Browsing by Author "Cloete, Trevor"

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    Open Access
    A Non-linear Visco-elastic Model for Dynamic Finite Element Simulation of Bovine Cortical Bone
    (2021) Blignaut, Caitlyn; Ismail, Ernesto; Cloete, Trevor
    Modelling and simulation of the human body during an impact situation such as a car accident, can lead to better designed safety features on vehicles. In order to achieve this, investigation into the material properties and the creation of a numerical model of cortical bone is needed. One approach to creating a material model of cortical bone suitable for these situations is to describe the material model as visco-elastic, as reported by Shim et al. [1], Bekker et al. [2] and Cloete et al. [3]. The work by Shim et al. and Bekker et al. developed three-dimensional models, but do not accurately capture the transition in behaviour in the intermediate strain rate region, while Cloete et al. developed a phenomenological model which captures the intermediate strain rate behaviour in one dimension. This work aims to verify and extend these models. The intermediate strain rate regime (1 s−1 to 100 s−1 ) is of particular interest because it is a key characteristic of the behaviour of cortical bone and several studies have been conducted to gather experimental data in this region [3, 4, 5, 6]. The behaviour can be captured using non-linear viscoelastic models. This dissertation focuses on the development and implementation of a material model of cortical bone based on non-linear visco-elastic models to capture the intermediate strain rate regime behaviour. The material model was developed using uni-axial test results from cortical bone. The model by Cloete et al. has been improved and extended, and issues of local and global strain rate with regards to the viscosity have been clarified. A hereditary integral approach was taken in the analysis and implementation of discrete models and was found to be consistent with mathematical models. The model developed was extended to three dimensions in a manner similar to that of Shim et al. and Bekker et al. for implementation in commercial finite element software (LS-Dyna and Abaqus).
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    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
    Design, build and commission a shock tube apparatus for autoignition research
    (2009) Downey, Michael; Yates, Andrew; Cloete, Trevor
    Includes abstract. Includes bibliographical references (p. 65-67).
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    Experimental and numerical study on the effect of strain rate to ductile damage
    (2009) Bowden, Andrew Scott; Cloete, Trevor
    Ductile fracture modelling is extensively used in the automotive, aerospace, aluminium and steel industries. However, these models are often only validated in a limited region of stress states, for example tensile failure by void growth but not shear. In addition, the predictions generally do not include strain rate or temperature effects. Quasistatic tests are often used in calibration, even though many applications such as automotive accidents and ballistic impact operate in the dynamic range. Thus the aims of this thesis were to develop a system to test the damage properties of materials at both quasistatic (≈ 1 s-¹) and dynamic (> 1 x 10³s-¹) strain rates, and then to determine the innfuence of strain rate to ductile fracture.
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    Experimental and numerical study on the effect of strain rate to ductile damage
    (2009) Bowden, Andrew Scott; Cloete, Trevor
    Ductile fracture modelling is extensively used in the automotive, aerospace, aluminium and steel industries. However, these models are often only validated in a limited region of stress states, for example tensile failure by void growth but not shear. In addition, the predictions generally do not include strain rate or temperature effects. Quasistatic tests are often used in calibration, even though many applications such as automotive accidents and ballistic impact operate in the dynamic range. Thus the aims of this thesis were to develop a system to test the damage properties of materials at both quasistatic (≈ 1 s−1 ) and dynamic (> 1×103 s −1 ) strain rates, and then to determine the influence of strain rate to ductile fracture. From the literature the Bai-Wierzbicki damage model was identified as being applicable to the widest range of loading conditions. Thus tests to calibrate this failure locus were conducted on sheet specimens with notches cut into each to introduce non-axial stresses, resulting in a range of loading conditions. This testing procedure involved experimental testing combined with finite element analysis (FEA) to determine the stress and strain state at the position of fracture initiation. All specimens used material from the same sheet of mild steel. To break the dynamic specimens a tensile split Hopkinson pressure bar, or TSHB, was optimized and built. Hopkinson bars are the standard method of conducting high strain rate characterisation tests, however, there is no universal design to examine tensile deformation. The apparatus built used a tubular striker and produced a square input pulse with low noise as desired. Sheet specimens were glued into slotted sections of threaded bar, which in turn screwed into the split Hopkinson bars. This method was successful as in every case the specimens broke before the epoxy. FEA modelling techniques were optimized to minimize computation time. The most important was the use of infinite elements to simulate the bars which, when calibrated, were found to be the ideal method of modelling split Hopkinson bars. Ultimately it was found that strain rate does influence ductile damage. The dynamic specimens failed at a lower strain than the quasistatic equivalents. This indicates that, at high strain rates, fracture strain decreases with strain rate. In contrast, in the quasistatic range strain rate tends to decrease displacement to fracture and thus it is proposed that at quasistatic strain rates, fracture strain increases with strain rate. It is speculated that the degree that strain rate influences ductile fracture is related to the Lode angle, which is a measure of the third deviatoric stress invariant.
