Buckling and vibration behaviour of syntactic foam core sandwich beam with natural fiber composite facings under axial compressive loads

Abstract

An experimental study of 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 is obtained 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.

Introduction

Sandwich composites with lightweight core find applications in marine, wind energy, aerospace and civil engineering structures due to their lower specific weights. Utilising low strength honeycomb cores or foams, than the metallic honeycomb core helps in reducing weight, manufacturing processes and resources [1]. Sandwich composites comprises of two thin and stiff skins with thick and lightweight core materials stacked in sequence as skin-core-skin. Many variations of this definition are available but the key factor in making this type of materials remains the lightweight core, which reduces the overall density of the material and stiff skins provide strength. Syntactic foams are a type of closed cell foams wherein closed porosity is present in the microstructure. Syntactic foams are two-component composite material system where hollow spheres are embedded into the matrix resin [2]. Syntactic foams are used as cores in sandwich structures owing to their high specific strength coupled with lower density. The use of closed cell structured core materials provides distinct advantages over other type of core materials, such as good adhesion between the skins and core [3]. The weakest point of the sandwich structures made with honeycomb core, when subjected to different loading conditions is debonding (delamination) of skins from the core material and wrinkling of the compressed side skins under compressive loads [3,4]. This motivates the researchers to adopt different processing routes for making cost-effective sandwich structures. Fiber reinforced polymers are used widely as skins in sandwich composites due to their low density and high specific strength. Another advantage offered by the use of polymer composites in skins is that the same polymer can be used to make the skin and the core. Cross-linking of polymer between core and skin would provide adhesion strength level equal to the strength of the polymer. This provides possibility of making the skin an integral part of the structure eliminating the requirement of the adhesive [5].

Natural fibers are low cost fibers with low density that possess properties comparable to those of man-made synthetic fibers [6,7]. Natural fiber composites find application in automotive, civil and footwear industries [8,9]. The commercial use of naturally available sisal fiber reinforced in polymer matrix composites are increasing due to its strength, low density, environmental friendliness and cost effectiveness [10,11]. Tensile, flexural and dielectric properties of vakka, banana, bamboo and sisal fiber reinforced polymer based composites reveal superior properties as a function of volume fraction. Sisal fiber reinforced polyester composites show higher specific flexural properties compared to the other fibers [12]. Venkateshwaran et al. [13] investigated mechanical and water absorption properties of banana/sisal fiber reinforced with epoxy resin. They observed that sisal fiber reinforced composites exhibited lower water absorption than banana fiber reinforced composites. Among different natural fibres, sisal fibre appears to be promising as they possess higher tensile strength than banana, silk, coir and cotton fibers [6,14]. The effect of gauge length (10–60 mm) on the sisal fiber are reported and found that the elastic modulus increases with gauge length with insignificant change in tensile strength [15]. Towo and Ansell [16] reported fracture and Dynamic Mechanical Analysis (DMA) of untreated and Sodium hydroxide treated sisal fiber reinforced with polyester and epoxy resin. They observed that the fiber content and fiber treatment enhanced the properties due to increased stiffness and proper interfacial bonding between the constituents. Mechanical properties of sisal-jute-glass fiber reinforced polyester composites are investigated by Ramesh et al. [17]. Their results reveal that jute-sisal mixed with glass fiber reinforced composites show increased flexural strength, whereas sisal fiber mixed with glass fiber reinforced composites presented higher impact strength. Li et al. [18] investigated tensile, flexural and DMA of sisal fiber reinforced in polylactide resin using injection moulding. They reported that the surface modified sisal fiber polylactide composites offered superior properties than untreated ones.

