Now showing 1 - 10 of 43
  • Publication
    Accurate prediction of blood flow transients : a fluid-structure interaction approach
    (Computational & Mathematical Biomedical Engineering (CMBE), 2009-07-01) ; ;
    Numerical studies are widely employed in establishing blood flow transients in arteries. Unfortunately, many of these are based on rigid arterial geometries where the physiological interaction between the flowing blood and the dynamics of a deforming arterial wall is ignored. Although many recent studies have adopted a fluid-structure interaction (FSI) approach, they lack the necessary validation and, thus, cannot guarantee the accuracy of their predictions. This work employs a well-validated FSI model to establish the dependency of WSS transients on arterial flexibility and predict flow transients in arterial geometries. Results show a high dependency of WSS transients on arterial wall flexibility, with hoop strains of as low as 0.15% showing significant differences in these transients compared to that seen in a rigid geometry. It is also shown that flow in the atherosclerosis susceptible regions of the vascular tree is characterised by a highly disturbed flow. In these regions, WSS magnitudes are at their lowest, while the WSS spatial gradients, rate of change and oscillatory shear index are at their highest.
  • Publication
    Meso-scale thermal and solidification modelling for metallic additive manufacturing processes
    The emergence of additive manufacturing (AM) in recent decades signifies a paradigm shift in how we think about manufacturing. Throughout history, breakthroughs in manufacturing were focused on mass production, with a “one size fits all” mentality. Whilst for large batch size applications this has invariably decreased unit manufacturing costs, increased throughput and decreased prices for customers, it also imposes significant limitations for small batch production. Conventional manufacturing requires many highly specialised steps and equipment, requiring significant resources to establish and setting the barrier to entry unfeasibly high for fledgeling SMEs to enter the manufacturing space. Coupled with this, it inevitably forces manufacturers to be unresponsive to their customers’ needs, as changes to a product or manufacturing process are costly, and require significant machine downtime. Additive manufacturing on the other hand offers virtually limitless freedom to the manufacturer to make changes to a product, even for a one-off bespoke application, without significant machine downtime or costly modification to the manufacturing process. Perhaps even more importantly, since parts are generated additively many of the restrictions that traditional machining imposes on part design no longer apply, allowing for near-net- shape, highly optimised structures to be realised. However, these advantages do not come without a cost. Widespread adoption of AM is still hampered by less than ideal mechanical performance.
  • Publication
    A fluid-structure interaction study of biofilm detachment
    (Computational & Mathematical Biomedical Engineering (CMBE), 2009-07-01) ; ; ;
    During the biofilm development process, bacterial cells may detach from the biofilm into the surrounding fluid. The key question in relation to detachment from bacterial biofilm is the mechanical response to hydrodynamic forces. In this study, a Finite Volume Method (FVM) based Fluid-Structure Interaction (FSI) solver in OpenFOAM package has been developed to model the biofilm response to flow [1]. Dynamic interaction was simulated between an incompressible Newtonian fluid and a bacterial biofilm described as a linear viscoelastic solid. Viscoelastic response of the biofilm was represented by the hereditary integral form of constitutive relation [2] while tensile relaxation modulus was expressed by the Generalised Maxwell Model (GMM) in the form of Prony series (a discrete retardation spectrum). GMM was obtained from the rheometry creep experimental data using a three-step method proposed by Dooling et al. [3]. The creep curves were all viscoelastic in nature and approximated by a linear viscoelastic model represented by Generalised Voigt Model (GVM). Elastic shear modulus (G), obtained from the three-step method, ranged from 583Pa to 1368Pa which were similar to the previous rheometry studies. In this two-dimensional model, biofilm was considered as semi-semispherical shape (thickness of 100μm and width of 346μm) attached to the center of the bottom boundary of the square cross-section flow cell. Fluid flow through the flow cell was in laminar regime. Simulation results predicted the potential site for biofilm detachment subjected to increasing fluid flow rate through the flow cell.
  • Publication
    Mode-mixity in Beam-like Geometries: Linear Elastic Cases and Local Partitioning
    This work is conducted as a part of a wider international activity on mixed mode fractures in beam-like geometries under the coordination of European Structural Integrity Society, Technical Committee 4. In its initial phase, it considers asymmetric double cantilever beam geometry made of a linear elastic material with varying lower arm thickness and constant bending moment applied to the upper arm of the beam. A number of relevant analytical solutions are reviewed including classical Hutchinson and Suo local and Williams global partitioning solutions. Some more recent attempts by Williams, and Wang and Harvey to reproduce local partitioning results by averaging global solutions are also presented. Numerical simulations are conducted using Abaqus package. Mode-mixity is calculated by employing virtual crack closure technique and interaction domain integral. Both approaches gave similar results and close to the Hutchinson and Suo. This is expected as in this initial phase numerical results are based on local partitioning in an elastic material which does not allow for any damage development in front of the crack tip.
