Field emission from tetrahedrally bonded amorphous carbon

The field emission behaviour of a series of Tetrahedrally Bonded Amorphous Carbon (ta-C) films has been measured. The films were produced using a Filtered Cathodic Vacuum Arc System. The threshold field for emission and current densities achievable have been investigated as a function of sp3/sp2 bonding ratio and nitrogen content. Typical as-grown undoped ta-C films have a threshold field of order 10-15 V/μm and optimally nitrogen-doped films exhibit fields as low as 5 V/μm. The emission as a function of back contact and front surface condition has also been considered and shows that the back contact has only a minor effect on emission efficiency. However, after etching in either an oxygen or hydrogen plasma, the films show a marked reduction in threshold field, down to as low as 2-3 V/μm, and a marked improvement in emission site density.

Tetrahedrally bonded amorphous carbon for field-emission displays

Field emission from a series of tetrahedrally bonded amorphous-carbon (ta-C) films, deposited in a filtered cathodic vacuum arc, has been measured. The threshold field for emission and current densities achievable have been investigated as a function of sp3/sp2 bonding ratio and nitrogen content. Typical as-grown undoped ta-C films have threshold fields of the order 10-15 V/μm and optimally nitrogen doped films exhibited fields as low as 5 V/μm. In order to gain further understanding of the mechanism of field emission, the films were also subjected to H2, Ar, and O2 plasma treatments and were also deposited onto substrates of different work function. The threshold field, emission current, emission site densities were all significantly improved by the plasma treatment, but little dependence of these properties on work function of the substrate was observed. This suggests that the main barrier to emission in these films is at the front surface.

The influence of peel angle on the mechanics of peeling flexible adherends with arbitrary load-extension characteristics

The peel test is commonly used to determine the strength of adhesive joints. In its simplest form, a thin flexible strip which has been bonded to a rigid surface is peeled from the substrate at a constant rate and the peeling force which is applied to the debonding surfaces by the tension in the tape is measured. Peeling can be carried out with the peel angle, i.e. the angle made by the peel force with the substrate surface, from any value above about 10° although peeling tests at 90 and 180° are most common. If the tape is sufficiently thin for its bending resistance to be negligibly small then as well as the debonding or decohesion energy associated with the adhesive in and around the point of separation, the relation between the peeling force and the peeling angle is influenced both by the mechanical properties of the tape and any pre-strain locked into the tape during its application to the substrate. The analytic solution for a tape material which can be idealised as elastic perfectly-plastic is well established. Here, we present a more general form of analysis, applicable in principle to any constitutive relation between tape load and tape extension. Non-linearity between load and extension is of increasing significance as the peel angle is decreased: the model presented is consistent with existing equations describing the failure of a lap joint between non-linear materials. The analysis also allows for energy losses within the adhesive layer which themselves may be influenced by both peel rate and peel angle. We have experimentally examined the application of this new analysis to several specific peeling cases including tapes of cellophane, poly-vinyl chloride and PTFE. © 2005 Elsevier Ltd. All rights reserved.

Extraction of fibre network architecture by X-ray tomography and prediction of elastic properties using an affine analytical model

This paper proposes a method for extracting reliable architectural characteristics from complex porous structures using micro-computed tomography (μCT) images. The work focuses on a highly porous material composed of a network of fibres bonded together. The segmentation process, allowing separation of the fibres from the remainder of the image, is the most critical step in constructing an accurate representation of the network architecture. Segmentation methods, based on local and global thresholding, were investigated and evaluated by a quantitative comparison of the architectural parameters they yielded, such as the fibre orientation and segment length (sections between joints) distributions and the number of inter-fibre crossings. To improve segmentation accuracy, a deconvolution algorithm was proposed to restore the original images. The efficacy of the proposed method was verified by comparing μCT network architectural characteristics with those obtained using high resolution CT scans (nanoCT). The results indicate that this approach resolves the architecture of these complex networks and produces results approaching the quality of nanoCT scans. The extracted architectural parameters were used in conjunction with an affine analytical model to predict the axial and transverse stiffnesses of the fibre network. Transverse stiffness predictions were compared with experimentally measured values obtained by vibration testing. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Design and process selection for joints in flexible materials

The application of high performance textiles has grown significantly in the last 10 to 15 years. Various research groups throughout the United Kingdom, such as the Department of Trade and Industry, have identified technical textiles as a field for future development. There is little design guidance for joining of flexible materials or general property models that can be applied to theses materials. This lack is due to the large diversity of properties, structures and resulting behaviours of the materials that are classified as "Flexible Materials". This dissertation explores the issues that are involved in characterising the materials at the fibre, bulk and textile levels. Different units of measurement are used for each stage of the manufacturing process of flexible materials and this disparity creates problems when trying to make general comparisons (e.g. comparing textiles to polymer films). Thus, a possible solution to this is to create selection charts that allow designers to compare the strength of materials for a given mass per unit area. A design tool was created using the Cambridge Engineering Selector (CES) software to enable the selection of joining processes for material. The tool is effective in selecting a reduced number of viable joining processes. Through case studies it was shown that designers are required to examine the selected processes (identified by the software) in greater detail - in particular the economics and geometry of the joint - in order to identify the optimum joining process.

