Aeronautical Engineering

The production of advanced fine-grained alumina by carbon nanotube addition

Fawad Inam et al. — Fri, 14 Sep 2012 04:40:09 PDT

Alumina and alumina+1vol.% carbon nanotube (CNT) composites were fully densified by spark plasma sintering. Post-sintering heat treatments (1300–1500 ◦C) were performed to completely oxidize CNTs and then densify the remaining 1 vol.% to produce fine-grained ceramics. The grain size and Vickers hardness of the heat-treated composites were compared with the monolithic alumina sintered without CNT addition. Compared to the initial powder particle size of alumina (D50: 356±74 nm), minimal grain growth (∼450 nm) was observed for the fully dense heat-treated composites. A 25% improvement in Vickers hardness and >10 times finer average grain size were observed for alumina produced by the heat treatment (1300 ◦C) of alumina+1vol.% CNT composite, compared to alumina sintered without CNTs.

Improving Oxidation Resistance of Carbon Nanotube Nanocomposites for Aerospace Applications

Fawad Inam et al. — Mon, 10 Sep 2012 08:27:24 PDT

Carbon nanotubes (CNTs) based materials possess strong potential to substitute various functional materials developed exclusively for aerospace applications. However, because of the low oxidation temperature of CNTs (400-500 oC), using CNT based ceramic nanocomposites in high temperature applications can be problematic. Making ceramic-CNT nanocomposites by atomic layer deposition (ALD) method and field assisted sintering technology (FAST) is a good route to improve oxidative stability of CNTs. In this study, thermo-gravimetric analysis (TGA) of alumina coated CNTs (prepared by ALD) and alumina-CNT nanocomposites (prepared by FAST) were carried out. 16% improvements were observed in the oxidation resistance for alumina-CNT nanocompo-sites prepared by ALD and SPS techniques. Different strategies to improve oxidation resistance are discussed.

Carbon Nanotubes for Epoxy Nanocomposites: A Review on Recent Developments

Fawad Inam et al. — Mon, 10 Sep 2012 07:54:43 PDT

Carbon nanotubes (CNTs) are one of the strongest and stiffest engineering fibres. Due to their unique combination of chemical and physical properties at an incredibly small size, they possess great potential to be used as nanofillers for many structural and functional materials, particularly in aerospace sector. Depending on the type, geometrical parameters, concentration, dispersion and many other factors, CNTs can significantly modify the mechanical, electrical and thermal properties of epoxy based materials. This review paper, covering methods of synthesis, composite processing techniques and properties, presents an overview of develop-ments in the field of CNT/ epoxy nanocomposites in recent years.

A Parametric Study of the Low-Impulse Blast Behaviour of Fibre-Metal Laminates Based on Different Aluminium Alloys

Thuc Vo et al. — Thu, 06 Sep 2012 02:37:07 PDT

A parametric study has been undertaken in order to investigate the influence of the properties of the aluminium alloy on the blast response of fibre-metal laminates (FMLs). The finite element (FE) models have been developed and validated using experi-mental data from tests on FMLs based on a 2024-O aluminium alloy and a woven glass-fibre/polypropylene composite (GFPP). A vectorized user material subroutine (VUMAT) was employed to define Hashin’s 3D rate-dependant damage constitutive model of the GFPP. Using the validated models, a parametric study has been carried out to investigate the blast resistance of FML panels based on the four aluminium alloys, namely 2024-O, 2024-T3, 6061-T6 and 7075-T6. It has been shown that there is an approximation linear relationship between the dimensionless back face displacement and the dimensionless impulse for all aluminium alloys investigated here. It has also shown that the residual displacement of back surface of the FML panels and the internal debonding are dependent on the yield strength of the aluminium alloy.

Vibration and buckling of composite beams using refined shear deformation

Thuc P. Vo et al. — Mon, 03 Sep 2012 07:52:00 PDT

Vibration and buckling analysis of composite beams with arbitrary lay-ups using refined shear deformation theory is presented. The theory accounts for the parabolical variation of shear strains through the depth of beam. Three governing equations of motion are derived from the Hamilton’s principle. The resulting coupling is referred to as triply coupled vibration and buckling. A two-noded C1 beam element with five degree-of-freedom per node which accounts for shear deformation effects and all coupling coming from the material anisotropy is developed to solve the problem. Numerical results are obtained for composite beams to investigate effects of fiber orientation and modulus ratio on the natural frequencies, critical buckling loads and corresponding mode shapes.

