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Articles by B. Nageswara Rao
Total Records ( 12 ) for B. Nageswara Rao
  K.G. Muthurajan , K. Sankaranarayanasamy , S.B. Tiwari and B. Nageswara Rao
  Studies are made to understand the delamination growth of general laminates with general loading conditions. Post-buckling solutions are obtained for a laminate with clamped ends applicable to thin film strip delamination in a base laminate under uniform membrane loads. The strain energy release rate at the crack-tip (G) is derived in terms of the critical equivalent base laminate strain at the onset of the buckling (ε*cr) and the applied equivalent strain (ε*). It is also expressed in terms of the maximum amplitude (Wmax) of the delaminated layer. A Griffith-type fracture criterion with constant specific fracture energy (Gc) of the material is used to govern the delamination growth. The stability characteristics of the delamination growth are discussed. The maximum amplitude (Wmax) of the delaminated layer increases with the applied load without enhancement in the length of the delamination for the values of G less than Gc. Initiation of the delamination growth can be expected when the value of G is very close to Gc.
  K.G. Muthurajan , K. Sankaranarayanasamy , S.B. Tiwari and B. Nageswara Rao
  Finite element analysis for adhesively bonded joints has been carried out by developing a special 6 node isoparametric element for the adhesive layer. The adhesive layer is assumed to be relatively thin and behaves elastically as simple tension-compression springs and shear spring connecting the adherends. The results of a metal-metal single lap joint are found to be in good agreement with the closed-form solution of Goland and Reissner. This adhesive element can be utilized for modeling adhesively bonded joints having shaped adherends for optimum joint efficiency.
  K.G. Muthurajan , K. Sankaranarayanasamy , S.B. Tiwari and B. Nageswara Rao
  Delamination is one of the most commonly observed failure modes in laminated composites. The existence of a delamination in a structure can significantly reduce the stiffness and strength of the structure. Various types of cracked specimens were designed to quantify the effect of delaminations in composites. The growth of delamination in composite structures starts when the strain energy release rate (G) under service loads exceeds the fracture energy (Gc). The measurement of fracture toughness of a material relies strongly on the method of data interpretation. In the original form, the experimental fracture data normally consists of a load-displacement record for a cracked specimen. Data reduction requires the knowledge of the specimen crack length and dimensions and a suitable method for calculating the fracture parameters. This study presents a simple and reliable procedure for the evaluation of mode I and mode II delamination fracture toughness from the test data of Double Cantilever Beam (DCB) specimens and Cracked-Lap Shear (CLS) specimens made of carbon/epoxy. Critical load estimation of DCB specimens from the measured specific fracture energy (GIC) of the materials is found to be in good agreement with test results.
  V.M.J. Sharma , V. Diwakar , K. Sree Kumar , B. Nageswara Rao and S.D. Pathak
  High strength materials are prone to failure in presence of flaws/cracks and this is expressed through the fracture toughness of the material. In fracture base design, the structure should be designed such that for a detectable minimum/or design allowable flaw size the structure under given service condition, the stress intensity factor (K) at the crack tip should always be less than KIc, the plane strain fracture toughness. In view of the sensitiveness of the parameter to the material conditions in terms of heat treatment, grain size, etc. it has become generally necessary to confirm the achieved KIc with respect to what is required by the design. It has been found extremely difficult and impractical to follow standard tests for KIc determination especially at various stages of fabrication/quality control, in view of involved procedures, for testing and to meet the validity conditions of the test. In fracture toughness testing, the Compact Tension (CT) specimen is recommended as one of the standard specimens. Many times the test becomes invalid as per ASTM E 399 standard. Since fracture toughness testing is a costly affair, it is preferable to minimize the number of repeat tests. A simple J-integral method is adopted for assessing the fracture toughness from the invalid test data of standard CT specimens.
  Renganathan K., , B. Nageswara Rao and M.K. Jana
  A simple procedure was established to obtain analytical solution for the general case of multi-layer thick cylindrical shell with each layer having different material properties under axial inertia loading. Design formulae in handbooks or monographs for a two-layer reinforced propellant grain, was shown to be a special case of the present general analytical solution. The solution of the problem was found to be useful for slump displacement evaluation of the propellant grain in a rocket motor under vertical storage condition. The slump displacement at the inner bore of the propellant grain was found to increase rapidly within an hour and later on increases slowly with time. When the elastic modulus varied monotonically in the ascending order from the inner radius to outer radius, the slump displacement at the inner bore of the propellant grain was found to increase compared to the case where the modulus varied monotonically in the descending order. Finite element solutions of all the above problems were found to be in good agreement with the present analytical solutions.
  K. Jayakumar , D. Yadav and B. Nageswara Rao
  This study presents a closed-form solution, utilizing a classical stress formulation approach to carry out elasto-electro-thermo analysis of generalized plane-strain of a right circular cylindrical shell. The present analytical solution holds good for thin as well as thick cylindrical shells made of metallic/composite/piezoelectric/incompressible materials. Standard finite elements are not suitable for modelling the incompressible nature of the solid propellant grains in rocket motors. An efficient axisymmetric hybrid-stress displacement formulation for compressible/nearly incompressible materials will provide accurate results for solid propellant rocket motors. The finite element analysis results for a propellant grain reinforced with a thin metallic casing under thermal and pressure loads are found to be in good agreement with the present analytical solution. Stress analysis results are also presented for a piezoelectric cylindrical shell to examine piezoelectric effects when the shell is subjected to internal pressure. These analytical solutions can provide not only a validation for the finite element model but also provide a means of parametric study, which is useful in the preliminary design stage.
