Parametric Analysis on the Vibration Characteristics of Sandwich composites Structure

Transcription

1 Parametric Analysis on the Vibration Characteristics of Sandwich composites Structure Amith Kumar S J 1, Sabeel Ahmed K 2 & Santosh M B 3 1, 2, 3 Department of Mechanical Engineering Jawaharlal Nehru National College of Engineering, Shimoga , Karnataka, India Visveswaraiah Technological University, Belgaum-Karnataka, India joanesamith@gmail.com 1, sabil_k@yahoo.com 2, santhoshmb22@gmail.com 3 Abstract Syntactic foam sandwich composites are potential materials for aerospace and marine applications because of their high specific properties and better energy absorption characteristics. In the present work, syntactic foam is prepared by uniform mixing of dry fly ash cenosphere (solid waste material) and phenolic resin in equal proportions. The effect of stiffening the syntactic foam core by resin impregnated paper honeycomb (RIPH) structure on vibration characteristics of sandwich composite panels are experimentally investigated under freely suspended configuration. Uni-axial Accelerometer, 4 Chanel real time analyzer and impulse hammer are used to analyze vibration signals using half-power bandwidth method. The results reveals that, frequency and damping ratio of sandwich composites panels are significantly influenced by the cell size of resin impregnated paper honeycomb structure infused in syntactic foam core. The effects of cell size on the core density of sandwich panels and its influence vibration response are also discussed. Index Terms Frequency response, Half power Bandwidth, Sandwich Panels, vibration signals. I. INTRODUCTION Composite materials offer high strength, higher modulus and lower density. This is of utmost important in many applications such as aerospace, marine and automotive structures. Depending on whether the structural design is strength-critical or stiffness-critical, the material used should therefore have a high strengthto-weight ratio or a high stiffness-to-weight ratio. Sandwich composites are produced by attaching two thin but stiff skins to a lightweight thick core. Further, the sandwich construction must exhibit good mechanical response under static loading [1]-[3] and impact loading [4]-[5]. Foam cored sandwich structures are increasingly used in the aforementioned sectors for its low density characteristics. Structural response of the sandwich composite depends mainly on the strength properties of the foam core material [6]. The purpose of the core is limited to transmit shear stresses between the face skins and to keep the skin distance approximately constant during the deformation under transverse loading condition [7]. Syntactic foam is one such core material which yields higher strength and stiffness properties. Syntactic foam based sandwich composites are presently being used in wide variety of marine and naval engineering applications [8]. Some of the researchers investigated dynamic response of composite structures using various techniques [9]-[10]. A review of the literature reveals that, syntactic foams of different materials have been successfully used as structural materials. However, most of the studies deals with the effect of change in volume fraction, density, microsphere particle size (radius ratio) etc on mechanical properties of syntactic foams. The benefit of stiffening the syntactic foam by the incorporation of resin impregnated paper honeycomb (RIPH) core structure and their influence on vibration characteristic is not found in the literature till date. The present work explores this possibility by investigating the vibration signal frequency response of sandwich composites with core of syntactic foam integrated with RIPH structure. II. MATERIALS AND METHODS Materials used in the present investigations are, dry fly ash cenosphere of density 450 kg/m 3 supplied by M/S Cenosphere India Pvt Ltd, Kolkota and phenolic resin of density 1120 kg/m 3 supplied by M/S Romit Resins Pvt Ltd, Raigad- Maharastra are used for the 80

