Stiffened composite cylindrical panels�Optimum lay-up for buckling by ranking

Buckling of discretely stiffened composite cylindrical panels made of repeated sublaminate construction is studied using a finite element method. In repeated sublaminate construction, a full laminate is obtained by repeating a basic sublaminate, which has a smaller number of plies. This paper deals with the determination of the optimum lay-up for buckling by ranking of such stiffened (longitudinal and hoop) composite cylindrical panels. For this purpose we use the particularized form of a four-noded, 48 degrees of freedom doubly curved quadrilateral thin shell finite element together with a fully compatible two-noded, 16 degrees of freedom composite stiffener element. The computer program developed has been used, after extensive checking for correctness, to obtain an optimum orientation scheme of the plies in the sublaminate so as to achieve maximum buckling load for a specified thickness of typical stiffened composite cylindrical panels.

Effect of sliding speed on friction and wear of uni-directional aramid fibre-phenolic resin composite

A previous study on the tribological performance of a compression-moulded aramid fibre-phenolic resin composite, containing 30% continuous fibre, showed that this composite provides a reasonable combination of the friction coefficient and wear rate to be used as a friction component, such as a brake shoe. In the present work, the effect of sliding speed on the friction and wear behaviour of this composite has been investigated. The sliding experiments were conducted in a speed range of 0.1-6 m s(-1) at two normal pressure levels of 1.0 and 4.9 MPa. The coefficient of friction was found to be stable over a wide range of sliding speeds and normal pressures. The wear of the composite was found to be insensitive to changes in the speed in the higher speed range. The results have been supplemented with scanning electron micrographs to help understand possible friction and wear mechanisms.

Estimation of low-velocity impact damage in laminated composite circular plates using nonlinear finite element analysis

Nonlinear finite element analysis is used for the estimation of damage due to low-velocity impact loading of laminated composite circular plates. The impact loading is treated as an equivalent static loading by assuming the impactor to be spherical and the contact to obey Hertzian law. The stresses in the laminate are calculated using a 48 d.o.f. laminated composite sector element. Subsequently, the Tsai-Wu criterion is used to detect the zones of failure and the maximum stress criterion is used to identify the mode of failure. Then the material properties of the laminate are degraded in the failed regions. The stress analysis is performed again using the degraded properties of the plies. The iterative process is repeated until no more failure is detected in the laminate. The problem of a typical T300/N5208 composite [45 degrees/0 degrees/-45 degrees/90 degrees](s) circular plate being impacted by a spherical impactor is solved and the results are compared with experimental and analytical results available in the literature. The method proposed and the computer code developed can handle symmetric, as well as unsymmetric, laminates. It can be easily extended to cover the impact of composite rectangular plates, shell panels and shells.

Some observations on the ignition of composite solid propellants

Heat-up times derived from studies on the ignition characteristics of a few model composite solid propellants, containing polystyrene, carboxy-terminated polybutadiene, plasticised polyvinyl chloride and polyphenol formaldehyde as binders, show that they are directly proportional to the mass of the sample and inversely proportional to the hear flux. Propellant weight-loss prior to ignition and high pressure ignition temperature data on the propellants, ammonium per chlorate, and binders show that the ignition is governed by the gasification of the binder pyrolysis products. The activation energy for the gasification of the pyrolysed polymer products corresponds to their ignition behaviour suggesting that propellant ignition is controlled by the binder.

Large deformation finite element analysis of laminated circular composite plates

A 48 d.o.f., four-noded quadrilateral laminated composite shell finite element is particularised to a sector finite element and is used for the large deformation analysis of circular composite laminated plates. The strain-displacement relationships for the sector element are obtained by reducing those of the quadrilateral shell finite element by substituting proper values for the geometric parameters. Subsequently, the linear and tangent stiffness matrices are formulated using conventional methods. The Newton-Raphson method is employed as the nonlinear solution technique. The computer code developed is validated by solving an isotropic case for which results are available in the literature. The method is then applied to solve problems of cylindrically orthotropic circular plates. Some of the results of cylindrically orthotropic case are compared with those available in the literature. Subsequently, application is made to the case of laminated composite circular plates having different lay-up schemes. The computer code can handle symmetric/unsymmetric lay-up schemes. The large displacement analysis is useful in estimating the damage in composite plates caused by low-velocity impact.

