6,378 results for Conference item

  • Stresses and cracks in a theoretical model of a metal-backed tibial knee replacement component

    Kelly, Piaras; O'Connor, JJ (1996)

    Conference item
    The University of Auckland Library

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  • Stress distributions within rapidly loaded cartilage - possible contributions to arthrosis

    O'Connor, JJ; Kelly, Piaras (1994)

    Conference item
    The University of Auckland Library

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  • Challenges for Modelling Filling during Liquid Composite Moulding Processes

    Bickerton, Simon; Buntain, MJ; Kelly, Piaras (2003)

    Conference item
    The University of Auckland Library

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  • Modelling the Viscoelastic Compression Behaviour of Fibrous Reinforcing Fabrics

    Bickerton, Simon; Kelly, Piaras; Buntain, MJ (2002)

    Conference item
    The University of Auckland Library

    The majority of fiber reinforced plastic manufacturing processes involve compressive deformation of the reinforcing material to be included in the product. The development of accurate process simulations will in many cases be dependent on our ability to provide good models for the deformation of these reinforcing structures. The viscoelastic behavior of preform materials in the absence of any matrix has been investigated, and the implications for Liquid Composite Molding (LCM) processes considered. Extending existing Resin Transfer Molding simulations to predict local and total tooling forces will require the development of viscoelastic preform deformation models. For other LCM processes that involve cavity thickness changes during mould filling, an accurate preform deformation model is essential in order to simulate mould filling. An experimental study is presented demonstrating the strong time dependent viscoelastic compression behavior of common LCM preform materials. A nonlinear viscoelastic model has been developed for constant speed preform compaction. The required empirical parameters have been derived from a series of characterization experiments performed on the preform materials studied.

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  • Modelling Forces Acting on Rigid, Non-planar Liquid Composite Moulding Tools

    Bickerton, Simon; Kelly, Piaras (2005)

    Conference item
    The University of Auckland Library

    The term Liquid Composite Molding (LCM) encompasses a growing list of composite manufacturing processes, including Resin Transfer Molding (RTM), Injection/Compression Molding (I/CM), and Vacuum Assisted RTM (VARTM). The RTM and I/CM processes utilise two-piece rigid moulds, which are subject to force components due to reinforcement compaction and internally generated resin pressure. The focus of this paper is prediction of tooling forces for RTM, which will allow efficient structural design of moulds, and selection of cost effective process parameters. Previous experimental work has demonstrated the influence of reinforcement compaction behaviour, which is strongly non-elastic. A viscoelastic compaction model has been developed which addresses both dry and wet response, and is implemented in RTM simulations of simple flat parts. Non-planar geometries introduce a tangential stress acting on mould surfaces, due to shear of the reinforcement. The tooling force analysis is extended to complex parts using an existing RTM filling simulation, LIMS, which has been developed at the University of Delaware.

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  • Mechanical design of joints incorporating metal interlayers

    Kelly, Piaras; Hills, DA; Nowell, D (1989)

    Conference item
    The University of Auckland Library

    We undertake a theoretical study of the strength of a joint modelled as two rigid blocks (ceramic components) bonded by a thin, comparatively soft metal interlayer, loaded in tension. Slip-line field theory and the upper bound theorem enables us to suggest firm guidelines for the maximum thickness of the interlayer which enables the full strength of the ceramic to be achieved, by exploiting the constraint implicit in a thin layer. We go on to investigate the influence of flaws (poorly bonded regions) and show how a comparatively small region of poor adhesion (~5% debonding) can significantly reduce the strength of the joint. Quantitative information about the amount of de-bonding which can be tolerated in a particular configuration without loss of strength will be presented.

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  • The analysis of surface-breaking interface cracks

    Kelly, Piaras; Hills, DA; Nowell, D (1992)

    Conference item
    The University of Auckland Library

    This paper discusses the analysis of a crack along an interface between two dissimilar elastic quarter planes for cases where the crack breaks the free surface. A dislocation density method of analysis is employed and the appropriate dislocation solution and far-field stress expressions are highlighted. Models for crack tip behaviour are discussed and it is shown that a simplified quadrature can be used to extract the crack extension force. Sample results for an interface crack loaded by constant crack face pressure are given.