  • Publication
    Mode-mixity in beam-like geometries:  global partitioning with cohesive zones
    In-service adhesive joints and composite laminates are often subjected to a mixture of mode I (tensile opening) and mode II (in-plane shear) loads. It is generally accepted that the toughness of such joints can vary depending on the relative amounts of mode I and mode II loading present. From a design perspective, it is therefore of great importance to understand and measure joint toughness under a full range of mode-mixities, thus obtaining a failure locus ranging from pure mode I to pure mode II. The pure mode toughnesses (I, II) can be measured directly from experimental tests. The most common tests being the double cantilever beam (DCB) for mode I and end loaded split (ELS) for mode II. Unfortunately, the analysis of a mixed mode test is not straightforward. In any mixed mode test, one must apply a partition in order to estimate the contributions from each mode. The particular test under study in this work is the fixed ratio mixed mode test (FRMM) with a pure rotation applied to the top beam (fig. 1). In this test, a range of mode-mixities can be obtained by varying γ, where γ is the ratio of h1/h2. This test is normally analysed using analytical or numerical methods, each of which suffers from a number of uncertainties. The present work attempts to shed some light on both analytical and numerical approaches and ultimately develop a testing protocol and recommendations for the accurate determination of modemixity in this FRMM test and other similar beam-like geometries.
  • Publication
    Characterisation of a surrogate lung material made of polyurethane foam and fluid-filled gelatine microcapsules
    (Computational & Mathematical Biomedical Engineering (CMBE), 2009-07-01) ; ;
    In this study, a surrogate lung material, developed to mimic the lungs behaviour in low and high rate impact tests in order to better understand the damage mechanism in the lungs resulting from car crashes, collisions and explosion [1], is tested and characterised. This aims to eliminate the practice of live animal testing. The surrogate lung consists of polyurethane foam mixed with gelatine microcapsules filled with Barium Sulphate solution. Thus, both the foam and microcapsules must be individually characterised in addition to the surrogate lung itself when treated as a continuum material. For this, a number of compression tests were carried out on each material to ascertain their mechanical properties. On the other hand, the damage to the surrogate lung specimens as represented by burst microcapsules was analysed by carrying out CT scans before and after testing. The results show that the modulus of elasticity increases with the test speed. CT scan results clearly demonstrated the magnitude and distribution of damage within the specimen.
  • Publication
    Modelling the Fracture Behaviour of Adhesively-Bonded Joints as a Function of Test Rate - A Rate Dependent CZM is Required to Predict the Full Range of Behaviour
    Adhesive bonding of lightweight, high-performance materials is regarded as a key enabling technology for the development of vehicles with increased crashworthiness, better fuel economy and reduced exhaust emissions. However, as automotive structures can be exposed to impact events during service, it is necessary to gain a sound understanding of the performance of adhesive joints under different rates of loading. Therefore, characterising the behaviour of adhesive joints as a function of loading rate is critical for assessing and predicting their performance and structural integrity over a wide range of conditions. The present work investigates the rate-dependent behaviour of adhesive joints under mode I loading conditions. A series of fracture tests were conducted using tapered double-cantilever beam (TDCB) specimens at various loading rates [1-2]. The experiments were analysed analytically and numerically. The full details of the analysis strategy employing analytical approaches for different types of fracture are presented in [1]. The numerical modelling of the TDCB experiments was performed using the finite-volume based package ‘OpenFOAM’ [3].
  • Publication
    The Bond Gap Thickness Effect on the Fracture Toughness of Nano-Toughened Structural Epoxy Adhesives
    (Adhesion Society, 2010-02-19) ; ; ;
    This work employs combined experimental and numerical studies to investigate the effect of bond gap thickness on the fracture behaviour of a nano-toughened epoxy adhesive produced by Henkel. Tapered-double cantilever-beam (TDCB) joints were subjected to a constant low loading rate. The mode I fracture behaviour of the joints was investigated as a function of bond gap thickness, which was varied from 0.25 to 2.5 mm. A detailed analysis of the fracture surfaces was carried out using scanning electron microscopy (SEM) and variations of the microstructural features with the bond gap thickness, and corresponding constraint, were revealed. The Rice and Tracey void growth model is used to relate the local plastic strain calculated by measuring the size of the voids on the fracture surfaces with the constraint calculated numerically for each bond gap thickness. It was shown that the difference in constraint imposed by different bond gap thicknesses is responsible for the observed dependency of GIC.