2. Thesis and Dissertations

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    Vibro-Acoustic Response of Aerospace Structure Under Non-Uniform Edge Loads: an Analytical Investigation
    (National Institute of Technology Karnataka, Surathkal, 2022) G, Vijay; P., Jeyaraj
    Analytical investigation carried on vibro-acoustic characteristics of plates under different non-uniform uniaxial edge loads, subjected to steady-state mechanical and acoustic waves excitation is presented. Owing to their high stiffness to weight ratio, these functionally graded-graphene reinforced nanocomposites (FG-GRC) are used as structural members in unified wing aircrafts, space launchers etc., these Graphene reinforced nanocomposites plate is assumed to be a layered structure, in which weight fraction (WGNP) of the graphene nanoplatelets (GNPs) continuously vary in each layer through the plate thickness. It is also assumed that GNPs are evenly distributed in longitudinal direction, but randomly slanted towards the transverse direction of plate. By using 2D continuum orthotropic plate model, the effective material properties of graphene reinforced nanocomposites with different grading pattern/weight fraction of GNPs are obtained by combining the modified Halpin-Tsai model and rule of mixture. In order to model the porous graphene reinforced nanocomposites, closed-cell cellular solids under Gaussian Random Field (GRF) are used. An analytical method based on the strain energy approach is adopted to estimate the buckling load (Pcr). Free and forced vibration responses of the plate are obtained, by using an analytical method based on Reddy’s third-order shear deformation theorem (TSDT). Further, vibration response of the plate is given as an input to the Rayleigh integral code built-in-house using MATLAB® to obtain the acoustic response characteristics. Validation studies carried out to ensure the accuracy of results based on 2D continuum orthotropic plate model. The predicted buckling, vibration and acoustic characteristic results of graphene reinforced nanocomposites plates by using the 2D continuum orthotropic plate model is compared with the published results and shows the good agreement with the present approach. iv Initially, influence of non-uniform uniaxial edge (NUE) loads on vibration and acoustic response isotropic plates is investigated. The results reveals that the buckling load (Pcr) is significantly influenced by the nature of NUE loads. Similarly, natural frequencies reduce with an increase in axial compressive load due to a reduction in structural stiffness. Vibration and acoustic resonant amplitudes are affected by the intensity of the compressive load. Sound transmission loss reduces with an increase in compressive load magnitude and the effect is significant in the stiffness dominant region. Followed by this, free vibration and buckling characteristics of graphene reinforced nanocomposites under the different NUE loads, four distinctive gradings (i.e., UD, X, O, and C patterns of GNPs) and different WGNP are studied. Results revealed that buckling and free vibration behaviour of the plate is significantly influenced by the GNPs dispersion pattern and weight fraction under non-uniform edge loads. It is also observed that buckling mode and the fundamental vibration mode of the plate under combined tensile-compression load (i.e., load factor (α) = 2) is entirely different from the other NUE load cases. Furthermore, to understand the vibro-acoustic characteristics of graphene reinforced nanocomposites under the different NUE loads, same grading patterns and different WGNP have been selected from the previous studies on free vibration and buckling characteristics. It is found that, the nature of edge load variation on buckling and vibro-acoustic response is significant. Free vibration mode shape changes with an increase in edge load and consequently affects the resonant amplitude of responses also especially for the plates with a higher aspect ratio. WGNP and dispersion grading pattern of GNPs also influences the resonance amplitudes. Plate with FG-GRCC dispersion pattern and higher WGNP has improved buckling and vibro-acoustic response behaviour. Similarly, change in sound transmission loss level is significant in the stiffness region compared to the damping and mass dominated region. Finally, a detailed investigation of porosity grading and coefficients on vibro- acoustic characteristics of graphene reinforced nanocomposites under the different NUE loads is presented. Three types of porous distribution patterns, in which the porosity changes latterly the thickness bearing of the graphene reinforced v nanocomposites, are considered. Plates with porosity is less at the surface and more at the centre is termed as VPC (increasing porosity towards the centre). Plates with less porosity at the centre and more at the surfaces is termed as VPS (increasing porosity towards the surface) and the plate with uniform porosity is termed as UP. It is observed that, the WGNP and grading pattern of GNPs reinforcement causes the stiffness hardening effect, whereas porosity distribution and coefficients cause the stiffness softening effect on the graphene reinforced nanocomposite plate. It is found that the plate with symmetric distribution of GNPs with more concentration at the surface and symmetric porosity variation with more porosity at the centre radiates less sound power (i.e., with higher WGNP%).
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    Buckling and Dynamic Characteristics of Non-Uniformly Heated FGCNT Polymer Nanocomposite Plate
    (National Institute of Technology Karnataka, Surathkal, 2017) George, Nivish; Jeyaraj, P.; Murigendrappa, S. M.
    Nature of temperature variation influences buckling and dynamic behaviour of structures under thermal load. However, studies on buckling and dynamic behaviour of non-uniformly heated structures are very limited. In present work, influence of non-uniform temperature variation on buckling strength of beams made of aluminium and laminated glass-epoxy materials are investigated experimentally. A novel experimental set-up, built in-house, is developed to perform this investigation. The load vs deflection curve obtained experimentally is used to predict the thermal buckling strength using inflection point method. Non-linear finite element analysis, considering the initial geometric imperfection, has been carried out to compare the experimentally obtained typical load-deflection curve. Experimental and numerical results revealed that critical buckling temperature of the non-uniformly heated beam greatly differs from the uniformly heated beam. It is also observed that different locations of heat source and resulting non-uniform temperature variations influence the critical buckling temperature significantly depending on the location of heat source. With the confidence gained from the results obtained from experimental investigation, a detailed numerical investigation is carried out on Functionally Graded Carbon Nanotube (FG-CNT) reinforced polymer composite plate to obtain the influence of non-uniform heating on the buckling, free vibration and forced vibration response. The effective material constants of the plate are obtained using the extended rule of mixture along with efficiency parameters of the CNT (to include geometry-dependent material properties). Influence of boundary conditions, iiaspect ratio, functional grading of the CNT, non-uniform heating on thermal buckling, free and forced vibration behaviour of the heated plate are analysed. The acoustic response of the plate is analysed by solving the Rayleigh integral. It is observed that temperature fields and functional grading of CNTs influences the critical buckling temperature of the plates. Further, nature of functional grading showed significant change in buckling mode shapes irrespective of the boundary conditions. The first few natural frequencies of the plate under thermal load decreases as the temperature increases and they are influenced significantly by the nature of temperature field. The free vibration modes of the rectangular plates are sensitive to the nature of temperature field whenever there is a free edge associated with the boundary condition. It is observed that, the plates with FG-X type CNT distribution showed better thermal buckling strength and free vibration characteristics in comparison to other types of functional grading. The resonant amplitude of vibration and acoustic response are significantly influenced by the nature of different functional grading and rise in thermal load. This reflects in the band wise calculation of sound power also which recommends the CNT functional grading with X distribution along the thickness direction for lower frequency level. Considerable increase in sound power level has been observed with increase in thermal load in the lower frequency range due to the variation in the stiffness associated with the plate. Similar variation in vibroacoustic response has been observed with increase in the CNT loading also.