Browsing by Author "Gibson, Michael C."

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    Determination of Residual Stress Distributions in Autofrettaged Thick Cylinders
    (Cranfield University, 2008-10-07T12:21:57Z) Gibson, Michael C.; Hameed, Amer
    High pressure vessels such as gun barrels are autofrettaged in order to increase their operating pressure and fatigue life. Autofrettage causes plastic expansion of the inner section of the cylinder – setting up residual compressive stresses at the bore after relaxation. Subsequent application of pressure has to overcome these compressive stresses before tensile stresses can be developed, thereby increasing its fatigue lifetime and safe working pressure. A series of Finite Element (FE) models of hydraulic autofrettage were created, to establish the correct boundary conditions required and means of developing accurate but computationally efficient models. Close agreement was observed between the solutions obtained from the developed models and those from existing analytical and numerical models. These initial models used a simplistic bi- linear stress-strain material representation; this deficiency was then addressed through the development of two means of creating radial position dependent non-linear material behaviour within FE, crucial for accurate prediction of residual stresses. The first utilised a method of altering the elastic properties of the material to achieve nonlinear stress-strain response. This provided accurate results that compared well with existing methods, but was unable to be used in simulation of swage autofrettage due to its elastic nature. The second method achieved non- linear behaviour through direct manipulation of the stress and plastic strain states of the FE model at a fundamental level. This was hence suitable for arbitrary loading procedures, including swage autofrettage. A swage-like model that applied deformation via a band of pressure was developed, to investigate the influence of localised loading and shear stresses that result on the residual stress field. A full model of swage autofrettage was then developed, which was optimised on the basis of accuracy and solution effort. It was then used to investigate the effects of various mandrel and contact parameters on the creation of residual stresses. The model is suitable for use in future optimisation studies of the swage autofrettage procedure.
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    Effects of μCT and FE resolution in expressing anisotropic properties in vertebral cancellous bone
    (Institute of Naval Medicine, 2016-09-15) Shanker, Tobias; Franceskides, Constantinos; Gibson, Michael C.; Clasper, J.; Adams, George; Zioupos, Peter
    With an aging population lower back pain is a growing concern amongst many people. Recent developments in FE have made possible the simulation of complex geometries, such as trabecular bone. Most current techniques homogenise vertebrae into solids with averaged material properties. This is undesirable as analysis on the effects within trabecular tissue is impossible. As vertebral tissue is highly anisotropic this study investigates the effects on anisotropy when the mesh resolution and orientation are varied. Trabecular cubes were taken from a human donor at different orientations around the medial-lateral axis (0°, 45°, 90°) and tested in all three axes. Prior to testing they were CT scanned (X-Tek), reconstructed (CTPro) for and meshed (ScanlP). The full size scans were linearly downsampled to 32pm, 50pm, 64pm, 128pm and 256pm. Using a power-law based on material properties in the literature (E=15GPa, p=1800 g/cm3 and v=0.3) each mesh was quasi-statically compressed in all three directions. Our finite-element analysis shows good agreement with the experimental results, showing that a pixel resolution of 64pm is good for preserving anisotropy in vertebral bone. This model was further validated against the other models at different orientations also showing a good agreement with the experimental results.
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    Evaluation of bone excision effects on a human skull model - I: Mechanical testing and digital image correlation.
    (Sage, 2019-12-06) Franceskides, Constantinos; Leger, Thibault; Horsfall, Ian; Tozzi, Gianluca; Gibson, Michael C.; Zioupos, Peter
    The mechanisms of skull impact loading may change following surgical interventions such as the removal of bone lesions, but little is known about the consequences in the event of subsequent head trauma. We, therefore, prepared acrylonitrile butadiene styrene human skull models based on clinical computed tomography skull data using a three-dimensional printer. Six replicate physical skull models were tested, three with bone excisions and three without. A drop tower was used to simulate the impact sustained by falling backwards onto the occipital lobe region. The impacts were recorded with a high-speed camera, and the occipital strain response was determined by digital image correlation. Although the hole affected neither the magnitude nor the sequence of the fracture pattern, the digital image correlation analysis highlighted an increase in strain around the excised area (0.45%–16.4% of the principal strain). Our approach provides a novel method that could improve the quality of life for patients on many fronts, including protection against trauma, surgical advice, post-operative care, advice in litigation cases, as well as facilitating general biomechanical research in the area of trauma injuries.
