanical response was determined using two methods, namely discrete and continuum modelling approaches. The discrete approach characterised the lattice with each strut modelled as a Timoshenko beam element solved in LS-DYNA. To capture the anisotropy, the lattice had to be loaded at arbitrary angles, which was achieved by a rotating the whole lattice and cropping it to a specified test region using custom code. The continuum modelling approach used a homogenisation approach by treating the lattice as a solid material with effective properties, this was solved in a custom implicit solver written in MATLAB using solid elements. The anisotropy was modelled by transforming the elasticity tensor to arbitrary coordinate systems to load the model in arbitrary directions. The discrete modelling approach suffered from high computational costs and difficulty in removing the boundary effects, all of which would be worsened for models of real trabecular bone. However the discrete approach did accurately captured the mechanical behaviour of the lattices tested. The continuum approach accurately captured some of the responses but failed to capture all behaviour caused by confinement. The continuum model could not capture the switch in predominant deformation mode of the 2D hexagonal lattice caused by lateral confinement, and failed to accurately capture the symmetry of the highly anisotropic 3D cubic lattice. The mean intercept length analysis failed to capture the anisotropic response of simple periodic lattices, showing no significant difference between the octet-truss and cubic lattices, despite them having a very large difference in mechanical anisotropy. It also showed that the Kelvin cell lattice had the highest degree of geometric anisotropy, which is compared to having the lowest mechanical anisotropy being the only fully isotropic 3D lattice investigated. The mechanical investigation showed that the lateral confinement has a large effect, significantly scaling the response of isotropic lattices whilst distinctly changing the anisotropic behaviour of the cubic and octet-truss lattice. The mean intercept length analysis cannot capture the mechanical confinement effect from geometry alone, and thus fails to capture the mechanical response due to confinement Overall, the continuum modelling approach showed difficulty in capturing the confinement effect in all lattices and thus a more robust method is required. The mean intercept analysis proved unsuccessful in capturing the mechanical response of three periodic idealised trabecular structures. A new microstructural index that can capture the mechanical anisotropy is required, with the ability to consider the effects of confinement on the structure.
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    Open Access
    An investigation into the characterisation of the spalling behaviour of Polycrystalline Diamond Compact(PDC) cutters under impact loading conditions
    (2002) Williams, Justin Craig; Nurick, Gerald
    An experimental investigation into the identification of the spalling mechanisms of polycrystalline diamond compact (PDC) cutters and the effect of stress wave shape on the degree of spalling was performed using the split-Hopkinson pressure bar (SHPB) apparatus. The PDC cutters were loaded under specific impact conditions and their reactions monitored and characterised. In order to isolate and identify the mechanism responsible for the spalling of the PDC cutters required them to be tested for compression waves, reflected tensile waves, contact stresses, resultant forces and normal forces. It was determined that the spalling phenomenon observed in PDC cutters was due to the application of excessive normal forces. The results obtained during the impact testing of the PDC cutters were of projected damage area and mass losses, which were converted to volume losses and average crack depth measurements. The loading conditions were characterised as the peak force, average force and impulse experienced by the PDC cutters which were then plotted as a function of each other as well as of the damage measurements. It was found that the degree of spalling is dependent on the amplitude of the applied stress wave and independent of the period. For this reason traditional drop testing methods cannot be used to describe the impact behaviour of PDC cutters and the SHPB apparatus therefore offers an alternative experimental technique.