Transition regime analytical solution to gas mass flow rate in a rectangular micro channel

Kokou S. Dadzie et al. — Thu, 09 Aug 2012 04:00:17 PDT

We present an analytical model predicting the experimentally observed gas mass flow rate in rectangular microchannels over slip and transition regimes without the use of any fitting parameter. Previously, Sone reported a class of pure continuum regime flows that requires terms of Burnett order in constitutive equations of shear stress to be predicted appropriately. The corrective terms to the conventional Navier-Stokes equation were named the ghost effect. We demonstrate in this paper similarity between Sone ghost effect model and newly so-called ‘volume diffusion hydrodynamic model’. A generic analytical solution to gas mass flow rate in a rectangular micro channel is then obtained. It is shown that the volume diffusion hydrodynamics allows to accurately predict the gas mass flow rate up to Knudsen number of 5. This can be achieved without necessitating the use of any adjustable parameters in boundary conditions or parametric scaling laws for constitutive relations. The present model predicts the non-linear variation of pressure profile along the axial direction and also captures the change in curvature with increase in rarefaction.

Effect of microwave post-curing upon the micromechanics of model Kevlar/epoxy composites

Richard Day et al. — Mon, 30 Jul 2012 08:26:11 PDT

Microwave processing of materials has the potential to deliver several major advantages over conventional thermal processing. One of these is an decrease in the time necessary for manufacture since the microwave energy is absorbed throughout the body of the material rather than relying on thermal conduction and convection. Another potential advantage is that the power is directed to the sample, this together with the decrease in processing time leads to lower energy being consumed. One question which needs to be addressed in the case of polymer composites is whether microwave processed materials are of as good quality as the thermally processed ones. In this work the interfacial properties of model Kevlar fibre reinforced epoxy composites post-cured by both conventional and microwave heating have been examined. Raman spectroscopy was employed to measure the fibre strain distributions along embedded fibres and from this information the interfacial shear stress distribution was calculated. The results show that the interfacial shear strengths and critical lengths of the microwave post-cured composites are comparable to those for thermally post-cured ones. This is potentially of interest in the commercial manufacture of composites since the process could be considerably shortened by the use of microwave post-curing leading to lower cycle times and costs without any deterioration in the interfacial properties of the composites.

A Review of Cutting-edge Techniques for Material Selection

Noori Brifcani et al. — Mon, 30 Jul 2012 07:55:58 PDT

Selecting the optimum material for a given application is a complex task for engineers and designers across all industrial fields. There are a huge number of materials now available with a range of different properties and behaviours and so it has become even more necessary to carry out a systematic process in order to screen and/or rank the materials to give a promising number of candidates. The output of the material selection process depends upon which method is used. In some methods, a chart can be used to identify promising candidates whereas in others a single ‘optimum’ material may be chosen or a ranked list of candidates identified. This paper aims to summarise the documented techniques for material selection, evaluating the methods that are currently available, and compare the methods for consistency and effectiveness.

Development of a microwave calorimeter for simultaneous thermal analysis,

Alan Nesbitt et al. — Mon, 30 Jul 2012 06:38:21 PDT

An instrument has been developed for monitoring cure processes under microwave heating conditions. The main function of the instrument was a calorimeter for performing microwave thermal analysis. A single model resonant cavity was used as the heating cell in the microwave calorimeter.

Thermal analysis measurements were obtained by monitoring the variation in the microwave power that was required to maintain controlled heating of the sample. The microwave thermal analysis data were analogous to conventional differential scanning calorimetry measurements. The dielectric properties of the sample, as a function of the extent of cure, have been obtained using perturbation theory from the changes in resonant frequency and quality factor of the microwave cavity during heating. Additionally, remote sensing fibre-optic probes have been employed to measure real time in situ infrared spectra of the sample during the cure reaction. In this paper, we describe the design and operation of the microwave calorimeter. Examples of experimental results are also presented.

Comparison of the Curing Kinetics of the RTM6 Epoxy Resin System Using Differential Scanning Calorimetry and a Microwave-Heated Calorimeter

Parnia Navabpour et al. — Mon, 30 Jul 2012 05:51:16 PDT

The cure of a commercial epoxy resin system, RTM6, was investigated using a conventional differential scanning calorimeter and a microwave-heated calorimeter. Two curing methods, dynamic and isothermal, were carried out and the degree of cure and the reaction rates were compared. Several kinetics models ranging from a simple nth order model to more complicated models comprising nth order and autocatalytic kinetics models were used to describe the curing processes.