2 fabrication of syntactic foam based Eco-core (blend of dry fly ash cenosphere and phenolic resin).e-glass fabric of 185 gsm and Epoxy resin LY 556 with Hardener HY 951 (in the ratio 10:1) supplied by M/S Insulation House Bangalore are used for the fabrication of face sheets. Fig. 1 shows the warm press molding technique used for the preparation of syntactic foam core. The mold consists of two M.S plates of size ( ) mm. The spacing between the mold plates is maintained at 12.5 mm using a metal frame. Mold surfaces are coated with silicon grease and then wrapped with aluminium foil to ensure easy removal of foam core after curing. Syntactic foam is made by the blend of dry fly ash cenosphere and phenolic resin uniformly mixed in the ratio of 50:50 by weight. The mixture is then thoroughly packed in the inner cavity of the metal frame which is kept on the lower mold plate. After packing the blend, the two mold plates are firmly clamped and then heated. The core is cured at C for 15 minutes in between the mild steel mold plates followed by room temperature curing for at least 24 hours before its use in the fabrication of sandwich composite. Provision is also made in the mold to allow the hot gases to escape during the process of curing. The developed composite out of this process called as stiffened-syntactic foam core. The configuration of developed sandwich composites is shown in Fig. 3. Table 1 details the developed sandwich composite panels. Syntactic foam + RIPH Heat Top Mold Plate Bottom Mold Plate Mold plates are clamped using C-clamps Metal Frame Heat Fig.1 Warm Press Molding Process For fabrication of sandwich composites, glass/epoxy laminates with fiber mass fraction of about 0.5 is first prepared by wet layup technique and then vacuum bonded to the core to form the face sheets as shown in Fig. 2. The thickness of the face sheets is maintained at 1.5 mm. The sandwich composites are cured for 24 hours at room temperature and then post cured at 100 ± 3 0C for one hour before being used for the preparation of test panels. Fig. 3 Configuration of sandwich composites panels with core of (a) Syntactic foam (b) Syntactic foam with kraft RIPH structure Table 1 Details developed of sandwich composite panels Glass/epoxy face sheets + syntactic foam Vacuum Bag To Vacuum pump Mold Plate Metal Frame with gasket seal Fig. 2 Vacuum bagging technique 81

3 III. EXPERIMENTATION The vibration testing instrument used in the experiment is calibrated and adjusted. It mainly consists of 4 channel real time analyzer, uni-axial accelerometer and impulse hammer. The configuration of experimental setup is shown in Fig.4. Fig. 5 Half power band width Fig. 4 Experimental setup The sandwich panels to be tested are hanged from a frame by thin strings. This simulates free boundary conditions and therefore prevents the introduction of unwanted stresses into the test panels that would need to be accounted for in the calculations. The test panel is tapped to induce the required vibration excitation using impulse hammer, the frequency domain response measured through accelerometer is displayed in terms of acceleration v/s frequency. The frequency response function (FRF) data is stored and processed to calculate the natural frequency and damping ratio. The bandwidth method is used to estimate damping ratio (ζ) of the test panels from frequency domain response. Bandwidth ( ω) is defined as the width of the frequency response magnitude curve when the magnitude is 1 2 times the peak value. Equation (1) is used to determine the damping ratio of the test panels. The variable parameter considered in the present investigation is the cell size of resin impregnated paper honeycomb structure which is integrated with syntactic foam core. The effect of cell size on natural frequency and damping ratio of syntactic foam core sandwich composites is the aim of investigation. ζ = ω 2ω n (1) Where, ω n is the resonance frequency ω = ω R ω L, ω R and ω L are the right and left frequency at 3 db below the resonance amplitude as shown in Fig.5. V. RESULTS AND DISCUSSION The uni-axial accelerometer and impulse hammer are interfaced with computer using the data acquisition system (four channel real time analyzer) with NVgate version 6.0 software. Frequency response function (FRF) generated by the software is shown in Fig 6 a. The degree of correlation of one signal with a second signal is confirmed with coherence which is related to the FRF. Coherence of the results varies from 0.94 to 0.99 shown in Fig. 6 b and is a function of frequency. In modal analysis, this function shows the quality of a measurement. A good impact produces a vibration response that is perfectly correlated with the impact, indicated by a coherence plot that is near one over the entire frequency range. If there is some other source of vibration, or noise, or the hammer is not exciting the entire frequency range, the coherence plot will drop below one in some regions. Natural frequency and damping ratio of different sandwich panels at respective modes are determined using half power band width method. Table 2 details the summary of natural frequency and damping ratio of the sandwich panels. Fig.7 a and 7 b shows the mode1 and mode2 plots respectively represented in terms of natural frequency and damping ratio of different sandwich panels. High damping and low natural frequency is encountered in the case of SF panel. This may be due to the fact that, low density and low stiffness of SF panel. However, marginally increase in density and very high stiffness of SFK5 panel results in low damping and high natural frequency. Test panels with 10mm, 15mm and 20 mm cell size of RIPH structure in syntactic foam (SFK10, SFK15 and SFK20) core sandwich panels has shown stable damping and natural frequency. 82

