[Show abstract][Hide abstract] ABSTRACT: In recent years cohesive elements, coupled with a finite-element analysis (FEA) approach, have become increasingly popular for simulating both delamination in composite materials and fracture in adhesively-bonded joints. However, the industrial application of Cohesive Zone Models to model large and complex structures has been hindered by the requirement of extremely fine meshes along the crack propagation path. In the present work two-dimensional linear and quadratic (i.e. second-order) cohesive elements to model crack initiation and growth have been implemented in Abaqus using a user subroutine. These elements, which have a modified topology that allows a user-defined number of integration points, have been employed to model the fracture response of various mode I test specimens consisting of metallic substrates bonded with a structural film-adhesive. The effects of the mesh-density, element order and number of integration points on the numerical solution have been investigated. Whilst the linear models have shown the typical mesh-size dependent behaviour, the results obtained with their quadratic counterparts have been found to be independent of the element size. Furthermore, it is shown that increasing the number of integration points improves the stability, convergence and smoothness of the solutions. The mesh-size independent response obtained with the quadratic models arises from more accurate simulation of the deformed profile of the substrates and a more accurate calculation of the energy dissipated in the process zone due to damage. Overall, it is demonstrated that the quadratic cohesive-element formulation enables the use of much coarser meshes, resulting in shorter simulation times, and will therefore allow an increase in the industrial application of Cohesive Zone Models.
[Show abstract][Hide abstract] ABSTRACT: The paper describes the results of a comprehensive study of turbulent mixing, fuel spray dispersion and evaporation and combustion in a gas-turbine combustor geometry (the DLR Generic Single Sector Combustor) with the aid of Large Eddy Simulation (LES). An Eulerian description of the continuous phase is adopted and is coupled with a Lagrangian formulation of the dispersed phase. The sub-grid scale (sgs) probability density function approach in conjunction with the stochastic fields solution method is used to account for sgs turbulence-chemistry interactions. Stochastic models are used to represent the influence of sgs fluctuations on droplet dispersion and evaporation. Two different test cases are simulated involving reacting and non-reacting conditions. The simulations of the underlying flow field are satisfying in terms of mean statistics and the structure of the flame is captured accurately. Detailed spray simulations are also presented and compared with measurements where the fuel spray model is shown to reproduce the measured Sauter Mean Diameter (SMD) and the velocity of the droplets accurately.
[Show abstract][Hide abstract] ABSTRACT: Aromatics form an integral part of typical aviation fuels with n-propyl benzene selected as a representative molecule for inclusion in several EU and US surrogate blends used for design calculations. Despite the practical relevance, kinetic and thermodynamic data obtained using comparatively accurate ab initio methods have to date not been compared with currently used reaction class based estimates. The use of ab initio methods for comparatively complex molecules also necessitates an assessment of the relative benefits of higher levels of theory as it is typically desirable to balance the accuracy of the treatment of individual reactions with the need to consider more complete reaction sequences. The current study examines six hydrogen extractions, via the hydrogen or methyl radicals from the n-propyl side chain. Potential energy surfaces were determined using 10 different approaches, including state-of-the-art DFT (M06, M06-2X and M08-SO) and contemporary composite methods (G4, G4MP2, CBS-QB3 and CBS-4M). Results are presented relative to data obtained using the CCSD(T)/jun-cc-pVTZ//M06-2X/6-311++G(3df,3pd) coupled cluster based method. Rate parameters were determined using transition state theory combined with (i) small curvature tunnelling and energetics at the M06-2X/6-31G(2df,p) level and (ii) Eckart tunnelling corrections and energetics at the CCSD(T)/jun-cc-pVTZ level. Results were found to agree comparatively well with modest differences in rates for several reactions. However, it is also shown that substantial deviations can arise with respect to reaction class based estimation techniques.
