Buckling and Dynamic Characteristics of Cenosphere/Epoxy Syntactic Foam Composites and Sandwiches under Mechanical and Thermal Loads

Abstract

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.