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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012

Short Beam Testing on Adhesive Butt Joints for Aluminum Hexagonal Core Sandwich Panels with Different Edging Configurations S. Rajkumar1, Dr D. Ravindran2 and Dr V.P.Raghupathy3 1

Chettinad College of Engineering and Technology, Karur, India Email: ccetraj@gmail.com 2 National Engineering College, Kovilpatti, India 3 P.E.S. Institute of Technology, Bangalore, India Email:{ ravinec85@gmail.com, drvpr2002@yahoo.co.in} vibrations, isolate sound, maintain temperature, retard fire, besides possessing extremely lowdensity. Therefore, its application is significant in space, automobile, marine and packaging industries. A typical honeycomb structure consists (Figure 1 and Figure 2) of two high strength thin sheets called face sheets sandwiching a soft, light and relatively thicker layer termed as core which ensures the two skins to work together. The honeycomb core is manufactured from a single sheet of Aluminum by successive in-line cutting, bending, folding and gluing [3]. This allows for continuous production resulting in high speed and low cost of production. Furthermore, production line can be extended to a continuous sandwich panel production by adding a skin bonding step. Secondary processing of sandwich panels like cutting, forming and jointing have been dealt with extensively in the literature [4]. In this paper, adhesive joining technique to secure butt joints with different edging and configuration details will be investigated,

Abstract — Aluminum honeycomb core sandwich panels offer the advantage of lightness, compression and shear resistance, fire and corrosion resistance and are used in a number of applications, for example, in public transport industry, in aeronautical sector, for tool machines, for serigraphy and building industry. Aluminum honeycomb is used as core material for sandwich panels: floors, roofs, doors, partitions, working surfaces for automatic machines and for all products which require an optimal stiffness-to-weight-ratio. These panels have thickness ranging from 3 to 300 mm with cell sizes ranging from 6 to 25 mm, resulting in core density ranging from density from 20 to 80 kg/m3. In most of the applications cited above, extensive joining will be required so that the panels can be fabricated to complex shapes. In this paper, various edging and configuration details that have been investigated to secure butt joints with adequate soundness through adhesive joining methods are presented. The shear strengths of the joints have been determined experimentally. The merits and demerits of each configuration are discussed and best combinations of edging and configuration are recommended for successful fabrication of complex assemblies with Aluminum honeycomb core sandwich panels. Index Terms — Honeycomb, Edging, Butt joint, Shear Strength, Resin, Bonding, Adhesive

I. INTRODUCTION Sandwich structures consist of a lightweight core material which is covered by face sheets on both sides. Although these structures have a low weight, they have high flexural stiffness and buckling strength. Hence, sandwich structures are an essential part of modern lightweight construction. Resin impregnated woven fabrics or other carrier materials (so-called prepregs) can be used as face sheets. Core materials include rigid polyurethane foams, honeycombs, nap cores, and corrugated thermo-plastic cores. The basic properties of sandwiches, like flame retardance, mechanical properties, and surface quality, are adjusted by selecting appropriate face and core materials according to the desired application. In recent times, sandwich structures have been widely used in load-bearing structures due to their high specific stiffness and high specific strength [1, 2].Honeycomb sandwich structure possesses high specific rigidity and it can reduce Š 2012 AMAE DOI: 02.AETAME.2012.3.16

Fig. 1. Schematic of Al hexagonal core sandwich panel

Fig. 2. Geometric details of Al Hexagonal core sandwich panel

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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012 II. JOINING PROCEDURE FOR SECURING SOUND BUTT JOINTS A. Configuration “A” In this configuration, the two faying faces of the panels are filled with small pieces of Polyurethane foam impregnated with resin AV 138 and further the resin is applied. Once the resin is applied the two specimens are joined together and the joint is tightened using a clamp as a supporting device. This set up is then exposed to the normal atmosphere for 4 to 5 hours. Later the clamps are removed and the joined specimens are heated in the oven at a temperature of 80°C for one hour. The joint details and the macro photograph of the joint are given in Figures 3 and 4.

Fig. 6. Macro photograph of Butt Joint - Configuration B

The two faying faces of the panels are filled with Polyurethane foam impregnated with resin AV 138 and further the resin is applied. Once the resin is applied at the bonding locations, the two panels are joined together as per the procedure described in Configuration ‘A”. The joint details and the macro photograph of the joint are given in Figures 7 and 8.