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    An experimental and theoretical study on the effect of strain rate on ductile damage
    (2016) Weyer, Matthew; Cloete, Trevor; Govender, Reuben Ashley
    Simulation of fracture in ductile materials is a challenging problem, since it typically occurs at length scales that are orders of magnitude smaller than that of the structures in which the fracture is occurring and, hence, difficult to resolve . One approach is to avoid modelling the micro-mechanics of ductile fracture by describing the macroscopic effects of fracture using damage parameters. Damage in metals can be defined as a measure of discontinuous deformation of a body. Many numerical models include some measure of damage to predict when a material will fracture under certain conditions, however there is little consensus as to what measures and parameters will accurately predict the onset of fracture. Most notably, the effect of strain rate at the point of fracture is significant and must be taken into account. The literature indicates that in the quasistatic regime where inertial effects are negligible, an increase in strain rate increases the strain at fracture. However, the research conducted in this dissertation suggests the opposite is true. The aim of this research is to conduct further high strain rate ductile damage experiments so as to extend the available data set, and develop a pragmatic damage model to relate the plastic strain at fracture to material parameters such as triaxiality, lode angle and strain rate in a specimen, which is verified using experiments performed under various loading conditions and strain rates.
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    An experimental investigation into the anisotropic behaviour of bovine femoral cortical bone
    (2017) Roginsky, Andrew; Cloete, Trevor; Ismail, Ernesto Bram
    To increase our level of knowledge of the human body for various applications, the behaviour of cortical bone needs to be understood. To understand and model the behaviour of cortical bone, knowledge of the strain rate dependent behaviour is required. Many authors have investigated these properties, however, the literature appears to be ambiguous and incomplete, with little focus being placed upon the intermediate strain rate regime (1s⁻¹ to 100s⁻¹). The ambiguity arises as each author presents an averaged data set which does not describe the level of scatter or precise testing methods, nor does it correspond with other authors work [33, 56, 27, 2, 62]. Furthermore, bone should display distinct anisotropic properties due to the microstructural layout. However, no author has published or recorded a complete data set detailing the anisotropy of bone across any species. The intermediate strain rate regime is of particular interest due to Paul [50], capturing a distinct transitional behaviour of cortical bone between low and high strain rates. The apparent lack in intermediate regime research is due to the difficulty in attaining constant strain rate testing conditions within this region using conventional methods. Consequently, due to the absence of data, no accurate model has been developed to simulate the behaviour observed. The focus of this dissertation will therefore be to redesign and fabricate the previously used intermediate strain rate testing device, provide an accurate data set across both quasi-static and dynamic regimes, and a phenomenological model which is able to capture this strain rate dependent behaviour. In order to develop an understanding of the scatter presented in each orientation, light microscopy, inverse light microscopy, and SEM of the specimens is performed. What is observed is that each orientation displays a distinct microstructural layout with fractures propagating in a distinctly different manner based on the strain rate regime. Furthermore, counter to previous findings, the strength of bone across a variety of samples does not appear consistent, however, the longitudinal and radial orientations still display strain rate sensitivity (per sample) which was captured using the improved phenomenological viscoelastic model.
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    Extending the annular in-plane torsional shear test specimen to applications at high strain rates
    (2022) Osman, Muhsin; Ismail, Ernesto; Cloete, Trevor
    The in-plane torsion test is a well-established test used for the characterisation of sheet metals. The specimen is intended to deform in planar simple shear and is designed to be machined with a continuous annular shear zone. As a result, there are no “edge effects” or geometric discontinuities to generate instabilities, thus large true strains up to 1 can be achieved. Before this research, the specimen had only been used for material characterisation in the quasi-static regime. The aim of this research was to conduct further quasi-static testing using the in-plane torsion test and to extend its use into the dynamic regime. Quasi-static tests were performed on a quasi-static torsional (QST) system that was designed to be integrated onto a Zwick universal testing machine. Dynamic tests were performed on a modified torsional split Hopkinson bar (TSHB) system. The TSHB system adopted a nested configuration which allowed for a longer incident bar, and thus larger obtainable strains. Two quick-release mechanisms were used, one using a novel reusable wedge and the other using fracture-pins. All specimens were manufactured from Al 1050 H14. Typical results agreed with material test data available in the literature. Both systems attained large strains at near-constant strain rates and together, allowed for material characterisation over a large range of strain rates. Near-uniform deformations were observed for specimens with lower strain gauge widths. An added feature of the specimen was the flat reverse face, which together with the nested configuration of both systems allows for the possibility for full-field DIC measurement in the future. An estimation method for steady-state flow stress is presented with the steady-state flow stress found to be rate dependant. Finally, a relationship between the steady-state flow stress and strain rate for all experimental results is proposed.
