2. Thesis and Dissertations

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    Study of Vibration Characteristics of Skew Laminated Composite Sandwich Plates and Shells Operating In Hygro and Thermal Environments
    (National Institute of Technology Karnataka, Surathkal, 2022) Kallannavar, Vinayak Basavanth; Kattimani, Subhaschandra
    This dissertation presents an investigation of the influence of temperature and moisture on free vibration characteristics of laminated composite, hybrid composite, and sandwich panels. The composite sandwich panels (plates and shells) considered to be made of laminated composite material face sheets, and softcore materials as the core materials. A finite element (FE) formulation is developed for the whole model using a layerwise first-order shear deformation theory (FSDT) considering the uniform temperature and moisture concentration rise. The influence of temperature and moisture on the natural frequencies of various shell structures such as cylindrical, ellipsoid, hyperbolic, and spherical shells are investigated and compared with the frequencies of flat sandwich plates. Effects of length to width ratio, length to thickness ratio, radius to length ratio, the ratio of core thickness to the thickness of the face sheet, skew angle, boundary conditions on the vibration characteristics of the skew laminated composite sandwich panels are studied. Additionally, an attempt has been made to investigate the challenges in an experimental vibration study of the laminated composite sandwich plates with a 3D printed Polylactic acid (PLA) core. Furthermore, an artificial neural network (ANN) based predictive model is established to understand the influence of temperature and moisture on the skew sandwich plates. Further study has been extended to investigate the performance of active constrained layer damping (ACLD) of a laminated composite sandwich plate operating in a thermal environment. Additionally, an experimental investigation is performed to understand the influence of temperature on the natural frequency of laminated composite sandwich plates operating in sub-ambient temperatures. Consequently, the present investigation is believed to be enormously helpful in the field of computational mechanics and structural health monitoring of composite sandwich structures operating in challenging environments suitable for various industrial applications.
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    Buckling and Dynamic Characteristics of Cenosphere/Epoxy Syntactic Foam Composites and Sandwiches under Mechanical and Thermal Loads
    (National Institute of Technology Karnataka, Surathkal, 2019) Waddar, Sunil Shankar.; Pitchaimani, Jeyaraj; Doddamani, Mrityunjay
    Polymer matrix composites provide lower weight structures and result in improved efficiency and performance in many transportation engineering applications. Thermosetting polymers reinforced with suitable hollow particle constituents, higher specific properties can be achieved. Development of syntactic foams with cenospheres serves dual purpose of beneficial utilization of industrial waste fly ash and reduction in the component cost in addition to weight reduction. In the present study, LAPOX L-12 epoxy resin is used as the matrix material and fly ash cenospheres in as received and silane modified conditions are used as filler material. Manual stirring method is employed for developing cenosphere/epoxy syntactic foams with as received and surface treated cenospheres in 20, 40 and 60 volume %. Sandwich composites are also prepared using sisal fiber woven fabric reinforced in epoxy as facings and syntactic foam as core. With increasing cenosphere content, density of untreated and silane treated foams decreases in general. Influence of cenosphere surface treatment and volume fraction of cenospheres in epoxy matrix on buckling and dynamic characteristics are experimentally investigated in this work. Buckling and free vibration behavior of cenosphere/epoxy syntactic foams under mechanical and thermal loadings are investigated experimentally in this work. Buckling load is obtained from the load-deflection curve based on the Double Tangent Method (DTM) and Modified Budiansky Criteria (MBC). Further, the influence of axial compression load on the natural frequencies associated with the first three transverse bending modes is analyzed. Finally, the buckling loads predicted using DTM and MBC are compared with the buckling load calculated based on the vibration correlation technique (VCT). It is observed that the buckling loads