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    Evaluation of bone excision effects on a human skull model - II: Finite element analysis
    (Sage, 2019-12-09) Franceskides, Constantinos; Gibson, Michael C.; Zioupos, Peter
    Patient-specific computational models are powerful tools which may assist in predicting the outcome of invasive surgery on the musculoskeletal system, and consequently help to improve therapeutic decision-making and post-operative care. Unfortunately, at present the use of personalized models that predict the effect of biopsies and full excisions is so specialized that tends to be restricted to prominent individuals, such as high-profile athletes. We have developed a finite element analysis model to determine the influence of the location of an ellipsoidal excision (14.2 mm × 11.8 mm) on the structural integrity of a human skull when exposed to impact loading, representing a free fall of an adult male from standing height. The finite element analysis model was compared to empirical data based on the drop-tower testing of three-dimensional-printed physical skull models where deformations were recorded by digital image correlation. In this bespoke example, we found that the excision site did not have a major effect on the calculated stress and strain magnitudes unless the excision was in the temporal region, where the reduction in stiffness around the excision caused failure within the neighboring area. The finite element analysis model allowed meaningful conclusions to be drawn for the implications of using such a technique based on what we know about such conditions indicating that the approach could be both clinically beneficial and also cost-effective for wider use
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    On differences in the equation-of-state for a selection of seven representative mammalian tissue analogue materials
    (Elsevier, 2017-10-10) Appleby-Thomas, Gareth J.; Fitzmaurice, Brianna; Hameed, Amer; Painter, Jonathan; Gibson, Michael C.; Wood, David C.; Hazael, Rachael; Hazell, Paul J.
    Tissue analogues employed for ballistic purposes are often monolithic in nature, e.g. ballistic gelatin and soap, etc. However, such constructs are not representative of real-world biological systems. Further, ethical considerations limit the ability to test with real-world tissues. This means that availability and understanding of accurate tissue simulants is of key importance. Here, the shock response of a wide range of ballistic simulants (ranging from dermal (protective / bulk) through to skeletal simulant materials) determined via plate-impact experiments are discussed, with a particular focus on the classification of the behaviour of differing simulants into groups that exhibit a similar response under high strain-rate loading. Resultant Hugoniot equation-of-state data (Us-up; P-v) provides appropriate feedstock materials data for future hydrocode simulations of ballistic impact events.
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    Simulated impact response of a 3-D printed skull, with an ellipsoidal excision, using finite element analysis
    (European Society of Biomechanics, 2016-07) Gibson, Michael C.; Franceskides, Constantinos; Zioupos, Peter
    This paper investigates methods of determining the influence of an ellipsoidal excision (14.2x11.8 mm occipital region) on the structural integrity of a human skull when exposed to impact loading. Experimental and simulation-based analyses were conducted, using 3-D printed replicas and a finite element model; both were derived from a clinical CT scan of the patient (28 YO MC, with no previous health concerns). Previous simulation studies [1] have achieved managed to predict skull fracture locations effectively for nonexcised skulls.
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    Spinal Motion Segments — I: Concept for a Subject-specific Analogue Model
    (Springer, 2020-06-24) Franceskides, Constantinos; Arnold, Emily; Horsfall, Ian; Tozzi, Gianluca; Gibson, Michael C.; Zioupos, Peter
    Most commercial spine analogues are not intended for biomechanical testing, and those developed for this purpose are expensive and yet still fail to replicate the mechanical performance of biological specimens. Patient-specific analogues that address these limitations and avoid the ethical restrictions surrounding the use of human cadavers are therefore required. We present a method for the production and characterisation of biofidelic, patient-specific, Spine Motion Segment (SMS = 2 vertebrae and the disk in between) analogues that allow for the biological variability encountered when dealing with real patients. Porcine spine segments (L1–L4) were scanned by computed tomography, and 3D models were printed in acrylonitrile butadiene styrene (ABS). Four biological specimens and four ABS motion segments were tested, three of which were further segmented into two Vertebral Bodies (VBs) with their intervertebral disc (IVD). All segments were loaded axially at 0.6 mm·min−1 (strain-rate range 6×10−4 s−1–10×10−4 s−1). The artificial VBs behaved like biological segments within the elastic region, but the best two-part artificial IVD were ∼15% less stiff than the biological IVDs. High-speed images recorded during compressive loading allowed full-field strains to be produced. During compression of the spine motion segments, IVDs experienced higher strains than VBs as expected. Our method allows the rapid, inexpensive and reliable production of patient-specific 3D-printed analogues, which morphologically resemble the real ones, and whose mechanical behaviour is comparable to real biological spine motion segments and this is their biggest asset.