Dynamic characterisation and longitudinal strength of swaged joints

Swaging is a cold working process involving plastic deformation of the work piece to change its shape. A swaged joint is a connection between two components whereby a swaging tool induces plastic deformation of the components at their junction to effectively bind them together. This is commonly used when welding or other standard joining techniques are not viable. Swaged joints can be found for example, in nuclear fuel assemblies to connect the edges of thin rectangular plates to a supporting structure or frame. The aim of this work is to find a model to describe the vibrational behaviour of a swaged joint and to estimate its strength in resisting a longitudinally applied load. The finite element method and various experimental rigs were used in order to find relationships between the natural frequencies of the plate, the joint stiffness and the force required to shift the plate against the restraining action of the swage connection. It is found that a swaged joint is dynamically equivalent to a simple support with the rotation elastically restrained and a small stiffness is enough to resist an important load. © 2011 Elsevier Ltd. All rights reserved.

Precracked reinforced concrete T-Beams repaired in shear with bonded carbon Fiber-Reinforced Polymer sheets

This study investigates the structural behavior of precracked reinforced concrete (RC) T-beams strengthened in shear with externally bonded carbon fiber-reinforced polymer (CFRP) sheets. It reports on seven tests on unstrengthened and strengthened RC T-beams, identifying the influence of load history, beam depth, and percentage of longitudinal steel reinforcement on the structural behavior. The experimental results indicate that the contributions of the external CFRP sheets to the shear force capacity can be significant and depend on most of the investigated variables. This study also investigates the accuracy of the prediction of the fiber-reinforced polymer (FRP) contribution in ACI 440.2R-08, UK Concrete Society TR55, and fib Bulletin 14 design guidelines for shear strengthening. A comparison of predicted values with experimental results indicates that the guidelines can overestimate the shear contribution of the externally bonded FRP system. © 2012, American Concrete Institute.

Characterization of impact induced damage modes in single lap joints of woven GFRP composites

Single lap joints of woven GFRP composites have been investigated for impact induced damage modes using C-scan, X-ray micro tomography, imaging and finite element (FE) modelling. This has allowed for damage modes to be observed in 3D from macro to micro level-resulting in much better understanding of damage mechanisms and realistic FE modelling.

Shear transfer across a diagonal crack in reinforced concrete strengthened with externally bonded FRP fabric

Aging concrete infrastructure in developed economies and more recently constructed concrete infrastructure in the developing world are frequently found to be deficient in structural strength relative to current needs. This can be attributed to a variety of factors including deterioration, construction defects, accidental damage, changes in understanding and failure to design for future loading requirements. Strengthening existing concrete structures can be a cost and carbon effective alternative to replacement. A competitive option for the strengthening of concrete slab-on-beam structures that are deficient in shear capacity is the U-wrapping of the down-stand beam portion of the shear span with externally bonded FRP fabric. While guidance exists for the strengthening of reinforced concrete by U-wrapping, the interaction between internal steel reinforcement, concrete and external FRP in the presence of a dominant diagonal shear crack is not well understood. An approach adopted in previous work has been to explore this interaction through conventional push-off testing. In conventional push-off testing, unlike in a beam, the shear plane is parallel to the direction of loading and perpendicular to the principal fibre orientation. This paper presents a novel push-off test variation in which the shear plane is inclined at 45° to the direction of loading and the principal fibre orientation. A variety of reinforcement ratios, FRP thicknesses and FRP end conditions are modelled. The implications of inclined cracking on debonding of FRP are investigated. The suitability and relevance of inclined push-off tests for further work in this area is also assessed. © 2013, NetComposite Limited.

Performance of joints in reinforced concrete slabs for two-way spanning action

A series of tests on filigree slab joints was performed with the aim of assessing whether such joints can be reliably used in the construction of two-way spanning reinforced concrete slabs. The test results were compared with code requirements. Adequate joint performance is shown to be achievable when the joints are appropriately detailed. Further research is recommended for the formulation of a more generic understanding when the design parameters are varied from those studied in this work.