[Show abstract][Hide abstract] ABSTRACT: BACKGROUND:An incorrect femoral tunnel position or inappropriate graft tensioning during medial patellofemoral ligament (MPFL) reconstruction may cause altered patellofemoral joint kinematics and contact mechanics, potentially resulting in pain and joint degeneration. HYPOTHESIS:Nonanatomic positioning of the tunnel or graft overtensioning during MPFL reconstruction will have an adverse effect on patellar tracking and patellofemoral joint contact mechanics. STUDY DESIGN:Controlled laboratory study. METHODS:Eight fresh-frozen cadaveric knees were placed on a customized testing rig, with the femur fixed and the tibia mobile through 90° of flexion. Individual heads of the quadriceps muscle and the iliotibial band were separated and loaded with 205 N in anatomic directions using a system of cables and weights. Patellofemoral contact pressures and patellar tracking were measured through the flexion range at 10° intervals using Tekscan pressure-sensitive film inserted between the patella and trochlea and an optical tracking system. The MPFL was transected and then reconstructed using a double-strand gracilis tendon graft. Pressures and kinematics were recorded for reconstructions with the graft positioned in anatomic, proximal, and distal tunnel positions. Measurements were then repeated with an anatomic tunnel and graft tension of 2 N, 10 N, and 30 N, fixed at 3 different flexion angles of 0°, 30°, and 60°. Statistical analysis was undertaken using repeated-measures analysis of variance, Bonferroni post hoc analysis, and paired t tests. RESULTS:For a graft tensioned to 2 N, anatomically positioned MPFL reconstruction restored intact medial and lateral joint contact pressures and patellar tracking (P > .05), but femoral tunnels positioned proximal or distal to the anatomic origin resulted in significant increases in peak and mean medial pressures and medial patellar tilt during knee flexion or extension, respectively (P < .05). Grafts tensioned with 10 N or 30 N also caused significant increases in medial pressure and tilt. Graft fixation at 30° or 60° restored all measures to intact values (P > .05), but fixation at 0° caused significant increases (P < .05) in medial joint contact pressures compared with intact knees. CONCLUSION:Anatomically positioned reconstruction with 2-N tension fixed at 30° or 60° of knee flexion restored joint contact pressures and tracking. However, graft overtensioning or femoral tunnels positioned too proximal or distal caused significantly elevated medial joint contact pressures and increased medial patellar tilting. The importance of a correct femoral tunnel position and graft tensioning in restoring normal patellofemoral joint kinematics and articular cartilage contact stresses is therefore evident. CLINICAL RELEVANCE:A malpositioned femoral tunnel or overtensioned graft during MPFL reconstruction resulted in increased medial contact pressures and patellar tilting. This may lead to adverse outcomes such as early degenerative joint changes or pain if occurring in a clinical population.
[Show abstract][Hide abstract] ABSTRACT: Corrosion is a major issue in industry and inspection and monitoring for wall thickness loss are important to assess the structural integrity of pipework and process vessels. Manual ultrasonic thickness measurements are widely used; however, they are also notoriously unreliable because of operator errors. Therefore, automated inspection scans and monitoring at fixed locations with permanently installed sensors are becoming more attractive; they help to remove some of the error introduced by operators. However, this raises the question of what the underlying uncertainties of automated ultrasonic wall thickness measurements are. A key contributor to the uncertainty is the surface roughness condition and the authors have been researching this topic for some time. This talk will give an overview of the physics of scattering of ultrasonic waves from rough corroded surfaces. The different scales of roughness will be discussed, and a simulation technique based on the Distributed Point Source Method (DPSM) to model the scattering and its experimental validation will be presented. The need for statistical results makes both the speed and accuracy of the simulation very important. Finally, based on the simulations, results of the estimated ultrasonic measurement errors due to roughness are presented.
The Journal of the Acoustical Society of America 11/2013; 134(5):4132.
[Show abstract][Hide abstract] ABSTRACT: Percutaneous needle insertions are a common part of minimally invasive surgery. However, the insertion process is necessarily disruptive to the substrate. Negative side effects are migration of deep-seated targets and trauma to the surrounding material. Mitigation of these effects is highly desirable, but relies on a detailed understanding of the needle-tissue interactions, which are difficult to capture at a sufficiently high resolution. Here, an adapted Digital Image Correlation (DIC) technique is used to quantify mechanical behaviour at the sliding interface, with resolution of measurement points which is better than 0.5mm, representing a marked improvement over the state of the art. A method for converting the Eulerian description of DIC output to Lagrangian displacements and strains is presented and the method is validated during the simple insertion of a symmetrical needle into a gelatine tissue phantom. The needle is comprised of four axially interlocked quadrants, each with a bevel tip. Tests are performed where the segments are inserted into the phantom simultaneously, or in a cyclic sequence taking inspiration from the unique insertion strategy associated to the ovipositor of certain wasps. Data from around the needle-tissue interface includes local strain variations, material dragged along the needle surface and relaxation of the phantom, which show that the cyclic actuation of individual needle segments is potentially able to mitigate tissue strain and could be used to reduce target migration.
