A honeycomb sandwich structure is a special class of materials which is fabricated by attaching two thin skins to a lightweight core. The core material is usually thick as compared to the skin material. The overall strength of honeycomb sandwich structure depends upon the strength of skin, core and interfacial bonding between core and skins. The honeycomb sandwich structure is used in aircrafts, ships and bridges due to their excellent structural efficiency and strength to weight characteristics [1]. The solid structures are replaced by composites due to their improved properties. The properties of honeycomb sandwich structure can be improved by varying core height, core shape and thickness of the face material [2]. Mechanical performance of sandwich structure is studied by three-point bending. The sandwich panels are designed such that failure occurs in the core of the panel. The honeycomb sandwich structure has better corrosion resistance, lower structural vibrations under same operational conditions due to this they replace their metallic counter parts [3]. The materials used for core are foam, balsa, corrugated aluminium, Nomex, glass thermoplastics [4], [5]. Sandwich panel consists of two skins (i. e., upper skin and lower skin) and core as shown in figure 1. In figure 1, A-honeycomb panel, B- upper plate and lower plate, C- core. The plates are attached to the core by using suitable adhesives. Aluminium alloys, titanium and high tensile steels can be used as skin materials. Wang et al. (2011) described the mechanical response of square honeycomb sandwich plate with asymmetry face sheet when subjected to blast load [6]. Square honeycomb sandwich plate is analysed for different sizes of upper face sheet to lower face sheet, to find maximum deflection and energy absorption. The results show that, the efficiency of energy absorption increases with decrease in thickness ratio at same blast loading. An increase in core height of a honeycomb panel will increase its energy absorbing capacity so it can be used for applications like automobile bumper [7]. Keskes et al. (2007) discussed the effects of defect in static and fatigue studies of honeycomb core sandwich panels. Two tests, static and fatigue were performed on aluminium honeycomb sandwich panel specimen with and without defect. The result shows that there were no effects of defect observed in the case of the static study. In fatigue study, the fatigue

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a defect. The results of the acoustic emission show the crack initiation and propagation and can be used as a reliable survey method for the damage mechanisms [9]. Fig. 1 Honeycomb sandwich panel 1) Tounsi et al. (2015) investigated experimentally the dynamic behavior of an aluminium alloy honeycomb under mixed shear-compression loading with a special attention on the combined effects between the cells in-plane orientation and the loading angles. An improvement of an existing experimental set-up is proposed and an original measurement technique based on an electro optical extensometer is used to overcome a separation phenomenon observed during the test [11].

Sandwich panel offers excellent rigidity at minimal weight. The behavior of the honeycomb structures is isotropic; hence the panels react differently depending on the orientation of the structure. Therefore, it is necessary to distinguish between the directions of symmetry, the so-called L and W-direction [12]. The L-direction is the strongest and the stiffest direction. The weakest direction is at 60° from the L-direction (in the case of a regular hexagon) and the most compliant direction is the W-direction. Another important property of honeycomb sandwich core is its compression

The honeycomb sandwich structures can be made of variety of materials and panel configurations. The sandwich structure provides great versatility as different range of core and facing material combinations can be selected. The following two criteria should be considered in selection of facing skin, core and adhesive. Structural Considerations: In this the structural efficiency, relative stiffness of material and adhesive performance should be considered. Honeycomb cores and facing material‟s strength depends on their mechanical properties. Stiffness of honeycomb sandwich structure is maximized at very low weights. While selection of adhesive material care is taken that, there is a rigid bonding between the facing and core material in order to transmit load from one face to the other. Environmental Considerations: The temperature, heat transfer rate, flammability, humidity are the factors categorized under these criteria. The sandwich structures with metallic facing and metallic cores maximize heat flow characteristics. Non-burning, self-extinguishing, flammable materials selected for sandwich construction based on application. The selected material offers excellent resistance to degradation due to humidity and moisture. In selection of a material for honeycomb sandwich structure economic criteria is also considered. Materials are available in a wide range including: -Aluminium, Nomex Fibreglass, Korex, Kevlar, Carbon.

