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 55–75% of the nominal area. However, the total real contact length within the tow contacts, associated with microscopic contact at the fibre level, is only 4–8% 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$_a$ values from 0.005 to 3.2 $\mu$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$_a$ 0.1 $\mu$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 ‘‘Institut PPrime”, 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 load–displacement 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 load–displacement 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

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

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 0.7–1. 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 ‘constant interface strength’ 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