438 research outputs found
Multiscale tool-fabric contact observation and analysis for composite fabric forming
This paper provides measurements and analysis at the meso and microscopic scales of the real contact
area between twill carbon fabric and a flat glass counterface. The mesoscopic contact area associated with
tow contacts is about 55ā75% of the nominal area. However, the total real contact length within the tow
contacts, associated with microscopic contact at the fibre level, is only 4ā8% of the idealised contact conditions
with parallel touching fibres, for a nominal contact pressure of around 2 kPa. The dependence of
real contact area on fabric shear angle is also investigated. The estimated real contact pressure is 15,000
times higher than the nominal contact pressure. Models or experiments of friction in composites forming
which do not take into account the real contact situation, which is very far from an idealised packing
arrangement, may fail to capture the essential tribological mechanisms.The authors are very grateful to the Tribology groups of the
LTDS laboratory in Lyon and Imperial College, London for help
with the semi-reflective coatings. We also acknowledge the
contribution of our industrial partners Jaguar Land Rover and
Granta Design Ltd, as well as the academic partners from the
Composites group at the University of Nottingham. This work
was funded by an Engineering and Physical Sciences Research
Council grant (reference EP/K032798/1).This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S1359835X15000986#
Novel Experimental Method for Microscale Contact Analysis in Composite Fabric Forming
This paper describes a novel experimental rig and associated experimental method developed to investigate composite fabric/tool contact at the microscopic scale. A key feature of this method is that it enables direct observation of real contact at the scale of fibres and the evolution of this contact under simultaneous application of shear and compression loads. To observe the contact, an optical semi-reflective coating is used. An algorithm is developed to analyse the contact images and measure the real contact length and orientation of individual fibres. The method is applied to microcontacts of carbon twill fabric. The real contact length under an apparent pressure of 1.9 kPa is surprisingly small compared to the apparent contact length. Transverse forces associated with friction are also measured. However these results are difficult to interpret as the test generates friction forces which differ from those which would be seen in conventional sliding friction tests.The authors are very grateful to Nazario Morgado and Dr Juliette Cayer-Barrioz from the LTDS laboratory in Lyon (France) for the calculations of the optical properties of semi reflective coatings, and to the Tribology Group of Imperial College London for help with these coatings. We also acknowledge the contribution of our industrial partners Jaguar Land Rover and Granta Design Ltd, as well as the academic partners from the Composites group at the University of Nottingham. This work was funded by an Engineering and Physical Sciences Research Council grant (reference EP/K032798/1).This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s11340-015-0044-y. Related research data is available at: https://www.repository.cam.ac.uk/handle/1810/24724
Effect of tool surface topography on friction with carbon fibre tows for composite fabric forming
The effect of tool surface roughness topography on tow-on-tool friction relevant to the dry forming of composite fabrics is investigated. A comprehensive range of tool average surface roughness R values from 0.005 to 3.2 m was used in friction testing with carbon fibre tows. The measured slope of these surfaces, which is the critical surface topographical characteristic, increased significantly with increasing roughness amplitude. Friction was found to be sensitive to roughness topography for very smooth surfaces (R 0.1 m), friction was relatively insensitive to roughness slope and amplitude. A finite element idealisation of the tow-on-tool contact was used to explain these results in terms of the level of tow-tool conformance. Smooth surfaces have low slopes which allow good conformance, and hence high real contact area and friction. Rougher surfaces have high slopes, particularly at shorter wavelengths, which prevents good conformance. In this case, point contact between fibres and surface dominates, leaving the resulting friction less sensitive to roughness.The authors would like to acknowledge the assistance of the Engineering and Physical Sciences Research Council (EPSRC) for supporting the present work under grant Ref. EP/K032798/1 (Friction in Composites Forming). We would also like to acknowledge the contribution of our industrial collaborators at Jaguar Land Rover and Granta Design Ltd, as well as our academic partners from the Composites Research Group at the University of Nottingham (Prof. Andy Long, Prof. Nick Warrior and Prof. Davide De Focatiis). Dr Olga Smerdova of āāInstitut PPrimeā, ISAE-ENSMA, Poitiers is thanked for useful discussions throughout the work. Hexcel are thanked for supplying the tow material