There are different methods by which we can manufacture honeycomb sandwich panel. Primary methods used to manufacture honeycomb are: (a) Adhesive bonding and expansion (b) Corrugation and adhesive bonding (c) Corrugation and braze welding (d) Extrusion. In case of high density honeycombs Corrugation method is used but it is slower one. Ceramic and thermoplastic honeycombs are manufactured by Extrusion process. The expansion and corrugation method for manufacturing of honeycomb panels are shown in figure 2 and figure 3 respectively.

S. B. Akiwate1*, V. D. Shinde2 1

Expansion process means that layers of materials with printed adhesive lines are first glued together. After that the block, consisting of a few layers of that material, is sliced into pieces of the required size. Finally each piece is expanded and several honeycomb cores are received. Expansion process consists of five stages, which are roll, sheet, HOBE (honeycomb before expansion) block, HOBE slice, expanded honeycomb panel as shown in figure 2 [13].

Corrugation process consists of folding of material into certain shape followed by gluing and slicing. Corrugation manufacturing in turn consists of roll, corrugating rolls, corrugated sheet, corrugated block, honeycomb panel. Shahdin A. et al. [8] fabricated sandwich beam specimens by using an autoclave and an aluminum mold. The two skins are bonded to each side of a light weight core with the help of co-curing process. Fabian R. et al. [10] presented the additive layer manufacturing technique for sandwich structures. This technique allows placing of the face sheets on a honeycomb core by a wet lay-up process followed by curing of sandwich plate in autoclave. Paika J. et al. [12] described the adhesive bonding and brazing method to join the honeycomb core material with the skin material. The adhesive films are inserted in bonding followed by heating in furnace.

Honeycomb sandwich plate consists of mainly three parts as explained earlier. Honeycomb sandwich panel is tested on three-point machine to check its bending strength. During testing, different failure modes are observed. Giglio. et al. [14] presented the three-point bending test on a sandwich panel made with aluminium skin and nomex honeycomb core. Such types of sandwich panels are used in aeronautical components. Various modes of failures are observed during three-point bending test on MTS Alliance RT/100 testing machine (100KN maximum load) are face yielding, intra-cell dimpling, core shear and local indentation. Fig. 4 Three-point bends test setup Properties of the face and the core materials, the geometry of structure and loading arrangement are the factors on which critical failure mode and its corresponding failure load depends. Yan et al. [15] fabricated sandwich beams having empty and aluminum foam-filled corrugated cores. 304 grade stainless steel was used to fabricate sandwich plate. Closed-cell aluminium alloy foam was selected as the filling material. Fabricated sandwich plates were tested on MTS-880 materials test system. The test result shows that, there are four main modes of collapse of sandwich beam (1) face yielding, (2) face elastic/plastic wrinkling, (3) indentation and (4) core shear. Crupi et al. [16] explained the collapse modes in aluminium honeycomb sandwich panels under bending and impact loading. The aluminium alloy (AA5052 and AA5754) sandwich panels with honeycomb core were tested on a servo-hydraulic load machine. It was concluded that, the honeycomb panels fail by the core shear failure mode. There is a perfect bond between the faces and the core and the possibility of delamination is eliminated, sandwich

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result of bending test also shows that the energy absorption capacity of sandwich panel is strongly affected by the size of the honeycomb cells. Banghai et al. [17] presented the failure modes of foam-core sandwich beams subjected to three-point bending test. The sandwich beams were made of face sheet having aluminium alloy (3003) and core material is closed cell aluminium foam. The failure modes observed in the quasi-static three-point bending tests are facing yield, core shear and indentation. Sandwich beams failure can depend on the geometry of the sandwich structure and the relative strength of the face sheets and foam core.

This section focuses on the theoretical analysis of honeycomb sandwich panel in three-point bending test. The honeycomb sandwich structure is made using aluminium alloys. The material used for core is aluminium alloy 3003 and for the upper and lower skin of panel is aluminium alloy 6061. The honeycomb core is attached to the upper and lower skin by epoxy film adhesive. Honeycomb core has a hexagonal cell structure. Sandwich structure specimens have a rectangular shape of 150×50mm., honeycomb cells orientation have ribbon direction, which is also called as L. The sample calculations of bending stiffness, shear stiffness and deflections of honeycomb sandwich panel are shown below. The dimensions of sandwich plate are, Upper and lower plate thickness is 0.75mm, core thickness is 10mm. The geometrical and mechanical properties of the sandwich panels are listed in Table 1.