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  • The quasi-static approximation for cracked interfaces in layered systems

    Pecorari, C; Kelly, Piaras (1999)

    Conference item
    The University of Auckland Library

    For the last two decades the quasi-static approximation (QSA) has been the most commonly used approach for describing the interaction of ultrasonic waves with imperfect interfaces. The QSA is a low-frequency approximation and it can be used when the thickness of the interface is much smaller than the wavelength of the waves used to inspect the interface. Its most complete formulation has been presented by Baik and Thompson [1], and models the real interfacial imperfections as continuous, uniform distributions of springs and masses along the interface plane (see Fig 1).

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  • Nonconforming elements for liquid composite molding process simulations

    Kelly, Piaras; Jennings, S (2006)

    Conference item
    The University of Auckland Library

    Liquid Composite Molding (LCM) processes are now a prevalent group of manufacturing methods for advanced composite materials. They offer many advantages over more traditional manufacturing methods, such as the ability to deal with large and complex shapes. Numerical simulations can lead to better predictions of process parameters. The standard procedure for the simulation of these processes is to use a Control Volume (CV) method. One problem with the CV method is that resin mass is not conserved on an element level, and this has consequences for accuracy. An attractive alternative to the CV method is to use a single grid of non-conforming finite elements. Such non-conforming elements encompass essential mass conservation properties. In this study it is shown how the standard non-conforming triangular element can be adjusted to ensure mass conservation on the element level and to ensure continuity of the fluid flux across inter-element boundaries. Numerical experiments are carried out which show that single grids of such elements, and nonconforming quadrilateral elements, produce accurate results in the case of the Injection Compression Molding process.

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  • Compaction of Dry and Wet Fibrous Materials during Infusion Processes

    Kelly, Piaras; Umer, Rehan; Bickerton, Simon (2004)

    Conference item
    The University of Auckland Library

    The objective of this study was to investigate the response of fibrous materials to compaction, since this response can affect significantly a number of important parameters, e.g. required tooling forces and fill-times, for some resin infusion manufacturing processes. A series of compression tests were carried out on both dry and wet (resin-impregnated) samples, at a number of different compaction speeds and to various final volume fraction levels. The materials were seen to exhibit a significant viscoelastic (stress relaxation) response, which changed according to whether the fibers were dry or wet. A thermomechanical model of fibrous material deformation was developed, incorporating the observed non-linear viscoelastic response, and the wet-dry change in response. The model is appropriate for the simple fibre compaction deformation which occurs during many of the liquid composite molding (LCM) processes. The model gives reasonably good results over a range of fiber volume fractions and compression speeds.

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  • Numerical Simulation of Liquid Composite Molding Processes

    Kelly, Piaras; Bickerton, Simon (2005)

    Conference item
    The University of Auckland Library

    Liquid Composite Molding (LCM) includes a range of composite materials manufacturing processes, for example Resin Transfer Molding (RTM), Injection Compression Molding (I/CM) and flexile-bag processes such as Vacuum Assisted Resin Transfer Molding (RTM). In these processes, resin is injected under pressure into a fully or partially-compacted fibrous preform before final cure. Mathematical models and numerical simulations of these manufacturing processes lead to better predictions of the filling times and preform final thicknesses, and of the optimal position of inlet gate location and inlet pressures. In this study, the governing equations of LCM are solved using a number of different variants of the Finite Element Method. In particular, a single fixed grid scheme which conserves fluid mass on the element level, and which ensures continuity of fluid flux across inter-element boundaries, is used and is shown to give accurate results.

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  • Modeling the viscoelastic behaviour of fibre reinforcing fabrics

    Kelly, Piaras; Umer, RA (2004)

    Conference item
    The University of Auckland Library

    The deformation of fibrous materials plays an important role in the Liquid Composite Molding (LCM) processes. These materials display markedly non-linear viscoelastic characteristics. A model of fibrous material deformation is developed, incorporating this non-linear viscoelastic response, with a view to being used in simulations of LCM processes. The model is one-dimensional and is appropriate for the simple fibre compaction deformation which occurs during LCM processes. A limited number of compaction experiments were carried out to determine the model parameters for a continuous filament mat. These included a rapid compaction (100mm/min), a number of slower compactions to the same final volume fraction, and then a number of tests at a constant compaction speed to different final volume fractions. The model gives reasonably good results over a range of compaction speeds and volume fractions.