Biomimetic-inspired CFRP to perforated steel joints
In many high-performance applications there is a need to join steel to CFRP parts. However the stiffness mismatch between these materials leads to high stress concentrations in such joints. This paper uses the biomimetics approach to help develop solutions to this problem. Nature has found many ingenious ways of joining dissimilar materials, with a transitional zone of stiffness at the insertion site commonly used. In engineering joints, one way to reduce the material stiffness mismatch is to gradually decrease the effective stiffness of the steel part of the joint by perforating it with holes. This paper investigates joining of flat perforated steel plates to a CFRP part by a co-infusion resin transfer moulding process. The possible effect of mechanical interlocking as resin fills the perforations is assessed by filling the holes with PTFE prior to moulding to prevent such resin ingress. The joints are tested under static tensile loading. The perforated steel joints show a 175% increase of joint strength comparing to non-perforated joints. Finite element analyses are used to interpret the experimental results. It has been found that the model is able to reproduce with accuracy the experimental loadādisplacement test curves and show the failure mechanisms of the joint.The authors acknowledge the financial support provided by the Engineering and Physical Sciences Research Council (EPSRC) and Dowty Propellers (part of GE Aviation).This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.compstruct.2016.06.01
Biomimetic-inspired CFRP to perforated steel joints
In many high-performance applications there is a need to join steel to CFRP parts. However the stiffness mismatch between these materials leads to high stress concentrations in such joints. This paper uses the biomimetics approach to help develop solutions to this problem. Nature has found many ingenious ways of joining dissimilar materials, with a transitional zone of stiffness at the insertion site commonly used. In engineering joints, one way to reduce the material stiffness mismatch is to gradually decrease the effective stiffness of the steel part of the joint by perforating it with holes. This paper investigates joining of flat perforated steel plates to a CFRP part by a co-infusion resin transfer moulding process. The possible effect of mechanical interlocking as resin fills the perforations is assessed by filling the holes with PTFE prior to moulding to prevent such resin ingress. The joints are tested under static tensile loading. The perforated steel joints show a 175% increase of joint strength comparing to non-perforated joints. Finite element analyses are used to interpret the experimental results. It has been found that the model is able to reproduce with accuracy the experimental loadādisplacement test curves and show the failure mechanisms of the joint.The authors acknowledge the financial support provided by the Engineering and Physical Sciences Research Council (EPSRC) and Dowty Propellers (part of GE Aviation).This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.compstruct.2016.06.01
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Experimental measurement of wrinkle formation during draping of non-crimp fabric
A rig and image analysis methodology is described to characterise wrinkle formation during draping of non-crimp fabrics. The circular fabric blank is draped over a male hemispherical mould, partly constrained by a circular clamping ring around the periphery of the blank. The three-dimensional shape of the fabric is derived from a shape-from-focus analysis of a stack of photographs of the deformed blank. Wrinkles are identified from the deviation of the shape from a smoothed shape. Wrinkle formation is strongly dependent on the fabric architecture and increases progressively with increased punch displacement. Triaxial fabrics have the highest wrinkle amplitude, unidirectional and 0/90Ā° biaxial fabrics the lowest amplitude. The clamping force reduces the wrinkling for some fabrics but, for the maximum force applied, is not effective at eliminating wrinkling.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.compositesa.2015.12.01
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Fibre behaviour in the spunbonding process. Part II: Modelling fibre dynamics in the diffuser
The behaviour of a fibre subject to the turbulent air flow in the diffuser in the spunbonding process is studied using a three-dimensional dynamics model in which the fibre is discretised as a chain of beads connected by linear and rotational springs. The turbulent air drag acting on the fibre is modelled as a random force, as a function of the mean air velocity, the turbulence intensity, and the spectrum of turbulence. The effect of the air flow parameters and the fibre diameter on the amplitude and the frequency of the fibre oscillations is analysed to understand how the fibre position at the exit of the diffuser is controlled by the turbulent air flow in spunbonding. This in turn will affect fibre laydown and the associated web formation. The authors are grateful to Fitesa Germany GmbH for the financial