Honeycomb Core Material Al3003 Thickness (mm) 10 Cell size (mm) 9 Density (kg/m3) 43.2 Bonding strength (kg /mm3) 36 Panel Skin Material Al6061 Upper and lower skin thickness (mm) 0.75 Density (kg/m3) 2700 Poissons ratio 0.33 Modulus of elasticity (GPa) 70

Shear modulus (MPa) 220

(1)

In equation 1, E- Modulus of elasticity, tf-Skin thickness; h-core and skin thickness; b-panel width. So, the bending stiffness of the honeycomb sandwich panel (150×50×11.5mm) is calculated as151.67 N.m2

• Shear stiffness:

It is calculated using equation 2. Where, Gc-Shear modulus (MPa); So, shear stiffness for same panel is 118.250×103N.

2) The total deflection at the center of honeycomb panel is sum of the deflections due to bending of upper plate and shearing of core [18]. The formula used to calculate deflection is given by equation 3. Where, F- Load, L-length of panel; kb and ks are deflection coefficients; D-bending stiffness and S-shear stiffness. The values of deflection coefficients when panel is simply supported are taken as1/48 and1/4 [15]. By using equation 3, deflection of honeycomb sandwich panel with 0.75mm face thickness and 10mm core thickness is calculated and the value is 0.89mm. The actual photographs of hexagonal honeycomb core and honeycomb sandwich panels are shown in Figure 5. Table 2 shows the deflections for the honeycomb sandwich panels with different core thickness, different upper and lower plate thickness which are theoretically calculated. The two cores were used in the theoretical analysis of honeycomb sandwich panel one is 10mm core and other is 14mm core. The densities of core are calculated and listed in the table 2 for all the specimens. The density for 10mm core is less than that of 14mm core. The bending stiffness for honeycomb sandwich panel made of aluminium alloys is shown in table 2, the values of bending stiffness increases as the load increases.

S. B. Akiwate1*, V. D. Shinde2 1

The numerical models of aluminium honeycomb sandwich panel were developed to investigate the bending behaviour of sandwich panel. The solid model is generated using CATIA and it is imported to Ansys workbench. The figure 6 shows the model of honeycomb sandwich panel and its analysis on ANSYS showing total deformation, Max. Von-misses stress. In analysis, the honeycomb sandwich panel made of aluminium alloys is simply supported on the two rollers. The diameter of roller is 10mm and its width is 50mm which is equal to width of sandwich panel. In three-point bending test the load is applied on sandwich panel at its centre by using roller having same dimensions as that of the supporting rollers. When the load is applied on the sandwich panel it shows the buckling behaviour. The buckling behaviour of panel is shown in the figure 6. The result of analysis shows that the maximum stress is 169 MPa when panel suffers from a 1137.52 N bending load. It can be seen that the failure of sandwich panel starts at its top plate first, as the load increases the core de-bonding will take place. The stress distribution on top plate is much higher than the bottom plate and core.

The total deformation of honeycomb sandwich panel is 0.16 mm. The honeycomb sandwich panel under loading conditions shows the failure mechanisms, the crack points were observed on the top plate of panel. The core of honeycomb panel is fails due to shear failure. The thickness of panel increases its energy absorbing capacity, as thickness increases time to reach peak load is increases.

The theoretical approaches have been used to find out the deflections of honeycomb sandwich structures, from that following points are concluded. manufacturing methods of honeycomb sandwich structures. The buckling behaviour of honeycomb sandwich panel is highlighted. 2. The bending stiffness, shear stiffness and deflections of sandwich panel were calculated by theoretical analysis. The analytical calculations show that deflections of sandwich panel for three-point bending is not much affected by varying core thickness and with different combinations of face sheets.

3. The numerical analysis of honeycomb sandwich panel for three-point bending shows the maximum stress on upper plate

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out through analysis.