4 (a) (a) (b) Fig. 6 Frequency response function and its coherence Table 2 Details summary of natural frequency and damping ratio Sandwich panels Natural frequency Hz Mode I Damping ratio Natural frequency Hz Mode II Damping ratio SF SFK SFK SFK SFK (b) Fig. 7 plots of natural frequency and damping ratio of different sandwich panels (a) mode I (b) mode II V. CONCLUSION Stiffened syntactic foam core sandwich composites are developed using RIPH structure in syntactic foam as core and GFRP laminates as face sheets. The developed composites are tested for vibration characteristics to determine natural frequency and damping ratio. Following important conclusions are drawn from this investigation. i. Density of RIPH integrated syntactic foam core is marginally increased with the decrease in the cell size of RIPH structure. ii. Stiffness of the sandwich panel is increases with the decrease in cell size of the RIPH structure in syntactic foam core. 83

5 iii. iv. Natural frequency of sandwich panels increases with the decrease in cell size of RIPH structure in syntactic foam core. Damping ratio increases with the increase in cell size of RIPH structure in syntactic foam core sandwich composites. However, damping ratio is very high in the case of sandwich composites with core of syntactic foam without RIPH structure. v. The variation in cell size of RIPH structure in syntactic foam significantly alters their vibration response; this in turn promises its application in stiffness critical structural applications. VI. REFERENCES [1] Hongneng Cai, Yasushi Miyano and Masayuki Nakada, Prediction of Long Term Flexural Fatigue Strength of Honeycomb Sandwich Composites, J. Reinf. Plast. Compos, vol. 29(2), pp , [2] Sezgin F.E. Tanoglu M. Egilmez O.O. and Donmez C, Mechanical behavior of polypropylene-based honeycomb-core composite sandwich structures, J. Reinf. Plast. Compos, vol. 29(10), pp , [3] E.A. Flores Johnson, Q.M. Li and R.A.W. Mines, Degradation of elastic modulus of progressively-crushable foams in uniaxial compression, J. Cell. Plast, vol. 44(5), pp , [4] C.C. Foo, L.K. Seah and G.B. Chai, Lowvelocity impact failure of aluminium honeycomb sandwich panels, Compos. Struct, vol. 85, pp , [5] Jeremy Gustin. Aaran Joneson. Mohammad Mahinfalah and James Stone, Low velocity impact of combination Kevlar/carbon fiber sandwich composites, Compos. Struct, vol. 69, pp , [6] G. Belingardi, M.P. Cavatorta, R. Duella, Material characterization of a composite foam sandwich for the front structure of a high speed train, Compos. Struct, vol. 61, pp , [7] Lorenzo Bardella, Francesco Genna, Elastic design of syntactic foamed sandwiches obtained by filling of three-dimensional sandwich-fabric panels, Int. J. of Solids Struct, vol. 38, pp , [8] Alberto Corigliano, Egidio Rizzi and Enrico Papa, Experimental characterization and numerical simulations of a syntactic-foam/glassfibre composite sandwich, Compos Sci Tech, vol. 60, pp , [9] Xiang, Guang, Zhang, and Zhu, Experimental vibration characteristics of sandwich beams with magnetorheological elastomers cores, J. Cent. South Univ. Tech, vol. 15, pp , [10] M.R. Doddamani, S.M. Kulkarni, Dynamic response of fly ash reinforced functionally graded rubber based sandwich composite, Int. J. of Engg Sci Tech, vol. 3, pp ,