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  • Towards a Complete Tooling Force Analysis of Liquid Composite Moulding Processes

    Bickerton, Simon; Kelly, Piaras (2007)

    Conference item
    The University of Auckland Library

    The term Liquid Composite Moulding (LCM) encompasses a growing list of composite manufacturing processes. Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM) utilise two-piece, rigid mould tools, and are the focus of this paper. Moulds used in these processes must supply forces to compact fibre reinforcement, and to balance resin pressures generated within the cavity. An RTM and I/CM simulation is presented, for which a complete tooling force analysis has been incorporated. Complex geometries require the consideration of both the normal and shear stress components acting on mould surfaces. Our previous experimental work has demonstrated the complex time dependency of tooling forces, due to non-elastic compaction behaviour of fibre reinforcements. A combination of elastic models is applied here, modeling phenomena such as stress relaxation and fluid lubrication of the reinforcement in a simple manner. A simple friction based model is included, allowing consideration of complex non-planar part geometries. Two industrial components are used to demonstrate the benefits of a complete tooling analysis. Such analyses are required as the application of the filling simulation is extended to processes within flexible and semi-rigid moulds.

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  • Predicting Stress Distributions Exerted on LCM Tools using Visco-elastic Compaction Models

    Walbran, William; Bickerton, Simon; Kelly, Piaras (2007)

    Conference item
    The University of Auckland Library

    Mould tools used for LCM processes such as Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM) must withstand the forces applied during compaction of the preform and injection of the resin. A simulation package has been developed at the University of Auckland to predict the forces applied to moulds during these processes. This will allow cost effective mould design and process selection. The simulation package utilizes a viscoelastic compaction model, which has been characterized for a glass-fibre chopped strand mat. Several RTM and I/CM experiments have been carried out, the local stress distribution being monitored throughout, as well as the total clamping force applied to the mould. The influence of the fluid pressure on the total stress experienced by the mould is shown to be a function of both the fibre volume fraction and the injection pressure, the latter being more pronounced at low volume fractions. The stresses observed during I/CM are significantly higher than during RTM, however the process times for I/CM are considerably lower. These experiments have been compared to initial calculations performed with the LCM simulation, and good agreement has been shown for the normal stress distributions and total clamping force evolutions recorded.

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  • Simulation of Resin Infusion Processes

    Kelly, Piaras; Johnson, C (2007)

    Conference item
    The University of Auckland Library

    This study is concerned with Resin Infusion Liquid Composite Molding processes such as Vacuum Assisted Resin Transfer Molding. The Resin Infusion process has been effectively simulated using two models, one based on the conventional Darcy's Law and one based on a height-averaged modified version of Darcy's Law. The finite elements themselves are used as control volumes and a mass conservation technique is applied to improve the accuracy of fluid flux evaluation. Elements at the flow front are temporarily subdivided to allow for convergence of the numerical scheme. It is found that the temporal height derivative is important and cannot be neglected if one is interested in an accurate simulation. Neglecting this term to form the quasi-static forms of the full governing equations significantly increases the mass balance error and can greatly reduce the mold fill time.

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  • A Fibre Compaction Model for Liquid Composite Moulding

    Kelly, Piaras (2007)

    Conference item
    The University of Auckland Library

    Liquid Composite Moulding (LCM) processes such as Resin Transfer Moulding (RTM) are widely-used to manufacture composite material components. The evaluation of resin fluid pressures and time to manufacture in these processes have been the study of many previous investigations. This study focuses on the forces which arise in LCM processes. A knowledge of these forces is of practical importance because they give the necessary tooling forces and also they help predict part thicknesses where force and thickness are coupled in complex processes such as Vacuum Assisted Resin Transfer moulding (VARTM). A phenomenological model is developed which predicts the forces required to compact certain fibrous materials in rigid moulds over a range of compaction velocities and volume fractions. Stress relaxation is accounted for. The model parameters can be obtained from a limited number of tests. The model is used to predict the stresses in dry and wet compacted fibrous materials, and the tooling forces in a complete LCM process. The model results agree well with experimental results.

