2. Thesis and Dissertations

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    Buckling and Free Vibration of Cylindrical Panels Under Non-Uniform Edge Loads
    (National Institute Of Technology Karnataka Surathkal, 2023) C M, Twinkle; P, Jeyaraj
    Introduction of lightweight materials for different structural members of aerospace, marine, civil and automobile sectors are being made possible by utilizing nano reinforcements and addition of porosity in the bulk composite. Cylindrical curved panel structures are extensively utilized in different engineering applications owing to their better structural stability characteristics. Stability and dynamic behaviour analysis of these lightweight cylindrical panel structures is essential for the satisfactory design. In general, the buckling and dynamic characteristics of these panels are mostly studied under uniform edge load (UEL) conditions. However, the panels are exposed to nonuniform and partial edge loads in practical situation. Hence, the prediction of buckling and free vibration characteristics of the panels under different non-uniform edge loads (NELs) will help the designers in avoiding the failure of these structures. The buckling and free vibration characteristics of different nano composite panels namely, GPL reinforced porous, GPL reinforced porous core sandwich, CNT and GOP reinforced cylindrical panels under NELs are calculated using semi analytical method in the present study. Considering a higher order shear deformation theory, Hamilton’s principle is used to formulate the governing differential equations and buckling and free vibration solutions are obtained by employing the semi analytical method based on Galerkin’s approach. Initially, the membrane stress resultants due to the applied edge loads are represented through Airy’s stress function expansion. Then the stress resultants are evaluated through the minimisation of strain energy. Followed by this, equations of motion are obtained based on Hamilton’s principle and the stress resultants. The Eigen value problems of buckling and free vibration are solved using the semi analytical method. Buckling and free vibration characteristics of graphene nano platelets (GPL) reinforced porous cylindrical panel under the inuence of NELs is studied rst. The distribution of GPL and porosity is varied in a layer wise fashion through the thickness. The effective mechanical properties are calculated using extended rule of mixture together with Halpin-Tsai micromechanics model and open-cell metal foam properties. It is found that the type of NEL greatly inuences the critical buckling load of the cylindrical panel. Further, the critical buckling load and natural frequency varies with a particular combination of porosity and GPL distributions. Next, a sandwich cylindrical panel with GPL reinforced porous core and metal facing sheets is analyzed. The effective mechanical properties are obtained by using properties of open cell foams and Halpin–Tsai micro mechanical model. Effects of nature of in-plane edge load, distribution of porosity and GPL, porosity coefcient, GPL loading, core to total thickness ratio are analyzed in detail. It is found that for the panel with high core thickness, even for the higher amount of porosity, the buckling resistance and free vibration frequency can be improved by properly tailoring the graded distribution of both the GPL and pores. Furthermore, a signicant variation in buckling load and free vibration frequencies is observed with respect to the type of in plane loading. Remarkable change in buckling mode and free vibration mode shape (with increase in the load intensity) is observed for panels having higher aspect ratio. The sandwich cylindrical panel with a core having a distribution of less porosity and high GPL content at the extreme surfaces provides maximum buckling strength and free vibration frequency value. Next, buckling and free vibration characteristics of agglomerated carbon nanotubes (CNTs) reinforced nano cylindrical panels are studied considering nonlocal elasticity theory. Effective material properties of the agglomerated CNT reinforced composite are obtained using a two-parameter micro-mechanics model while Eringen’s non-local theory is used to account the size effect. A comprehensive study is carried out to analyze the inuence of various degrees of agglomeration (complete, partial), nature of edge load , and non-local effects on the buckling and free vibration response of CNT reinforced nano cylindrical panel. The results revealed that non-local size effect leads to a reduction in stiffness and thus reduces buckling and dynamic characteristics. It is also observed that critical buckling load varies with type of in plane load. The reduction in natural frequency with increase in the edge load intensity is different for different type of NEL. Finally, the buckling and free vibration characteristics of graphene oxide powder (GOP) reinforced cylindrical panels are studied. Inuence of loading of GOP quantity, nature of grading of GOP, nature of non-uniform and partial edge loads on critical buckling coefcient and fundamental frequency and mode shapes are investigated. It is noted that the buckling and vibration characteristics are sensitive to the nature of GOP grading, GOP loading and nature of variation in edge loads. Furthermore, the fundamental buckling mode is not always the typical (1, 1) mode instead of that (2, 1) mode is observed as the buckling mode according to the variation in aspect ratio and nature of edge loads. It is found that near critical buckling load, the fundamental vibration mode changes to (2,1) from (1,1) for parabolic and partial edge loading cases for the panels with aspect ratio higher than 1.3.
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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.