predicted through the three different methods are in close agreement. Experimental results revealed that the buckling load and natural frequency of syntactic foams increase with cenosphere volume fraction. It is observed that natural frequencies reduce with increase in axial compression load for all the modes. However, rapid increase in the fundamental frequency is observed when the compressive load is near and beyond the criticalbuckling load. It is observed that silane modified cenosphere embedded in epoxy matrix registered superior performance (rise in critical buckling load and natural frequencies to the tune of 23.75 and 11.46% respectively) as compared to untreated ones. Experimental results are compared with the analytical solutions that are derived based on Euler-Bernoulli hypothesis and results are found to be in good agreement. Finally, property map of buckling load as a function of density is presented by extracting values from the available literature. Experimental investigation on deflection behavior of fly ash cenosphere/epoxy syntactic foam under thermal environment (three different heating conditions) is investigated. Three different heating cases (increase-decrease, decrease and decreaseincrease) are considered. Influence of fly ash cenosphere volume fraction and nature of temperature variation on deflection behavior of syntactic foam beam is discussed elaborately. The temperature rise on the test specimens are measured using K-type thermocouples and lateral deflections are measured using Linear variable differential transducer (LVDT). The data is collected with the help of in-built LabVIEW program to plot temperature deflection curve. Results reveal that the syntactic foam beam experience snap-through buckling under thermal environment and is reflected by two bifurcation points in temperature-deflection plot also. It is observed that the time duration for which the syntactic foam beam stays in the first buckled position increases with increase in cenosphere content. Thermal environment induces compressive stresses in the samples causing such snap-through buckling. However, such phenomenon is not observed when the syntactic foam beams are exposed to mechanical compressive loads. Temperature variation across the beam length strongly influences snap-through buckling in syntactic foams in addition to volume fraction of filler content. An experimental study on buckling and dynamic response of cenosphere reinforced epoxy composite (syntactic foam) core sandwich beam with sisal fabric/epoxy composite facings under compressive load is presented. Influence of cenosphere loading and surface modification on critical buckling load and natural frequencies of the sandwich beam under compressive load is presented. The critical buckling load isobtained from the experimental load-deflection data while natural frequencies are obtained by performing experimental modal analysis. Results reveal that natural frequencies and critical buckling load increase significantly with fly ash cenosphere content. It is also observed that surface modified cenospheres enhance natural frequencies and critical buckling load of the sandwich beam under compressive load. Vibration frequencies reduce with increase in compressive load. Fundamental frequency increases exponentially in post-buckling regime. Experimentally obtained load-deflection curve and natural frequencies are compared with finite element analysis wherein results are found to be in good agreement. Buckling behaviour of sandwich composites made of syntactic foam core and sisal fabric/epoxy composite facings subjected to non-uniform heating is investigated. The critical buckling and snap-initiation temperatures are found from the temperaturedeflection plots. It is observed that, the critical buckling temperature increase with the filler content in the core material and surface treatment show slightly higher buckling temperature. The sandwich beams undergo snap-through buckling at higher temperatures due to developed viscoelastic forces. Due to increase in stiffness of the beam with filler content the deflection of the beam found to be less. The sandwich beams showed higher buckling temperatures than the neat syntactic foam samples.
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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.
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    Buckling and Dynamic Behavior of Non-Uniformly Heated Cylindrical Panels
    (National Institute of Technology Karnataka, Surathkal, 2018) Bhagat, Vinod; Jeyaraj, P.; Murigendrappa, S. M.
