Debes Bhattacharyya

A study of hole drilling in Kevlar composites

Abstract Because of the high toughness and flexibility of the fibres, Kevlar-reinforced polymer composites are always difficult to machine, characterised by poor, fuzzy surface finish with partially pulled-out and crushed fibres. For drilling, tungsten-carbide-reinforced tooling in conjunction with very high rotational speeds are normally suggested. The present paper describes a simple technique of drilling Kevlar composites by using slightly modified high-speed steel (HSS) drill bits at commonly available rotational speeds. The investigation has been carried out at both ambient and cryogenic temperatures, the latter obtained through the application of liquid nitrogen at the drill site. Although there is an increase in drill thrust under cryogenic conditions enhancing the probability of delamination, this shortcoming is more than offset by the improved surface finish, hole quality and far superior tool life. Furthermore, delamination can be minimised by using a back support, a commonly followed practice in the industry. Interestingly, the poor surface quality and edge damage at the entry and particularly at the exit faces of the drilled holes can be minimised by the application of liquid nitrogen and are totally eliminated by the introduction of very thin resin-rich layers on the laminate surfaces. The delamination results are shown to agree reasonably well with the predictions made by a linear-elastic fracture mechanics model and are also investigated by using a three-dimensional, orthotropic, finite-element model. It is shown through finite-element analysis that any major surface delamination is unlikely to happen when a back support is used; however, crack propagation is still possible in the shearing mode.

Multiple regression and neural networks analyses in composites machining

The machining forces-tool wear relationship of an aluminium metal matrix composite has been studied in this paper using multiple regression analysis (MRA) and generalised radial basis function (GRBF) neural network. The results show that using the force-wear equation derived from MRA is a fairly accurate way of predicting the attainment of prescribed tool wear. However, the use of a neural network analysis can further improve the accuracy of the tool wear prediction particularly when the functional dependency is nonlinear. It is evident that the relationship derived from the feed force data is more accurate than that derived from the cutting force.

Donor-acceptor pair luminescence of nitrogen-implanted ZnO single crystal

Donor-acceptor-pair sDAPd luminescence is a direct probe of the acceptors in ZnO. We report the near-surface doping of a ZnO single crystal by ion implantation with nitrogen and titanium. Secondary-ion-mass spectroscopy shows that the doping depth is approximately 80 nm sNd and 50 nm sTid. The DAP photoluminescence centered at 3.232 eV is observed from both the undoped and doped ZnO single-crystal samples. The luminescence spectrum of the nitrogen-doped sample shows enhancement of the DAP transition compared to the “pure” ZnO sample. The acceptor energy is calculated to be 177 meV, consistent with nitrogen as the acceptor in DAP luminescence. The DAP recombination lifetime is found to be ,5.5 ns. The temperature evolution of spectra shows the gradual transition from DAP luminescence to electron+ acceptor recombination luminescence at temperatures above 37 K. Our experimental results suggest that ion implantation is an effective way of doping nitrogen into ZnO.

Chip formation in the machining of SiC-particle-reinforced aluminium-matrix composites

Abstract As a consequence of the widening range of applications of metal-matrix composites (MMCs), the machining of these materials has become a very important subject for research. Aluminium-matrix composites are widely used for their favourable specific strength/stiffness and corrosion resistance properties. This paper describes a study of chip formation during the machining of a DURALCAN® aluminium-matrix composite (A359/SiC/20p). For good machinability, it is desirable to have continuous chips in short segments without the use of a chip breaker. The chip-formation mechanism in machining this silicon-carbide-particle-reinforced aluminium MMC at three different cutting speeds has therefore been investigated by using an explosive charged quick-stop device. An improved quick-stop device has enabled research to be carried out more easily on the mechanism of chip formation by achieving better chip control during machining. During the chip-breaking process, the primary chip-forming mechanism involves the initiation of cracks from the outer free surface of the chip due to the high shear stress. Meanwhile, some small voids are formed by the separation of particles and the matrix material within the chip because of the stress concentration at the edges of the particles. The crack propagation is enhanced through the coalescence of these voids along the shear plane. The fracture and sliding of material then follow to form semicontinuous ‘saw-toothed’ chips.

Thermoforming woodfibre–polypropylene composite sheets

Abstract Thermoforming of woodfibre–polypropylene composite sheets made without any modification of the fibres or the polymer is the focus of this paper, the emphasis being on their formability and the associated issues. Both the degree to which a material conforms to the desired part geometry after deformation and the extent to which a sheet material may be deformed before unacceptable defects occur are considered. Four thermoforming processes such as V-bending, die-match forming, air pressure forming and deep drawing have been utilised to examine both single-curvature and double-curvature deformation conditions. The technique of Grid Strain Analysis (GSA) has been applied to quantify differences in strain distributions during sheet deformation. The effects of thermoforming process parameters and sheet composition on sheet formability are also discussed. Notably, this study considers composite sheets reinforced with wood fibres rather than woodflour, enabling the study of fibre layup and fibre interlocking effects. While the tensile strengths of the composite sheets increase marginally the stiffnesses increase significantly compared to those of unreinforced polypropylene. The key deformation mechanism for layered woodfibre–polypropylene composite sheets is inter-ply shear while intra-ply shear dominates the deformation of homogeneous sheets. Forming temperature and blank size have the most pronounced effects on the formability of these composite sheets.