  N.S. Babu , S.B. Tiwari and B. Nageswara Rao
  Studies are made on graphitised carbon fibres using wide-angle X-ray diffraction technique and Scanning/Transmission Electron Microscopy (SEM/TEM) to obtain structural parameters and morphological features of the fibres. Two important crystallite size structural parameters La and Lc can be generally obtained from X-ray diffraction technique. Lc values are evaluated by applying the necessary correction factors for the non-separation of Kα1 and Kα2 doublet and instrumental broadening. For simplicity empirical expressions are developed for these correction factors from the standard monograms. The surface features of pitch and PAN graphitised carbon fibres at high resolution are examined using SEM and orientation studies are made using TEM. Finally crystallite domain size parameters are related to the tensile modulus of the fibres. It is found that the crystallite domain size increases with the graphitisation temperature and stretching loads. This in turn leads to the enhancement in the tensile modulus of the fibre.
  K. Jayakumar , D. Yadav and B. Nageswara Rao
  Utilizing the stress formulation approach a closed-form solution is obtained for a long multi-layer cylindrical shell subjected to electro-thermo-mechanical loads. The present analytical solution holds good to examine the elastic behaviour of laminated composite shells with anisotropic piezoelectric layers acting as sensor and actuator under electro-thermo-mechanical loads. Standard finite elements are not adequate for modelling of the incompressible nature of the solid propellant grains in rocket motors. Utilizing MSC/MARC© software, finite element analysis has been carried out on a three-layered (solid propellant grain/insulation/metallic casing) cylindrical shell subjected to thermal and internal pressure loads. The structure is idealized using the eight node quadrilateral isoparametric Hermann element for incompressible materials like propellant grain and insulation and regular elements for casing material. The finite element analysis results are found to be in good agreement with the present closed-form solution. This analytical solution can serve as benchmark to finite element solutions. Stress analysis has also been carried out on a three-layer cylindrical shell to examine the piezoelectric effects of one of the layers as composed of piezoelectric layers.
  Swamy N.S., , H.R. Nataraja , K.S. Sai , S.B. Tiwari and B. Nageswara Rao
  This study considers the Rivlin-Ericksen constitutive equation for the Cauchy stress in the equation of motion to examine the flow of an incompressible second-grade fluid with an oscillating rigid moving plate. Simple and reliable numerical procedures are used to obtain the parameters in the analytical expressions for the velocity field and the shearing stress on the moving plate. The Doppler effect is noticed from the increased frequency due to the motion of the plate. The thickness of the boundary layer reduces with an increase in the magnetic interaction parameter.
  Sai K.S., , N.S. Swamy , H.R. Nataraja , S.B. Tiwari and B. Nageswara Rao
  This study presents an exact solution for the flow of two immiscible fluids under a general oscillatory time-dependent pressure gradient in a channel with one porous floor. The oscillatory behavior of the time-dependent pressure gradient is expressed in terms of Fourier series. At the interface, continuity of velocities and shear stresses is assumed. Equations governing the flow are solved using the slip condition at the permeable interface whereas the generalized Darcy`s law in the porous region. The unsteady flow depends upon the Reynolds numbers of the fluids, slip parameter and porous parameter. Analytical expressions are provided for the mass flow rate and wall shearing stresses. Numerical results are presented considering water and mercury as the two immiscible fluids for the uniform pressure gradient as well as for the sinusoidal time-dependent pressure gradient. Since the formulation of the problem is general, it is possible to examine the unsteady flow of any two immiscible fluids under any specified oscillatory time-dependent pressure gradient. This study will be useful in learning how the pressure and viscous forces exert their influence to produce different flow patterns.
  S.B. Tiwari , B. Nageswara Rao , H.R. Nataraja and K.S. Sai
  This research is concerned primarily with nonlinear oscillations of a modified van der Pol equation. The motion is represented by the harmonic oscillator equation, with the addition of a small nonlinear term. The governing differential equation falls under autonomous category. The solution of the problem is examined utilizing the method of slowly varying amplitude and phase (the Krylov-Bogoliubov-Mitropolsky technique). Stationary values of the amplitude are obtained and discussed their stability. It is noted that the stable limit-cycles of the differential equation in the higher order averaging method can be identified easily from the time derivative of the amplitude function and the sign of its derivative at the stationary value of the amplitude.
  K. Renganathan , B. Nageswara Rao and M.K. Jana
  Structural integrity under inertia loading was considered to be one of the design conditions for a solid propellant rocket motor. Grain structural integrity evaluations were generally based upon mechanical properties which were evaluated from tests of specimens removed from carton samples of propellant. Significant deviations were noticed while testing specimens from carton samples and from dissecting grains due to manufacturing process. Main grain to carton correlation factors established for a HTPB-based propellant and their adequacy examined measuring slumps in cylindrical grain segments. A simple methodology presented to carryout viscoelastic finite element analysis for slump estimations in rocket grains under vertical storage condition. Finite element analysis was carried out on a cylindrical segment grain of a typical steel casing rocket motor using the eight-node quadrilateral axi-symmetric Hermann element of the MARC computer program. A mathematical model proposed to represent the time dependent master stress relaxation modulus of a HTPB-based propellant grain essential for estimation of slump displacements in cast segment grains. Measured slump displacements in rocket motors of different segment grains at different storage times were found to be reasonably in good agreement with the finite element analysis results.
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