Al2O3---ZrO2 composite coatings for thermal-barrier applications

Graded alternate layers of Al2O3 and 8% Y2O3-ZrO2 and their admixtures were plasma sprayed onto bond-coated mild steel. They were evaluated for thermal-shock resistance, thermal-barrier characteristics, hot corrosion resistance (molten NaCl corrodant) and depth of attack, adhesion strength and the presence of phases. Although front-back temperature drops of 423-623 K were observed, some of the coatings showed good adherence even after 100 thermal shack cycles. In the sequence of the graded layers, the oxide which is directly in contact with the bond coat appears to influence the properties especially in coatings of 150 and 300 mu m thickness. Molten NaCl readily attacks the films at high hot-face temperatures (1273 K for 1 h) and the adhesive strength falls significantly by 50-60%. Diffusion of alkaline elements is also found to depend on the chemical composition of the outer coating directly facing the molten corrodant. (C) 1997 Elsevier Science Limited.

Stochastic Aeroelastic Analysis of Composite Helicopter Rotor

The effect of uncertainty in composite material properties on the aeroelastic response, vibratory loads, and stability of a hingeless helicopter rotor is investigated. The uncertainty impact on rotating natural frequencies of the blade is studied with Monte Carlo simulations and first-order reliability methods. The stochastic aeroelastic analyses in hover and forward flight are carried out with Monte Carlo simulations. The flap, lag, and torsion responses show considerable scatter from their baseline values, and the uncertainty impact varies with the azimuth angle. Furthermore, the blade response shows finite probability of resonance-type conditions caused by modal frequencies approaching multiples of the rotor speed. The 4/rev vibratory forces show large deviations from their baseline values. The lag mode damping shows considerable scatter due to uncertain material properties with an almost 40% probability of instability in hover.

Nucleation and growth characteristics of cavities during the early stages of tensile creep deformation in a superplastic zirconia-20 wt% alumina composite

Constant-stress tensile creep experiments on a superplastic 3-mol%-yttria-stabilized tetragonal zirconia composite with 20 wt% alumina revealed that cavities nucleate relatively early during tensile deformation. The number of cavities nucleated increases with increasing imposed stress. The cavities nucleate at triple points associated largely with an alumina grain, and then grow rapidly in a cracklike manner to attain dimensions on the order of the grain facet size. It is suggested that coarser-grained superplastic ceramics exhibit lower ductility due to the ease in formation of such grain boundary facet-cracks and their interlinkage to form a macroscopic crack of critical dimensions.

Regarding the effect of aging heat treatment on the failure behaviour of a SiC reinforced Al based metal matrix composite

In the present study, 6061 Al metallic matrix was reinforced by 12.2 wt% df SiC particulates using liquid metallurgy route. The composite material thus obtained was extruded and characterized in the as-solutionized and peak aged conditions in order to delineate the effect of aging associated precipitation of secondary phases on the tensile fracture behavior of the composite samples. The results' of microstructural characterization studies carried out using scanning electron microscope revealed the increased presence of precipitated secondary phases in the metallic matrix and a more pronounced interfacial segregation of alloying elements in case of peak aged samples when compared to the as-solutionized samples. The results of the fractographic studies conducted on the as-solutionized samples revealed that the failure was dominated by the SiC particulates cracking while for the peak aged samples the fracture surface revealed a comparatively more pronounced SiC/6061 Al debonding and reduced SiC particulates cracking. This change in the failure behavior was rationalized in terms of embrittlement of the interfacial region brought about by the aging heat treatment and is correlated, in addition, with the mechanical properties of the composite samples in as-solutionized and peak aged conditions.

Microstructure and properties of squeeze cast Cu-carbon fibre metal matrix composite

There have been reported attempts of producing Cu based MMCs employing solid phase routes. In this work, copper was reinforced with short carbon fibres by pressure infiltration (squeeze casting) of molten metal through dry-separated carbon fibres. The resulting MMC's microstructure revealed uniform distribution of fibres with minimum amount of clustering. Hardness values are considerably higher than that for the unreinforced matrix. Addition of carbon fibres has brought in strain in the crystal lattice of the matrix, resulting in higher microhardness of MMCs and improved wear resistance. Tensile strength values of MMCs at elevated temperatures are considerably higher than that of the unreinforced matrix processed under identical conditions. (C) 1999 Kluwer Academic Publishers.

Residual strength and fatigue life assessment of composite patch repaired specimens

Design, analysis and technology for the integrity enhancement of damaged or underdesigned structures continues to be an engineering challenge. Bonded composite patch repairs to metallic structures is receiving increased attention in the recent years. It offers various advantages over rivetted doubler, particularly for airframe repairs. This paper presents an experimental investigation of residual strength and fatigue crack-growth life of an edge-cracked aluminium specimen repaired using glass epoxy composite patch. The investigation begins with the evaluation of three different surface treatments from bond strength viewpoint. A simple thumb rule formula is employed to estimate the patch size. Cracked and repaired specimens are tested under static and fatigue loading. The patch appears to restore the original strength of the undamaged specimen and enhance the fatigue crack growth life by an order of magnitude. (C) 1999 Elsevier Science Ltd. All rights reserved.