    Today, curved panels especially cylindrical and conical are considered as a backbone of numerous engineering structures. Knowledge of buckling and dynamic behavior of structures over a range of temperature is essential for their better design. Most of the studies carried out on heated panels are based on uniform temperature distribution assumption. However, in real life application, the cylindrical panels employed in structures are exposed to non-uniform temperature variation due to the location of the heating source and thermal boundary conditions. In the present study, the thermal buckling strength of the non-uniformly heated metallic panel predicted numerically is validated experimentally using in-house developed experimental set-up. Further studies are extended to investigate the effect of non-uniform temperature variation on buckling strength and free vibration characteristics of metallic, laminated composite, and functionally graded carbon nanotube (FGCNT) reinforced polymer composite, cylindrical panels using the finite element method. Finally, the optimization of a non-uniformly heated laminated cylindrical panel against thermal buckling strength and fundamental natural frequency is also carried out. Typical variation of temperature-deflection plot for different temperature fields is obtained experimentally and further, inflection point method is used to predict the critical buckling temperature from temperature-deflection plot. Experimental studies are further extended to analyze the influence of geometrical parameters and structural boundary constraints on critical buckling temperature. Experimentation results reveal that the location of the heat source and resulting non-uniform ivtemperature field influences the thermal buckling strength significantly. Among three cases examined in experimentation for the position of heat source, minimal buckling strength is observed when the heater is located at the center of the panel while maximum buckling strength is observed when the heater is located at the forefront curved edge. It is also found that aspect ratio and structural boundary constraints play a major role in deciding the buckling strength of the panel. From the numerical studies carried out on non-uniformly heated panels, a relation known as magnification factor is established to evaluate the buckling strength of non-uniformly heated cylindrical panels knowing the buckling strength of uniformly heated panels. Among five cases investigated for the position of heat source, the highest magnification factor is observed for a panel with the heat source located at the forefront curved edge. It is observed that the free vibration mode shapes of the panel change significantly with increase in elevated temperature. The changes are observed in terms switching of modes with a significant change in modal indices. With the rise in temperature, nodal and anti-nodal positions of a particular free vibration mode shape are shifting towards the location where the intensity of the heat source is high and structural stiffness is low. It is found that for a stiffer panel, the buckling strength of the laminated and FG-CNT composite panels with temperature-dependent elastic properties is significantly lesser than that of the panels with temperature independent elastic properties. Panel with maximum area exposed to a peak temperature of particular non-uniform temperature fields shows lowest buckling strength. Functional grading of CNTs with more amount of CNTs located close to top and bottom of the panel (FG-X) results in higher buckling strength and free vibration frequencies compared to those panel with maximum CNTs distribution near the mid-plane. Free vibration frequencies of non-uniformly heated FG-CNT panel with temperature dependent properties is observed to decrease drastically with elevated temperature compared to the panel with temperature independent properties. Variation vin frequencies observed in a pre-stressed panel with temperature dependent and independent properties is more significant in stiffer panels. Irrespective of temperature dependent and independent properties, shifting of nodal and anti-nodal lines and change of modal indices are also observed at elevated temperature. Well-known and generally acknowledged optimization technique, particle swarm optimization is employed for the optimization of thermal buckling strength of laminated composite panels exposed to five different temperature fields. Two different optimization approach like single objective optimization approach and multiobjective optimization approach are employed. In single objective optimization, the panel is exposed known temperature field whereas, in multi-objective optimization, the panel is exposed to unknown temperature fields when in-service. It is found from the analysis that the variation in the optimum buckling strength of non-uniformly heated panels is more significant at lower curvature ratio. Whereas, variation in the optimum fiber orientation under different temperature fields is significant at higher curvature ratio. Multi-objective optimization approach has proved to be superior to that of single objective optimization approach when panels are exposed to the unpredictable thermal environment. Further, studies are carried out on optimization of both thermal buckling strength and fundamental free vibration frequency of heated panels using particle swarm optimization in conjunction with the artificial neural network. Multiobjective design index (MODI) has been derived for the panel considering buckling strength and fundamental frequency as objectives for optimization. It is found that MODI of the cylindrical panels under thermal load is complex and significantly influenced by the temperature fields, lamination scheme, in-plane boundary constraints, elevated temperature and geometric parameters. It is also observed that the MODI of the panel can be maximized by optimizing laminate orientations. Further, it is observed that panel with lamination scheme of (θ°/–θ°/θ°/–θ°)S gives higher value of MODI compared to other lamination schemes considered.