Fig. 3. Schematic of Butt joint – Configuration A Fig. 7. Schematic of Butt joint – Configuration C

Fig. 4. Macro photograph of Butt joint - Configuration A

B. Configuration “B” In this configuration, the edges of the panel are deformed and Al sheet is inserted. The two faying faces of the panels are filled with Polyurethane foam impregnated with resin AV 138 and further the resin is applied. Once the resin is applied at the bonding locations, the two panels are joined together as per the procedure described in Configuration ‘A”. The joint details and the macro photograph of the joint are given in Figures 5 and 6.

Fig. 8. Macro photograph of Butt Joint - Configuration C

D. Configuration “D” In this configuration, a transverse cut is made only along the face sheet in each of the panels to be joined and the edges are deformed to almost middle of the panel. Once the resin is applied at the bonding locations, the two panels are joined together as per the procedure described in Configuration ‘A”. The joint details and the macro photograph of the joint are given in Figures 9 and 10.

Fig. 5. Schematic of Butt joint – Configuration B Fig. 9. Schematic of Butt joint – Configuration D

C. Configuration “C” In this configuration, the edges of the panel are deformedand Al C Channel is inserted. © 2012 AMAE DOI: 02.AETAME.2012.3.16

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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012

Fig. 14. Macro photograph of Butt Joint - Configuration F Fig. 10. Macro photograph of Butt Joint - Configuration D

III. STATIC SHEAR TEST OF JOINTS

E. Configuration “E” In this configuration, the edges are deformed to almost middle of the panel without any transverse cut.. Once the resin is applied at the bonding locations, the two panels are joined together as per the procedure described in Configuration ‘A”. The joint details and the macro photograph of the joint are given in Figures 11 and 12.

Short Beam shear tests as per ASTM C393 were carried out to determine shear strength of Butt joints of various configurations. Typical size of the specimen is a 50 x 100 mm panel. In this test, it must be ensured that the moments produced at core failure do not stress the facings beyond the compressive or tensile proportional limit stress of the facings. If the facings are too thick, the shear load will be carried to a considerable extent by the facings, thus leading to high apparent core shear strength as computed by usual approximate methods. In this test, specimen is placed on the two supports as shown the diagram, the equipment arrangement is like simply supported beam. Then point load is applied on the specimen using digital flexural test system till Shearing of the specimen occurs. The test set up is indicated in Figures 15 and 16.

Fig. 11. Schematic of Butt joint – Configuration E

Fig. 15. Short beam test

Fig. 12. Macro photograph of Butt Joint - Configuration E

F. Configuration “F” In this configuration, an Al sheet is inserted transversely. Once the resin is applied at the bonding locations, the two panels are joined together as per the procedure described in Configuration ‘A”. The joint details and the macro photograph of the joint are given in Figures 13 and 14.

Fig. 16. Schematic of short beam test

The shear yield strength is computed as per the following equation [1]:

Fig. 13. Schematic of Butt joint – Configuration F

© 2012 AMAE DOI: 02.AETAME.2012.3.16

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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012 TABLE I. RESULTS OF SHORT BEAM TESTS

Where P is the load applied, d is sandwich panel thickness, c is the core height and b is the width of the sandwich panel.The results of the shear tests carried out on different joint configurations are indicated in Table 1. It can be seen from this table that shear yield strength of the sandwich panel and that of the joint with configuration “A” are marginally higher. The shear yield strength of the joints with configurations “B”, “C”, “D”, “E” and “F” are same and is marginally lower than that of the parent material.

joints compare well with that of the parent material and the failures occur at locations far away from the joint zone. ACKNOWLEDGMENT The authors thank the Management of Chettinad College of Engineering and Technology, Karur, National Engineering College, Kovilpatti and P.E.S. Institute of Technology, Bangalore for the encouragement. REFERENCES

CONCLUSIONS [1]

Hartsock, J. A. (1969). Design of Urethane Foam Sandwich Structures, Journal of Cellular Plastics, 5: page 188 – 193 [2] Swanson, S. R. and Kim, J. (2002). Optimization of Sandwich Beams for Concentrated Loads, Journal of Sandwich Structures and Materials, 4: 273–293 [3] Pflug J. & Leuven K.U, “Faltwabe und Verfahren zu deren Herstellung”, PCT Patent EP96/03121, German patent DE 197 16 637, 1996 – 1997 [4] http://www.nida-core.com/english/nidaprod_ honeyinfo_ working.htm.

Adhesive joining procedures for securing butt joints with six different configurations in Al hexagonal core sandwich panels have been investigated. The joining and configuration details need careful consideration and also the joint locations require filling with resin impregnated polyurethane foams to achieve thorough adhesion. The shear yield strength of the

© 2012 AMAE DOI: 02.AETAME.2012.3.16

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