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    Open Access
    Characterisation of Glass Fibre Polypropylene and GFPP based Fibre Metal Laminates at high strain rates
    (2011-12) Govender, Reuben Ashley; Langdon, Genevieive; Nurick, Gerald
    Fibre reinforced polymers (FRP) are finding increasing use in structures subjected to high rate loading such as blast or impact. Proper design of such structures requires thorough characterisation of the material behaviour over a range of loading rates from quasi-static to impact. This thesis investigated the quasi-static and impact response of Glass Fibre Polypropylene (GFPP) in compression, bending and delamination. The bending and delamination response of Fibre Metal Laminates (FMLs) based on GFPP and aluminium was also investigated at quasi-static and impact rates. High strain rate (5x10^2 to 10^3 /s) compression tests were conducted on GFPP using a compressive Split Hopkinson Pressure Bar (SHPB) and a Direct Impact Hopkinson Pressure Bar (DIHPB), in the through-thickness and in-plane directions. In both loading directions, the peak stress of GFPP increased linearly with the logarithm of strain rate. For in-plane loading, the failure modes were dominated by localised fibre buckling and kink bands, leading to delamination. The through thickness loading produced macroscopic shear and spreading failure modes. However, both of these failure modes are linked to in-ply fibre failures, due to through thickness compression causing transverse tensile strain. Previous studies of similar materials have not explicitly stated the link between through thickness compression and fibre failure associated with transverse tensile strain. A novel test rig was developed for Three Point bend testing at impact rates. The specimen was supported at the outer points on a rigid impacter and accelerated towards a single output Hopkinson Pressure Bar (HPB), which impacted the specimen at its midspan. Previous impact bend test rigs based on HPBs were limited to testing specimens with deflections to failure up to approximately 1mm, whereas the rig implemented herein measured deflections up to approximately 10 mm. This configuration permits the output HPB to be chosen purely on the magnitude of the expected impact force, which resulted in superior force resolution to configurations used in other studies. The HPB Impact Bend rig was used to test GFPP and aluminium-GFPP FML specimens, at impact velocities ranging from 5 to 12 m/s. The flexural strength of GFPP increased with strain rate, while the flexural response of the FML specimens was relatively insensitive to strain rate. v Several candidate delamination test geometries were investigated at quasi-static displacement rates (1 mm/min), and the Single Leg Bend (SLB) test was identified as suitable for adaptation to higher rate testing. Single Leg Bend delamination tests of both GFPP and FML specimens were performed using the HPB Impact Bend rig, at impact velocities of 6 to 8 m=s. The shape of the force displacement response for the high rate testswas markedly different from the quasi-static tests, for both the GFPP and FML specimens. Finite element (FE) simulation of the quasi-static and impact rate SLB tests on GFPP indicated that the difference was probably due to the interaction of flexural vibrations and stress waves in the specimen and the impacter cross member. The experimental results and FE analysis suggest that the delamination fracture toughness of GFPP decreases slightly as strain rate increases. High rate delamination tests on FML specimens resulted in unstable crack growth.
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    Circular plates subjected to localised blast loads : some insights into the mechanism of tearing and the energy required
    (2000) Wiehahn, Miles Alexander; Nurick, Gerald; Bowles, H C
    The scope of this study extends to experimental tests of plates subjected to blast loads with the intention of measuring the velocity of fragments and the subsequent modelling of the plates. A comparison of the results of the numerical and experimental results must be undertaken. An energy balance of the fractured plates must be undertaken with the aim of determining the energy of tearing.
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    Design, manufacture and commissioning of a low pressure quasistatic bulge tester for skin and membrane tissue
    (2020) Curry, Andrew Michael; Govender, Reuben; Nurick, Gerald
    The material properties of skin are of great importance to a variety of fields such as dermatology and reconstructive surgery. Relatively little infrastructure and expertise exists locally in South Africa for testing biological tissue. The difficulties of testing the material properties of skin are the non-uniformity and anisotropy across specimen location and subjects. This anisotropy is most commonly measured by tensile testing of samples cut in different orientations. However, the individual samples at different orientations would be extracted from slightly different locations on the same subject, which will naturally vary in response. Bulge testing is a method of determining response to tension in different directions at the same location, by applying biaxial tension. It uses a positive pressure applied to a peripherally clamped specimen to deform the specimen in a balloon type manner. In this project, bulge testing apparatus was designed and built for the purpose of testing skin and membrane tissue, under biaxial tension. The testing apparatus consists of a syringe pump to control inflation of a specimen, which is clamped in an inflation chamber. Digital Image Correlation (DIC) was used to capture the 3D deformation fields of the specimen, and hence infer the strain fields. To simplify commissioning testing, a commercial silicone elastomer suited for skin prosthetics, was used to manufacture specimens for uniaxial and bulge experimental testing. Two types of bulge specimens were manufactured, standard round specimens and elliptical specimens. The round specimens were used to compare their material response to uniaxial tests and the elliptical bulge specimens were used to simulate the anisotropic response of skin. The method of analysis used in this project is based on using DIC and curvature calculations at multiple points to calculate membrane stresses in principal directions. The method of calculating principal curvatures from DIC is adapted from the work by Machado et al. [1] that calculated Gaussian curvature using the first and second fundamental forms of a surface. In total 18 round, 6 elliptical and 10 uniaxial specimens were tested and the material properties were found to vary slightly between each specimen. The spread in data between the uniaxial and bulge tests was found to be very similar with the bulge data showing 10 % spread at 1.2 stretch and constant 8 % spread above 1.2 stretch and the uniaxial data showing increasing spread from 7 % to 15 %. The curvature results showed very clear principal directions of curvature for the elliptical specimens. This demonstrated that the method used in this project is capable of clearly extracting the orientations of stiffer fibre directions of skin and other collagenous tissue.