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  • Simulation of the Complete Resin Infusion Process

    Govignon, Quentin; Bickerton, Simon; Kelly, Piaras (2008)

    Conference item
    The University of Auckland Library

    Resin Infusion (a.k.a. VARTM) is one of the Liquid Composite Moulding processes, for which liquid resin is drawn into dry fibre reinforcement. Resin Infusion is a closed mould process in which half of the mould is formed by a flexible vacuum bag. Significant cavity thickness changes occur during processing, due to the flexibility of the vacuum bag and the complex stress balance within the laminate. While the magnitude of thickness change is often small, the influence is significant on reinforcement fibre volume fraction. Dynamic changes in permeability during mould filling and post-filling have the potential to significantly affect the process. To simulate this behaviour, it is important to accurately model the compaction and relaxation of reinforcement in the dry and wet state. A series of tests have been completed to determine the compaction behaviour of an isotropic glass fibre mat. From these tests several non-linear elastic compaction models have been determined, and applied within a new Resin Infusion simulation. The process simulation, which addresses the pre-filling, filling and post-filling stages, is compared to an experiment employing a full field cavity thickness measurement apparatus, as well as measurement of resin pressure at three discrete points within the laminate.

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  • A Compaction Model for Liquid Composite Moulding Fibrous Materials

    Kelly, Piaras (2008)

    Conference item
    The University of Auckland Library

    Liquid Composite Moulding (LCM) processes are a family of advanced composite materials manufacturing processes, which includes the Resin Transfer Moulding (RTM), Injection/Compression Moulding (I/CM) and Vacuum Assisted Resin Infusion (VARI) processes. In an LCM process, many important manufacturing parameters depend on the stresses taken up by the fibrous material before, during and after the fluid-filling stage. For example, the tooling forces in an RTM process and the fill-time and part-thickness in a VARI process depend on this fibre stress. Fibrous materials respond to load in a complex manner, exhibiting viscoelastic effects and undergoing permanent deformation. A new framework for the mathematical modeling of these materials is proposed based on thermomechanical arguments. Physical phenomena of the microscale such as fibre bending, fibre-to-fibre friction and the concept of ???frozen energy??? are incorporated. The framework is demonstrated for the case of a fibrous material undergoing permanent deformations during a compaction/unloading cycle and the results are compared with experiment.

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  • Simulation and Verification of Local and Global Forces Exerted on Rigid LCM Tools

    Walbran, William; Bickerton, Simon; Kelly, Piaras (2008)

    Conference item
    The University of Auckland Library

    The SimLCM code is being developed at the University of Auckland as a generic Liquid Composite Moulding (LCM) simulation. A comprehensive tooling force analysis has been implemented, providing local and global predictions. Complex geometries require the consideration of both the normal and tangential stress components acting on mould surfaces, due to the compaction of fibre reinforcement and internally generated resin pressures. This approach is required as SimLCM will be extended from the rigid tool (i.e. RTM, I/CM) to flexible (i.e. Resin Infusion, VARTM) and semi-rigid tool (i.e. RTMLight) LCM techniques. Several elastic and viscoelastic reinforcement compaction models have been developed to address both dry and wet response, and are implemented within SimLCM. A simple friction based model is included to model the tangential stress component, allowing consideration of complex non-planar part geometries. This paper will demonstrate current capability of the package, and present experimental verification studies.

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  • Predicting Stress Distribution Exerted on LCM Tools during Filling Phases

    Walbran, William; Bickerton, Simon; Kelly, Piaras (2008)

    Conference item
    The University of Auckland Library

    Mould tools used for LCM processes such as Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM) must withstand local forces due to resin pressure and compaction of the fibre reinforcement. Prediction of these tooling forces will allow cost effective mould design and process selection. A series of RTM and I/CM experiments have been undertaken, monitoring total clamping force and normal stress distribution acting on mould surfaces. A mixed elastic and a visco-elastic reinforcement compaction model have been used to model these processes, both being compared to experimental data. Both models show good agreement to experiment during compaction phases, however the visco-elastic model matches the experimental data significantly better during periods influenced by stress relaxation. Circumferentially averaged stress distributions are also compared at key points in the process, both models showing good qualitative agreement to experiment, and the RTM cases also matching well quantitatively. Overall, the RTM process has been modeled accurately, while some discrepancy exists for I/CM during secondary compaction, when fluid is compressed along with reinforcement.

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