[1] C. Lu, M. Zhao, L. Jie, J. Wang, Y. Gao, X. Cui and P. Chen, “Stress distribution on composite honeycomb sandwich structure suffered from bending load”, Procedia Engineering, Vol. 99, 2015, pp 405-412. [2] G. Nirupama, V.D. Reddy and G. Krishnaiah, “Design and fabrication of spot welded corrugated panel under three point bending by FEM”, Procedia Engineering, Vol. 97, 2014, pp 1282-1292. [3] E.P. Bowyer and V.V. Krylov, “Experimental investigation of damping flexural vibration in glass fibre composite plates containing one and two-dimensional acoustic black holes”, Composite structures, Vol. 107, August 2013, pp 406-415. [4] D.G. Vajma and G.G. Tejani, “Experimental Test on Sandwich Panel Composite Material”, International Journal of Innovative Research in Science, Engineering and Technology, Vol. 2, July2013, pp 3047-3054. [5] R. Harish and R.S. Sharma, “Vibration response analysis of honeycomb sandwich panel with varying Core Height”, International Journal of Emerging Technologies in Computational and Applied Sciences, Vol. 5, August 2013, pp 582-586. [6] T. Wang, M. Ma, W. Yu, S. Dong and Y. Gao, “Mechanical response of square honeycomb sandwich plate with asymmetry face sheet subjected to blast loading”, Procedia Engineering, Vol. 23, 2011, pp 457-463. [7] R.S. Sharma, V.P. Raghupathy, G.M. Priyamvada, A. A and M. Abhiraj, “Investigation of low velocity impact response of aluminium honeycomb sandwich panel”,ARPN Journal of Engineering and Applied Sciences, Vol. 6, November 2011, pp 7-13. [8] A. Shahdin, L. Mezeix, C. Bouvet, J. Morlier and Y. Gourinat, “Fabrication and mechanical testing of glass fiber entangled sandwich beams:A comparison with honeycomb and foam sandwich beams”, Composite Structures, Vol. 10, pp 43-51. [9] B. Keskes, Y. Menger, A. Abbadi, J. Gilgert, N. Bouaouadja and Z. Azari, “A Fatigue technology, Vol. 41, 2007, pp 157-161. [10] F. Riss, J. Schilp and G. Reinhart, “Load-dependent optimization of honeycomb for sandwich Components-New possibilities by using Additive Layer Manufacturing”, Physics Procedia, Vol. 56, 2014, pp 327-335. [11] R. Tounsi, E. Markiewicz, G. Haugou, F. Chaan and B. Zouari, “Dynamic behaviour of honeycomb under mixed shear compression loading: Experiments and Analysis of combined effects of loading angle and cells in plane orientation”, International journal of solids and Structures, Vol. 80, February 2016, pp 501-511. [12] J.K. Paik, A.K. Thyamballi and G.S. Kim, “The strength characteristics of aluminum honeycomb sandwich panels”, Thin-Walled Structures, Vol. 35, 1999, pp 205-231. [13] A. Ashab, D. Ruan, G. Lu, S. Xu and C. Wen, “Experimental investigation of the mechanical behavior of aluminum honeycombs under quasi-static and dynamic indentation”, Materials and Design, Vol. 74, March 2015, pp 138-149. [14] M. Giglio, A. Gilioli and A. Manes, “Numerical investigation of a three-point bending test on sandwich panels with aluminum skins and Nomex™ honeycomb core”, Computational Materials Science, Vol. 56, February 2012, pp 69-78. [15] L.L. Yan, B. Han, B. Yu, C.Q. Chen, Q.C. Zhang and T.J. Lu, “Three-point bending of sandwich beams with aluminum foam-filled corrugated cores”, Materials and Design, Vol. 60, April 2014, pp 510-519. [16] V. Cruip, G. Epasto and E. Guglielmino, “Collapse modes in aluminium honeycomb sandwich panels under bending and impact loading”, International Journal of Impact Engineering, Vol. 43, December 2011, pp 6-15. [17] J. Banghai, L. Zhibin and L. Fangyun, “Failure mechanism of sandwich beams subjected to three-point bending”, Composite Structures, 2015.

S. B. Akiwate1*, V. D. Shinde2 1

PG student, DKTES Textile and Engineering Institute, Ichalkaranji, India