The results showed that the resin cured isothermally showed similar cure times and final degree of cure using both conventional and microwave heating methods, suggesting similar curing mechanisms using both heating methods. The dynamic curing data were, however, different using two heating methods, possibly suggesting different curing mechanisms. Near-infrared spectroscopy showed that in the dynamic curing of RTM6 using microwave heating, the epoxy-amine reaction proceeded more rapidly than did the epoxy-hydroxyl reaction.

This was not the case during conventional curing of this resin.

Investigation of the microwave curing of the PR500 epoxy resin system

Mark Wallace et al. — Mon, 09 Jul 2012 08:53:40 PDT

Microwave heating has been used to cure a resin system, PR500 (3M). The same resin has been cured using a conventional oven. The cured resins have been compared using a number of techniques including modulated differential scanning calorimetry (MDSC), dynamic thermal analysis, infrared spectroscopy (IR) and solid-state NMR spectroscopy. The reaction path appears to be slightly different depending upon the nature of the heating. The epoxy-amine reaction occurs to a greater extent than the epoxy-hydroxyl reaction in the microwave cured resin compared to the thermally cured resin. The dielectric properties for the thermally and microwave cured materials were measured for degrees of cure greater than 75% and over this range are similar for materials cured by the two techniques and thus not sensitive to this change. Broadening of the glass transition for microwave-cured epoxy resins was observed. Since the IR and solid-state NMR results show small differences as does the DMA behaviour of materials cured using the two routes the broadening is attributed to a difference in network structure

Vibration and Buckling of Cross-Ply Composite Beams using Refined Shear Deformation Theory

Thuc Vo et al. — Fri, 06 Jul 2012 06:45:00 PDT

Vibration and buckling analysis of cross-ply composite beams using refined shear deformation theory is presented. The theory accounts for the parabolical variation of shear strains through the depth of beam. Three governing equations of motion are derived from the Hamilton’s principle. The resulting coupling is referred to as triply coupled vibration and buckling. A two-noded C1 beam element with five degree-of-freedom per node is developed to solve the problem. Numerical results are obtained for composite beams to investigate modulus ratio on the natural frequencies, critical buckling loads and load-frequency interaction curves.

Large Eddy Simulation (LES) of Glass Fibre Dispersion in an Internally Spout-Fluidised Bed for Thermoplastic Composite Processing

Xiaogang Yang et al. — Wed, 27 Jun 2012 07:53:15 PDT

Large eddy simulation (LES) has been conducted to investigate glass fibre dispersion in an internally spout-fluidised bed with draft tube and disk-baffle, which was used in the manufacture of long glass fibre reinforced thermoplastic composites. The LES results have demonstrated that the internally spout-fluidised bed with draft tube and disk-baffle can remarkably improve its hydrody-namic behaviour, which can effectively disperse fibre bundles and promote pre-impregnation with resin powder in manufacturing fibre reinforced thermoplastics. The hydrodynamics of the spout-fluidised bed has been investigated and reported in a previous paper (Hosseini et al., 2009). This study attempts to reveal important features of fibre dispersion and correlations between the fibre disper-sion and the characteristics of turbulence in the internally spout-fluidised bed using the LES modelling, focusing on the likely hydro-dynamic impact on fibre dispersion. The simulation has clearly indicated that there exists a strong interaction between the turbulent shear flow and transported fibres in the spout-fluidised bed. Fibre entrainment is strongly correlated with the local vorticity distribu-tion. The dispersion of fibres was modelled by a species transport equation in the LES simulation. The turbulent kinetic energy, Rey-nolds stress and strain rate were obtained by statistical analysis of the LES results. The LES results also clearly show that addition of the internals in the spout-fluidised bed can significantly change the turbulent flow features and local vorticity distribution, enhancing the capacity and efficiency of fibre flocs dispersion.

