Now showing 1 - 10 of 43
  • 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
    An open-source finite volume toolbox for solid mechanics and fluid-solid interaction simulations
    Over the past 30 years, the cell-centred finite volume method has developed to become a viable alternative to the finite element method in the field of computational solid mechanics. The current article presents an open-source toolbox for solid mechanics and fluid-solid interaction simulations based on the finite volume library OpenFOAM. The object-oriented toolbox design is outlined, where emphasis has been given to code use, comprehension, maintenance and extension. The toolbox capabilities are demonstrated on a number of representative test problems, where comparisons are given with finite element solutions.
  • 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
    The Effect of Prepeg Storage Humidity on Co-cured Composite Joints
    The increasing use of composite materials in the aerospace industry has driven a need for a greater understanding of bonded composite joints. There are generally two types of composite joint used in the aerospace industry; secondary bonded joints and cocured joints. Secondary bonded joints are produced by bonding two cured composite laminates together with an adhesive. However, when composites and adhesives are used to manufacture large parts in the aerospace industry, it is often convenient to co-cure the two materials at the same time. This helps to reduce the high costs associated with autoclave curing and also to reduce processing time. However, despite the apparent advantages, co-curing is not without its drawbacks. Any moisture stored in the composite material prior to co-curing is released during the cure cycle and has a negative effect on the joint. This can also result in interfacial failure. A way around this problem is to either dry the composite material prior to curing or to engineer the composite surface using a variety of surface treatments to promote adhesion, such as an atmospheric pressure plasma treatment [1]. The former option will be investigated in this work. The effects of moisture on the fracture performance of secondary bonded composite joints is well publicised. Moisture can be introduced into the composite laminate prior to [2] or after [3] secondary bonding. The moisture can plasticize the adhesive and reduce the glass transition temperature of the adhesive [4]. However, compared to secondary bonded joints, relatively little work has been carried out on co-cured joints. In the present work, the effect of the level of moisture in the composite prepreg prior to co-curing will be examined.
  • 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
    Evolution of dynamic fractures in PMMA : experimental and numerical investigations
    (WIT Press / Computational Mechanics, 2004-10-20) ; ; ; ;
    A combined experimental/numerical study has been conducted to investigate dynamic fractures in poly(methyl methacrylate) (PPMA). The results obtained from single-edge-notched-tensile (SENT) fracture tests support the idea that the evolution of fracture in PMMA is governed by nucleation, growth and coalescence of penny-shaped micro-cracks. The density of the microcracks and therefore the roughness of the fracture surface increase with the crack velocity. Both the surface roughness and the size of the process region increase with the crack length for a given specimen. Microscopy of the virgin material and fractured surfaces showed no consistent evidence of pre-existing flaws, dust particles or other impurities that would provide nucleation sites for the micro-cracks. Instead, it was observed that molecular weight significantly affects the fracture, and therefore must play an important role in the nucleation of micro-cracks. The crack velocity measurements show rapid initial crack acceleration followed by a nearly constant mean velocity, which was in some cases well above previously reported terminal crack speed. The mean velocity is found to increase with decreasing initial notch depth. Oscillations in the crack velocities were also observed and they were more pronounced at higher crack velocities. To a large extent, the degree of crack velocities oscillations is dependent on the filtering technique applied to process the raw experimental data. Therefore, no conclusive correlation between the fracture histories and fracture surfaces was obtained. Finite Volume (FV) method was developed for the numerical simulations of the experiments. Global material behaviour was approximated as linear elastic, while a Cohesive Zone Model (CZM) was used for defining the local separation process of the material. Numerical predictions show good agreement with experimentally observed variations of the process region and the crack velocity with initial crack length. Oscillations in the crack speed are also predicted.
  • Publication
    Fracture properties of PCBN as a function of loading rate and temperature
    (Trans Tech Publications, 2010-11) ; ; ;
    Polycrystalline Cubic Boron Nitride (PCBN) is a super-hard material used in some of the most demanding material removal operations today. These include turning of hardened steels, as well as the machining of highly abrasive alloys. In these applications the tools are subjected to high operating temperatures, abrasive and impact loading. This can lead to the brittle fracture of the tool. Accurate determination of the fracture toughness and mechanical properties of PCBN under a wide range of operating conditions is therefore essential in order to evaluate the performance of the tool under these highly demanding conditions. For this study, a laboratory scale three point bend test rig has been used for the fracture tests. The fracture toughness of two different grades of PCBN are measured at a range of loading rates and temperatures corresponding to the actual in-service conditions. The results show the measured properties of these materials vary with both loading rate and temperature. The fracture surfaces of the specimens are examined using scanning electron microscopy to determine dominant fracture mechanisms
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