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    Spinal Motion Segments — II: Tuning and Optimisation for Biofidelic Performance
    (Springer, 2020-06-24) Franceskides, Constantinos; Arnold, Emily; Horsfall, Ian; Tozzi, Gianluca; Gibson, Michael C.; Zioupos, Peter
    Most commercially available spine analogues are not intended for biomechanical testing, and the few that are suitable for using in conjunction with implants and devices to allow a hands-on practice on operative procedures are very expensive and still none of these offers patient-specific analogues that can be accessed within reasonable time and price range. Man-made spine analogues would also avoid the ethical restrictions surrounding the use of biological specimens and complications arising from their inherent biological variability. Here we sought to improve the biofidelity and accuracy of a patient-specific motion segment analogue that we presented recently. These models were made by acrylonitrile butadiene styrene (ABS) in 3D printing of porcine spine segments (T12–L5) from microCT scan data, and were tested in axial loading at 0.6 mm·min−1 (strain rate range 6×10−4 s −1 – 10×10−4 s−1 ). In this paper we have sought to improve the biofidelity of these analogue models by concentrating in improving the two most critical aspects of the mechanical behaviour: the material used for the intervertebral disc and the influence of the facet joints. The deformations were followed by use of Digital Image Correlation (DIC) and consequently different scanning resolutions and data acquisition techniques were also explored and compared to determine their effect. We found that the selection of an appropriate intervertebral disc simulant (PT Flex 85) achieved a realistic force/displacement response and also that the facet joints play a key role in achieving a biofidelic behaviour for the entire motion segment. We have therefore overall confirmed the feasibility of producing, by rapid and inexpensive 3D-printing methods, high-quality patient-specific spine analogue models suitable for biomechanical testing and practice
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    Subject-specific functional model of hard and soft tissues; skull and spine
    (Cranfield University, 2018) Franceskides, Constantinos; Zioupos, Peter; Gibson, Michael C.
    There is a strong demand for mechanically and morphologically accurate models of the human musculoskeletal system, particularly of the spine. Such models would have multiple applications, including surgical guides, the analysis of implant fitment and design, as well as individual strength evaluation. Current standards such as the ASTM F1717 (devised for the static and dynamic testing of implants) represent complex spine morphologies using simplified blocks of homogeneous material generally constructed from ultra-high-molecular-weight polyethylene (UHMWPE). These do not attempt to replicate morphological characteristics, and therefore do not reproduce mechanical loading properties, especially when considering the complexity of vertebral bodies and their facets. The work described in this thesis investigated the creation of a compressively accurate and validated model of a lumbar motion segment, specifically the validity of technologies such as computed tomography (CT) scanning, computer-aided scan reconstruction, rapid prototyping, digital image correlation (DIC) and finite element analysis (FEA) modelling. In particular, DIC (an optical measurement method) allowed full-field measurements of the displacements and strains. This was used to determine loading paths and magnitudes during the testing procedure. To complement this approach, FEA modelling identified the location and severity of maximum strains for subsequent comparison to the DIC and mechanical testing data. All FEA models were based on CT scan datasets of the modelled cadaveric material, and were validated against the ex vivo mechanical test measurements. The research followed a number of core stages: 1. First, the applicable technologies were tested and verified, with all channels indicating closely related data. This was achieved by the compressive loading of two types of analogue skulls, allowing the validation of DIC as a data acquisition technique in complex structures. Validation against FEA models demonstrated their potential to provide further insight into the experimental results. The initial testing identified a well-defined pathway for a sample manufacturing and preparation process, making it much easier to produce reliable analogues for subsequent experiments. ii 2. In the second stage, analogue motion segments (AMss) were created using the CT scan datasets obtained from the cadaveric porcine specimens. Motion analysis provided a better understanding of the loading paths again by using DIC as an appropriate data acquisition system. Following the creation of the AMS, different materials were considered for the creation of intervertebral discs (IVDs). The mechanically most biofidelic material was selected. 3. Finally, a sensitivity study was carried out to determine a relationship between the scanning resolution and model accuracy for both the mechanical analogue and the FEA model. The use of 3D printing was found to be an effective, efficient and economical strategy for the creation of accurate biomechanical analogues. Furthermore, DIC was a useful tool when looking at individual component strains and displacements. Finally, when considering a motion segment, the majority of the elastic loading – and thus its behaviour on the whole – was governed by the material properties of the IVD simulant. This research demonstrated a clear path towards the creation of a reliable, biofidelic motion segment, or even a partial lumbar spine analogue, that would comply in dynamic and static loading scenarios as well as conformity in compression. The capability of the techniques and the compliance and accuracy of the resulting models was confirmed by developing both analogue mechanical models and FE simulations. Given their potential advantages, it is only a matter of time before mechanical analogues and their corresponding digital models replace the outdated and inaccurate testing standards in our current medical facilities and research centres.