Investigation of the Micromechanics of the Microbond Test

Richard Day et al. — Mon, 21 May 2012 07:26:34 PDT

The microbond test is a method which is sometimes used for measuring interfacial shear strength. In the analysis of the data it is often assumed that the interfacial shear stress is constant and thus, by implication, that the strain in the fibre along the embedded fibre decreases linearly from the point of entry to the point of exit. In this paper the results of conventional microbond tests and simulated microbond tests performed under a Raman microscope on a Kevlar-49/epoxy system are reported. The conventionally performed tests show that the calculated interfacial shear strength for this system is approximately 16 MPa regardless of the position of the supporting knife edges. The strain distribution along the fibre during simulated microbond tests was studied as a function of knife edge position, interfacial area and level of load by means of Raman spectroscopy. It was found that the interfacial shear stress was not constant, as is frequently assumed, but was strongly dependent upon distance through the droplet, knife-edge position and applied load. At low loads the strain was a maximum at the point where the fibre entered the droplet and then dropped off sharply through the embedded length. This effect was enhanced when the knife-edge separation was reduced. The variation of the shape of the stress distribution was similar to that predicted by a linear finite element analysis. At higher load levels the onset of failure in the region closest to the point where the fibre entered the droplet could be observed.

Flexural and interlaminar shear strength properties of carbon fibre/epoxy composites cured thermally and with microwave radiation

C Nightingale et al. — Fri, 24 Feb 2012 03:31:34 PST

The ease of heating an epoxy resin with microwaves depends, among other factors, on the dielectric properties of its components at the frequency of the radiation used. The majority of the papers published on the microwave curing of reinforced epoxy resin composites have used widely available DGEBA type resins and amine hardeners such as 4,4’-diaminodiphenylsulphone (DDS). This paper investigates the use of two epoxy systems where the choice of resin and hardener was based on their measured dielectric loss factors. System 1 contained a resin and hardener with higher loss factors than those used in System 2. The two systems were formulated with polyetherimide (PEI) as a toughening agent. Unidirectional carbon fibre prepregs were prepared from both systems. Composites were laid up from these prepregs which were then cured in three different ways: autoclave curing, partial autoclave curing followed by microwave post-curing, and microwave curing. System 1 composites had greater flexural properties and interlaminar shear strengths than System 2 composites when autoclave cured. Flexural properties and interlaminar shear strengths were greater for System 2 in the microwave post-cured composites. When fully microwave cured the properties were similar. In the microwave cured composites the flexural and interlaminar shear properties were influenced by the structure of the phase separated PEI and the void content.

Comparison of the Curing Kinetics of a DGEBA/Acid Anhydride Epoxy Resin System Using Differential Scanning Calorimetry and a Microwave-Heated Calorimeter

P Navabpour et al. — Thu, 29 Sep 2011 01:43:47 PDT

The cure of an epoxy resin system, based upon a diglycidyl ether of bisphenol-A (DGEBA) with HY917 (an acid anhydride hardener) and DY073 (an amine–phenol complex that acted as an accelerator), was investigated using a conventional differential scanning calorimeter and a microwave-heated power-compensated calorimeter. Dynamic cure of the epoxy resin using four different heating rates and isothermal cure using four different temperatures were carried out and the degree of cure and reaction rates were compared.

The cure kinetics were analyzed using several kinetics models. The results showed different activation energies for conventional and microwave curing and suggested different reaction mechanisms were responsible for curing using the two heating methods. Resins cured using conventional heating showed higher glass transition temperatures than did those cured using microwave heating.

Comparison of the mechanical and physical properties of a carbon fibre epoxy composite manufactured by resin transfer moulding using conventional and microwave heating

D A. Papargyris et al. — Fri, 06 May 2011 06:53:27 PDT

Microwave processing holds great potential for improving current composite manufacturing techniques, substantially reducing cure cycle times, energy requirements and operational costs. In this paper, microwave heating was incorporated into the resin transfer moulding technique. Through the use of microwave heating, a 50% cure cycle time reduction was achieved. The mechanical and physical properties of the produced carbon fibre/epoxy composites were compared to those manufactured by conventional resin transfer moulding. Mechanical testing showed similar values of flexural moduli and flexural strength for the two types of composites after normalisation of the corresponding data to a common fibre volume fraction. A 9% increase of the interlaminar shear strength (ILSS) was observed for the microwave cured composites. This enhancement in ILSS is attributed to a lowering of resin viscosity in the initial stage of the curing process, which was also confirmed via scanning electron microscopy by means of improved fibre wetting and less fibre pull-out. Furthermore, both types of composites yielded minimal void content (