Contribution of Coalescence to Microfibril Formation in Polymer Blends during Cold Drawing

The main goal of this study is to obtain a morphological confirmation of the contribution of coalescence to the fibril formation mechanism in microfibril reinforced composites (MFC) and in this way to explain the large differences in sizes of the starting spheres and the resulting long microfibrils, at rather modest draw ratios (between 6 and 12). For this purpose, the peculiar property of compatibilizers has been exploited, namely coating the dispersed particles and thus preventing them from eventual coalescing after coming in contact. Using scanning electron microscopy (SEM) on samples at various stages of the cold drawing of polypropylene/poly(ethylene terephthalate) blends with and without a compatibilizer, the assumed mechanism has been established: the MFC from compatibilized blends show much shorter microfibrils because the compatibilizer inhibits the coalescence process. A qualitative model illustrating the transformation of the dispersed spheres into microfibrils due to coalescence during the cold drawing is also discussed. In Commemoration of the Contributions of Professor Valery P. Privalko to Polymer Science.

Effect of metal-ion doping on the optical properties of nanocrystalline ZnO thin films

Optical properties of metal (Al, Ag, Sb, and Sn)-ion-implanted ZnO films have been studied by ultraviolet-visible spectroscopy and spectroscopic ellipsometric techniques. The effects of metal-ion doping on the optical band gap (Eg), refractive index (n), and extinction coefficient (k) of nanocrystalline ZnO films have been studied for the similar implantation dose of all the metal ions. The ellipsometric spectra of the ion-implanted samples could be well described by considering an air/roughness/ZnO–M (layer 1)/ZnO (layer 2)/glass model. The band gap of ZnO films increases with Al ion doping and decreases with doping of Ag, Sb, and Sn ions. The refractive index of ZnO films in the visible spectral region increases substantially on Sb and Sn ion doping, while it decreases to some extent with Al ion doping.

A novel approach in organic waste utilization through biochar addition in wood/polypropylene composites

In an attempt to concurrently address the issues related to landfill gas emission and utilization of organic wastes, a relatively novel idea is introduced to develop biocomposites where biochar made from pyrolysis of waste wood (Pinus radiata) is added with the same wood, plastic/polymer (polypropylene) and maleated anhydride polypropylene (MAPP). Experiments were conducted by manufacturing wood and polypropylene composites (WPCs) mixed with 6 wt%, 12 wt%, 18 wt%, 24 wt%, and 30 wt% biochar. Though 6 wt% addition had similar properties to that of the control (composite without biochar), increasing biochar content to 24 wt% improved the composites tensile/flexural strengths and moduli. The biochar, having high surface area due to fine particles and being highly carbonised, acted as reinforcing filler in the biocomposite. Composites having 12 wt% and 18 wt% of biochar were found to be the most ductile and thermally stable, respectively. This study demonstrates that, WPCs added with biochar has good potential to mitigate wastes while simultaneously producing biocomposites having properties that might be suited for various end applications.

A direct comparison of matched-die versus diaphragm forming

Continuous fibre reinforced thermoplastic laminates, in comparison with monolithic metallic sheets, are effectively inextensible in the direction of their reinforcements and therefore severely limited in the range of possible deformations they can undergo during forming. Inter-ply and intra-ply shearing plays a major role in the determination of the degree of formability of such laminates. The present study focuses on the shear behaviour of CFRT sheets in inter-ply and intra-ply modes aiming to assess their significance in matched-die and double diaphragm forming processes. In order to gain a better understanding of the minimum stress levels necessary to induce the two different shear modes, basic shearing experiments on single plies and unidirectional reinforced laminates were conducted prior to forming spherical shaped dome parts. By forming down-scaled yacht hulls it was intended to apply the findings obtained with the spherical dome parts to other 3-D shapes and to demonstrate the versatility and limitations of both processes.

Parametric study of manufacturing poly(lactic) acid nanofibrous mat by electrospinning

Electrospinning is a versatile method for manufacturing polymer-based multi-functional and high-performance nanofibrillar network. Two important characteristics, namely minimum diameter variation and bead area, render the nanofibre mats acceptable for many membrane type applications, but the relationship between processing parameters and microstructures is still not well understood. This article outlines a systematic study via the design of experiments in the context of selecting process control parameters while electrospinning nonwoven mats of nanofibres from poly(l-lactic acid). The goals are to obtain a robust set of parameters to reduce the variation in product quality by performing the minimum number of experiments. A desirable combination has been found to be low concentration of polymer solution, low feed rate, comparatively high applied voltage and a large distance between the collector and the needle. However, a low concentration of polymer solution may result in some bead formation if other factors are not changed accordingly.