Sensing of delaminations in composite laminates using embedded magnetostrictive particle layers

This is an exploratory study to illustrate the feasibility of detecting delamination type of damage in polymeric laminates with one layer of magnetostrictive particles. One such beam encircled with excitation and sensing coils is used for this study. The change in stress gradient of the magnetostrictive layer in the vicinity of delamination shows up as a change in induced voltage in the sensing coil, and therefore provides a means to sense the presence of delamination. Recognizing the constitutive behavior of the Terfenol-D material is highly nonlinear, analytical expressions for the constitutive relations are developed by using curve fitting techniques to the experimental data. Analytical expressions that relate the applied excitation field with the stress and magnetic flux densities induced in the magnetostrictive layer are developed. Numerical methods are used to find the relative change in the induced voltage in the sensing coil due to the presence of delamination. A typical example of unidirectional laminate, with embedded delaminations, is used for the simulation purposes. This exploratory study illustrates that the open-circuit voltage induced in the sensing coil changes significantly (as large of 68 millivolts) with the occurrence of delamination. This feature can be exploited for device off-line inspection techniques and/or linking monitoring procedures for practical applications.

Studies on indentation fracture toughness on ceramic and ceramic composite using acoustic emission technique

This paper is aimed at investigating the acoustic emission activities during indentation toughness tests on an alumina based wear resistant ceramic and 25 wt% silicon carbide whisker (SIC,) reinforced alumina composite. It has been shown that the emitted acoustic emission signals characterize the crack growth during loading. and unloading cycles in an indentation test. The acoustic emission results indicate that in the case of the composite the amount of crack growth during unloading is higher than that of loading, while the reverse is true in case of the wear resistant ceramics. Acoustic emission activity observed in wear resistant ceramic is less than that in the case of composite. An attempt has been made to correlate the acoustic emission signals with crack growth during indentation test.

The studies on micro-structural defects in deformed composite matrix of Al-Si-Mg and SiC

Detailed Fourier line shape analysis has been performed on three different compositions of the composite matrix of Al-Si-Mg and SiC. The alloy composition in wt% is Al-7%Si, 0.35%Mg, 0.14%Fe and traces of copper and titanium (similar to 0.01%) with SiC varying from 0 to 30wt% in three steps i.e., 0, 10 and 30wt%. The line shift analysis has been performed by considering 111, 200, 220, 311 and 222 reflections after estimating their relative shift. Peak asymmetry analysis has been performed considering neighbouring 111 and 200 reflections and Fourier line shape analysis has been performed after considering the multiple orders 111 and 222, 200 and 400 reflections. Combining all these three analyses it has been found that the deformation stacking faults both intrinsic alpha' and extrinsic alpha " are absent in this alloy system whereas the deformation twin beta has been found to be positive and increases with the increase of SiC concentration. So, like other Al-base alloys this ternary alloy also shows high stacking fault energy, and the addition of SiC introduces deformation twin which increases with its concentration in the deformed lattices.

Enhanced lithium-ion transport in PEG-based composite polymer electrolyte with Mn0.03Zn0.97Al2O4 nanoparticles

The ion conduction and thermal properties of composite solid polymer electrolyte (SPE) comprising Poly(ethylene) Glycol (PEG, mol wt. 2000), lithium perchlorate (LiClO4) and insulating Mn0.03Zn0.97Al2O4 nanoparticle fillers were studied by complex impedance analysis and DSC techniques. The average size of the nanoparticles was determined by powder X-ray diffraction (XRD) using Scherrer's equation and was found to be similar to 8 nm. The same was also determined by TEM imaging and found to be similar to 12 nm. The glass transition temperature T, as measured by differential scanning calorimeter (DSC), showed a minimum at 5 mol% of narroparticles. Fractional crystallinity was determined using DSC. NMR was used to deter-mine crystallinity of a pure PEG sample, which was then used as the standard. Fractional crystallinity X. was the lowest for 5 mol% and beyond. The ionic conductivity of the composite polymer electrolyte containing 5 mol% Mn0.03Zn0.97Al2O4 nanoparticles was found to be 1.82 x 10(-5) S/cm, while for the pristine one, it was 7.27 x 10(-7) S/cm at room temperature. As a function of nanoparticle content, conductivity was observed to go through two maxima, one at around 5 mol% and another shallower one at around 12 mol%. The temperature dependence of conductivity could be divided into two regions, one consistent with Arrhenius behaviour and the other with VTF. We conclude that the enhancement of ionic conductivity on the addition of Mn0.03Zn0.97Al2O4 nanoparticles is a result of reduction in both the T, and the crystallinity. (C) 2002 Elsevier Science B.V. All rights reserved.