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    SCARAB : development of a rugged, low cost, inspection-class robotic platform
    (2015) Mathew, Thomas J; Cloete, Trevor; Booysen, Tracy
    This dissertation details the design and development of a prototype of a new robotic platform designed to carry a variety of sensors into environments that are too dangerous or confined for human workers, and forms part of a series of three concurrent M.Sc(Eng) dissertations which will integrate into a complete system. Ultimately this platform will be controlled and transported by the man-wearable harness and control station developed by W.K. Fong, and will gather data using the sensor payloads designed by G. Knox. Each dissertation, however, has independently quantifiable goals and results. An important application area for such a system is Urban Search and Rescue (USAR): the field of work concerned with the discovery, extrication, and treatment of survivors trapped in collapsed structures. These typically occur as a result of terrorist attacks, natural disasters, or engineering failure. Human workers, often assisted by dogs, are trained in this work but the danger of the working environments make USAR a key area where the use of robotic assistance can make a massive difference in helping to save lives - both those of rescuees and rescuers. A body of existing work, both in the commercial and academic spheres, has been done in this regard, and as a result there is much to be learned from the experiences of others. The history of robot-assisted USAR work, as well as the existing robots available, is surveyed and critically analysed. Significant challenges are noted: existing systems frequently lack sufficient mobility, are too large, difficult to transport and deploy, difficult to use, and very costly. Their cost has affected the prevalence of their use both as a barrier to acquisition but also during their use; robot operators frequently have their decisions constrained by the financial risk of losing or damaging a robot. Accordingly, it is proposed to develop a small, rugged, low-cost inspection-class robot that can be quickly and easily deployed in a variety of scenarios. This development work is covered in three sections; the mechanical and industrial design of the platform, its design, manufacture and assembly are considered first. This is followed by a description of the electrical and electronic systems needed to power and control the robot as it conducts inspections in challenging terrain. To protect the robot from damage in this terrain, impact-absorbing wheels are developed. The test-driven iterative design approach followed, as well as the equipment and methods used therein, constitute a large portion of this dissertation and are detailed in their own chapter which can be read as a sub-project within the main project. The finished prototype is tested against the developed specifications, and from these results conclusions are drawn and recommendations for future work made.
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    The influence of cylindrical charge geometry on the velocity of blast-driven projectiles in one dimension
    (University of Cape Town, 2020) Qi, Ruixuan; Langdon, Genevieve; Chung Kim Yuen Steeve; Cloete, Trevor
    The impact of improvised explosive devices (IEDs) on the safety of civilians can be devastating, especially when solid objects are inserted into the explosives. These inserts are propelled at high speed and increase the lethality of an IED detonation. Due to the wide range of possible IED configurations, a fundamental understanding of momentum transfer from explosives to the solid inserts is required. This project investigated the influence of charge geometry on the velocity of a 5 mm diameter stainless steel ball bearing. The ball bearing was half-buried and centrally placed on the at face of a cylindrical charge which was detonated centrally on the opposite face. The geometric parameters of interest were the charge diameter and the charge aspect ratio (length/diameter). Investigations were carried out in the project through blast and impact experiments as well as numerical simulations. The impact velocity of the explosively driven ball bearing was inferred using the impact crater depth on a witness plate. The correlation between crater depth and the impact velocity was determined using impact experiments which was performed using a gas gun. The average velocity (between detonation and impact) was captured by tracking the time of detonation and impact. The time of impact was recorded through a Hopkinson Pressure Bar (HPB) behind the witness plate. Additionally, the total axial impulse and the localised impulse, over the face of the HPB, were recorded by a ballistic pendulum and the HPB. Numerical simulations were conducted using a commercial software, Ansys Autodyn 18.0. The blast arrangement was simulated using a two-dimensional, axisymmetric model. The maximum velocity, average velocity, impact velocity, total axial impulse and localised impulse were 'extracted' from the simulations. The simulated velocities agreed well with experimental measurements, showing less than 2% variation. The deformed shape of the blasted ball bearings displayed similar characteristics to the model predictions. There were differences in the simulated impulse, with the numerical model predicting higher magnitudes but a less localised distribution. For a constant charge diameter, the bearing velocity increased in a nearly logarithmic manner with the increase in aspect ratio until a critical aspect ratio of 3/2 was reached. At a constant charge mass, the bearing velocity decreased with the increase in charge diameter. The numerical model suggested that the influence of charge geometry on the bearing velocity was likely caused by the shape of the detonation pressure waves. The detonation pressure profile is sensitive to the charge aspect ratio and the diameter.