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    Modelling directional casting processes in which heat conduction and cavity radiation are the dominant modes of heat transfer
    (2002) Kotschy, P J; Mitchell, G; Nurick, Gerald; Martin, John
    Directional investment casting processes involve complex interactions of various mechanisms of heat and mass thansfer in spatially complex domains and in the presence of a change of phase. In particular, the transfer of heat within the furnace occurs in the form of conduction, convection and radiation. This thesis addresses the development of computational techniques to simulate, at a macroscopic scale, such casting processes. In this study the conservation of heat energy within the casting is assumed to be maintained by conduction, accompanied by the release of latent heat energy during solidification. The overall state of the radiation in the furnace chamber is analysed in terms of the absorbed, emitted and reflected energies for each surface defining the geometry of the chamber. By applying a diffuse-grey approximation to these energies, an expression for the net flux for each surface is derived.
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    Projectile impact damage in filament wound CFRP tubes : effect of stacking sequence on projectile kinetic energy dissipation
    (2000) Will, Mark Alan; Nurick, Gerald; Franz, Thomas
    The effect a change in laminate stacking sequence has on its ability to dissipate projectile kinetic energy has been investigated. T800 carbon fibre filament wound tubes in an epoxy matrix are investigated, having the following lay-up; laminate A - [-35°/35°/90°3/-35°/+35°/90°3/-35°/+35°] and laminate B - [90°6/(-35°/+35°)3]. The laminates underwent projectile impact up to and beyond their ballistic limits and the significant damage mechanisms were then measured and their contribution to energy dissipation quantified. Instrumented drop tests were also performed in order to determine the normal laminate shear fracture energy release rate and energy dissipated by friction between the projectile and laminate.
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    The effect of venting configuration on the blast response of an aircraft unit load device
    (2019) Kriek, Shaun; Langdon, Genevieve; Nurick, Gerald
    The inclusion of venting areas in aircraft unit load devices (ULDs) as a potential blast mitigation technique is investigated in this work. Damage to the ULD, such as large deflections or container rupture, from an internal explosion threatens to tear the aircraft skin and cause fuselage decompression. The loading within a luggage container was expected to be reduced when the explosive products were vented into the adjacent ULDs. Although previous work has investigated the effect of venting on ULD blast loading, this has only considered a single venting side and not multiple venting configurations. To determine if a multiple-venting system would be beneficial in ULDs, experimental blast testing was performed by subjecting a 1:6 scaled ULD box to representative blast loads with different venting configurations. The blast response of the side of the ULD which would be positioned closest to the fuselage was measured. Numerical simulations were established to provide insight into the blast loading effects not measured experimentally. The loading within the ULD box, in terms of the number and magnitude of blast wave reflections, and internal pressure build-up, was reduced when introducing venting areas. Final deformations were reduced by 11% and 22% when using a single- and double-venting configuration, respectively. Further deformation reduction was expected if more venting area was made available: unconfined blasts tests (demonstrating complete absence of ULD confinement) reduced deformations by 44%. The fully-confined (no venting) blast test resulted in rupture failure when blasted with a 20 g explosive, whereas the vented tests exhibited no tearing when blasted with higher charge masses. The double-venting configuration demonstrated better blast mitigation than the single-venting configuration. However, since both reduced the deformations and rupture probability of the container, the implementation of a multiple-venting system within aircraft ULDs would improve the survivability of the ULD container during a blast event.