Studies on syntactic foam sandwich composites are available in literature wherein majority of research is focused on mechanical characterisation of foams and their sandwiches. Islam and Kim [19] investigated tensile and flexural response of sandwich composites prepared with paper skin and syntactic foam core. They observed that syntactic foams synthesized with lower particle size exhibits higher flexural properties than the sandwich with higher particle size. John et al. [20] investigated tensile and compressive properties of glass-microballoon/cyanate ester syntactic foam with carbon-cyanate ester skin and observed that the mechanical properties increases with resin content. Analytical approach to evaluate the buckling load of sandwich made of glass/carbon and boron fiber laminate skin and Poly Vinyl Chloride (PVC) foam is established by Aiello and Omres [21]. The theoretical model predicted better global buckling behaviour of sandwich panels for lower values of skin ratio thickness to overall sandwich thickness. Gupta et al. [22] investigated compressive properties of glass microballoon reinforced syntactic foam core with glass-epoxy and glass-carbon-epoxy skins. They observed delayed crack initiation for glass-carbon/epoxy hybrid skin than glass/epoxy ones. Recently waddar et al. [23,24] investigated buckling and vibration (free) behaviour of cenosphere embedded epoxy (syntactic foams) in bulk form and found that these properties show increasing trend with cenosphere content. Salleh et al. [25] investigated experimentally the mechanical properties of GFRP/vinyl ester skin with glass microballoon/vinyl ester syntactic foam core sandwich panels. They found that the properties are dependent on the weight fraction of the glass microballoons, void content and interfacial bonding between the constituents.

Buckling and free vibration studies of syntactic foam sandwich composites are scarce. Gonclaves et al. [26] investigated numerically buckling and free vibration of PVC foam core sandwich with steel face sheets using coupled stress finite element method. Microstructure dependent beam element predicted more accurate results than the classical Timoshenko beam model. Fleck and Sridhar [27] carried out experimental investigations on sandwich columns made of woven glass fibre epoxy skins and PVC foams with different densities. They observed that the columns undergo different types of buckling phenomenon (Euler macrobuckling, shear microbucking and face microbuckling) depending on the geometry of the sandwich columns. Grognec and Soaud [28] investigated numerically the elastoplastic buckling behaviour of sandwich beams with symmetric homogenous and isotropic core/skin layers subjected to axial compression. The results obtained numerically are in good agreement with the available analytical solutions. Grygorowicz et al. [29] presented analytical and numerical buckling analysis of sandwich columns with aluminium face sheet and aluminium alloy foam core. Mathieson et al. [30] investigated experimentally the effect of cross-sectional configuration and slenderness ratio on GFRP skin and polyurethane core sandwich composites. Lower slenderness ratios resulted in skin wrinkling mode of failure and length greater than critical slenderness ratios resulted in global buckling. Jasion and Magnucki [31] performed experimental, analytical and numerical analysis on buckling behaviour of aluminium foam core sandwich with aluminium face sheet subjected to axial compressive load. Experimentally obtained critical buckling loads are found to be closer to analytical and numerical results. Smyczynski and Magnucka-Blandzi [32] analysed buckling behaviour of simply supported sandwich beam with aluminium face and foam core numerically using transverse shear deformation effect. Sokolinsky et al. [33] investigated free vibration response of polymer foam core and steel face sheet cantilever sandwich beam analytically and experimentally. The results obtained using higher order theory are found to be in good agreement with experimental values. Tang et al. [34] investigated buckling behaviour of fixed-fixed and hinged-hinged calcium silicate face sheets sandwich panels with polyurethane foam core subjected to axial load. Buckling load values obtained through analytical, numerical (finite element method) and experimental routes matches closely. Wu et al. [35] investigated numerically the buckling and free vibration response of functionally graded carbon nanotube (CNT) reinforced composite face sheets with Titanium alloy core using Timoshenko beam theory. They observed that CNT volume fraction, end supporting conditions and slenderness ratio have significant influence on critical buckling loads and natural frequencies.

Literature review suggests that the sisal fiber reinforced skins with fly ash cenospheres reinforced in polymer matrix core should be explored for sandwich construction owing to higher specific properties finding applications in aerospace and marine industries. Main objective of the present work is to investigate buckling and dynamic behaviour of sandwich beam with fly ash cenosphere/epoxy syntactic foam as core with sisal fibre fabric composite laminate facings under compressive load. Effect of fly ash cenospheres loading and its surface modification on critical buckling load and free vibration frequencies of the sandwich beam under compressive load is studied in detail. Elastic properties of fly ash cenosphere and sisal fabric reinforced epoxy laminate are obtained experimentally. These values are further used to predict the critical buckling load and free vibration frequencies numerically. Finally, the numerical and experimental results are compared.