Journal of the mechanical behavior of biomedical materials. 10/2013; 30C:50-60.
[Show abstract][Hide abstract] ABSTRACT: The success of Total Shoulder Arthroplasty (TSA) is believed to depend on the restoration of the natural anatomy of the joint and a key development has been the introduction of modular humeral components to more accurately restore the patient's anatomy. However, there are no peer-reviewed studies that have reported the degree of glenoid component mal-position achieved in clinical practice and the clinical outcome of such mal-position. The main purpose of this study was to assess the accuracy of glenoid implant positioning during TSA and to relate it to the radiological (occurrence of radiolucent lines and osteolysis on CT) and clinical outcomes.
68 TSAs were assessed with a mean follow-up of 38+/-27 months. The clinical evaluation consisted of measuring the mobility as well as of the Constant Score. The radiological evaluation was performed on CT-scans in which metal artefacts had been eliminated. From the CT-scans radiolucent lines and osteolysis were assessed. The positions of the glenoid and humeral components were also measured from the CT scans.
Four position glenoid component parameters were calculated The posterior version (6°±12°; mean ± SD), the superior tilt (12°±17°), the rotation of the implant relative to the scapular plane (3°±14°) and the off-set distance of the centre of the glenoid implant from the scapular plane (6±4 mm). An inferiorly inclined implant was found to be associated with higher levels of radiolucent lines while retroversion and non-neutral rotation were associated with a reduced range of motion.
this study demonstrates that glenoid implants of anatomic TSA are poorly positioned and that this malposition has a direct effect on the clinical and radiological outcome. Thus, further developments in glenoid implantation techniques are required to enable the surgeon to achieve a desired implant position and outcome.
[Show abstract][Hide abstract] ABSTRACT: This article describes a novel method for image-based, minimally invasive registration of the femur, for application to computer-assisted unicompartmental knee arthroplasty. The method is adapted from the well-known iterative closest point algorithm. By utilising an estimate of the hip centre on both the preoperative model and intraoperative patient anatomy, the proposed 'bounded' iterative closest point algorithm robustly produces accurate varus-valgus and anterior-posterior femoral alignment with minimal distal access requirements. Similar to the original iterative closest point implementation, the bounded iterative closest point algorithm converges monotonically to the closest minimum, and the presented case includes a common method for global minimum identification. The bounded iterative closest point method has shown to have exceptional resistance to noise during feature acquisition through simulations and in vitro plastic bone trials, where its performance is compared to a standard form of the iterative closest point algorithm.
Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine. 08/2013;
[Show abstract][Hide abstract] ABSTRACT: Abstract In the sport of cricket the objective of the "no-ball" law is to allow no performance advantage through elbow extension during ball delivery. However, recently it has been shown that even bowlers with actions that are considered within the law show some elbow extension. The objective of this study was to investigate:  the effect of elbow orientation during anatomical landmark digitisation and  the choice of upper arm tracking cluster on the measurement of elbow angles during cricket bowling. We compared the mean elbow angles for four different elbow postures; with the joint flexed at approximately 130°, 90°, in full extension and with the elbow flexed with the humerus internally rotated, and two upper arm clusters in two different situations:  during a controlled movement of pure flexion-extension and  during cricket bowling. The digitised postures of the anatomical landmarks where the elbow was extended and at 90° of flexion were more repeatable than the other two postures. The recommendation of this study when analysing cricket bowling is to digitise the humeral epicondyles with the joint flexed at 90°, or in full extension, and to relate their positions to an upper arm cluster fixed close to the elbow.
[Show abstract][Hide abstract] ABSTRACT: This paper examines the fundamental mechanisms of synovial fluid lubrication in artificial joints. Film thickness measurements were made for bovine serum solutions in a model test device. In contact imaging was also carried out to aid interpretation of these results. The results indicated that two types of film are formed; a boundary layer of adsorbed protein molecules, which are augmented by a high-viscosity fluid film generated by hydrodynamic effects. The high-viscosity film is due to inlet aggregation of protein molecules forming a gel which is entrained into the contact preferentially at low speeds. As the speed increases this gel appears to shear thin, giving much lower lubricant film thickness. Results suggest that protein-containing fluids do not obey classical Newtonian EHL models.
Tribology International 07/2013; 63:97–104.
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