support.This is the author accepted manuscript. The final version is available from SAGE via http://dx.doi.org/10.1177/095440621561698
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Fibre behaviour in the spunbonding process. Part I: Characterisation of air flow and fibre motion in the diffuser
This is the author accepted manuscript. The final version is available from SAGE via https://doi.org/10.1177/0954406215616983 The random fibre oscillatory behaviour induced by turbulence in the diffuser of an industrial spunbonding rig is measured experimentally. The turbulent air flow is firstly characterised by constant temperature hot-wire anemometry: averaged flow quantities, such as the mean velocity and the turbulent kinetic energy, as well as time dependent quantities, such as the integral time and the energy spectrum, are measured. The influence of the turbulent flow on the dynamics of a nylon fibre of diameter 200āĀµm and a spunbonding fibre of diameter 18āĀµm in the diffuser is then investigated by extracting the transverse displacement from images acquired by a digital camera. The authors are grateful to Fitesa Germany GmbH for the financial
support
Friction of carbon fibre tows
The fundamental frictional behaviour of carbon fibre tows relevant to composite fabric forming is explored. Tow-on-tool and tow-on-tow contact are considered. For tow-on-tool contact, an experiment is devised to simultaneously observe the true filament contact length and measure the friction force over a range of normal loads. Filament contact length is not constant, as would be given from an idealised assumption of parallel touching filaments, but increases in a characteristic manner with normal load. Friction force follows a power law variation with normal load with exponent in the range 0.7ā1. Accounting for the evolving contact length in a Hertzian calculation of the real contact area produces a contact area versus load variation which differs only by a constant factor from the measured friction force curves. Thus, the results agree with a āconstant interface strengthā model of friction. Tow orientation and sizing are found to have a significant effect on friction by altering the real contact area.Engineering and Physical Sciences Research Council (Grant ID: EP/K032798/1 (Friction in Composites Forming)), Jaguar Land Rover, Granta Design Ltd, Composites Research Group at the University of Nottingham, Hexcel U
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Friction and lubrication in metal rolling
This dissertation is concerned with the physical processes which determine friction and
lubrication in metal rolling in the mixed lubrication regime, with particular attention paid to the conditions encountered when rolling aluminium foil. Two areas of relevance to the analysis of the rolling process are initially investigated. Firstly, the rheological properties of a typical aluminium foil rolling oil at high pressures and shear rates have been measured using a disc machine. The behaviour of the oil was found to be well described by the Eyring viscous model, at the shear rates and pressure likely to be found in metal rolling. Secondly, the deformation of asperities when the bulk material is deforming has been examined. The theory developed here was found to agree reasonably with experiments. The results of these investigations are used in the analysis of lubrication in metal rolling, considering the hydrodynamic buildup of oil pressure in the entry region and the crushing of the asperities both in the entry region and at the beginning of the work zone. The contact between roll and strip is divided into two regions, that under the asperities and that in the intervening valleys. Calculations for conditions appropriate to strip and foil rolling give the proportion of the two types of contact and the film thicknesses in each region. Measurements of film thicknesses with an experimental mill in a regime where roughness is unimportant were not found to agree well with an existing simple theory of lubrication. This was ascribed to uneven lubrication in the experiments. After taking this into account, the experiments in a regime where roughness was important were found to agree reasonably with the theory developed here. The effect of roughness on traction is measured in a disc machine with elastic contacts. Its behaviour is found to be determined by the bulk properties of the lubricant at the pressures and strain rates under the asperities. Theory and experiments presented in this dissertation lead to a greater understanding of the physical processes determining friction in metal rolling in the mixed lubrication regime. Film thicknesses and friction coefficients in metal rolling may now be estimated with more confidence
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