Dear, JP

Kaboglu, C

Morris, A

Pimenta, S

Spiral - Imperial College Digital Repository

The influence of different types of core materials on the impact behaviour of sandwich composites  Cihan Kaboglu *,  Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom Soraia Pimenta, Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom Andy Morris, Design Authority, EDF Energy, West Burton Power Station, Retford,Nottinghamshire, DN22 9BL,United Kingdom John P Dear, Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom  Abstract Sandwich structures have become very popular in applications where the weight of the component affects the efficiency of the application in the aviation and aerospace industries. The purpose of this study is to understand the impact behaviour of sandwich structures with different core materials by performing low velocity drop weight impact test. The sandwich structures were manufactured with Glass Fibre Reinforced Polymer (GFRP) skins and Balsa wood, Polyethylene terephthalate (PET) and Polyvinyl chloride (PVC) core through resin infusion under flexible tools (RIFT). The densities of the core materials are 148 kg/m3, 105 kg/m3 and 60 kg/m3 respectively.  In the study, three different core materials were tested and compared by using the damaged area of the back face of the sample. The effect of different core materials on the mechanical behaviour of the structures is crucial. Balsa wood cored sandwich composite structures behaved in a very brittle manner as can be seen in Figure 1. At an impact energy of 80 J, the percentage of debonding on the distal face of the balsa wood-plain cored sandwich composites is around 60% and the percentage of debonding on the distal face of PET-Plain and PVC-Plain cored sandwich composite structures is nearly 10% as seen in Figure 1. It can be reported that the microstructure of the core materials plays an important role, because even the density of the balsa wood is greater than the density of the PET and PVC, the structures which have PVC and PET as core materials have less damage than the structures which have balsa wood as a core material Keywords: Sandwich composite, low velocity impact, core materials.                                                          * ADDRESS FOR CORRESPONDENCE: Cihan Kaboglu, Mechanical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom     E-mail address: c.kaboglu13@imperial.ac.uk / Tel.: +44 (0)74042 40712    1 1. Introduction Sandwich composite structures have become very popular because of their static and dynamic mechanical properties. These structures are very popular in aerospace, automotive and wind energy industries where the weight of the components are crucial [1-8].  One of the biggest concern of the wind energy, aerospace and automotive industries is the strength and rigidity of the components. These kind of components faces low velocity impact most frequently due to dust impact, bird strikes, or dropped tools in service. Sandwich composite structure has an ability to absorb more energy due to core materials. Glass or carbon fibre reinforced polymers (GFRP-CFRP) are common used as a skin material in sandwich structures, and polymeric foam – such as polyvinyl chloride (PVC) or polyethylene terephthalate (PET), balsa wood and hybrid core (Tycor) – are used as a core material. The material combination improve the mechanical properties, rigidity and strength of the sandwich composite structures such as high strength to weight ratio, energy absorbing, bending stiffness [9-15].  In the research, the effects of changing core materials and different surface finishing options on the low velocity impact behaviour have been investigated by performing low velocity drop weight impact tests. Three different polymeric foams and five different surface finishing options with PVC were used as core materials. The damage area of the samples and comparison of the samples with each other are presented in the following sections.  2. Materials and testing Three different core materials were performed under low velocity drop weight test; Polyvinyl chloride (PVC), polyethylene terephthalate (PET) foams and balsa wood. The foam cores were supplied by AIREX AG. The mechanical properties of PVC are shown in Table 1.   TABLE 1. MECHANICAL PROPERTIES OF THE CORES[16, 17]  Core materials Density (kg/m3) Shear Strength (N/mm2) Shear Modulus (N/mm2) PVC Foam 60 0.85 22 PET Foam 110 0.8 20 Balsa Wood 148 2.6 187   Gurit XE603, which is a +/- 45 biaxial E-glass reinforcement fabric, was used as the reinforcing material of the skins. A mixture of Prime 20 LV epoxy resin and slow hardener (which is suitable for the production of sandwich composite structures) was used a matrix material of the skins [18].  The manufacturing process of the sandwich  composite structures was explained in the previous studies [7, 19] The dimensions of low velocity drop weight impact samples are 100 × 100 × 18 mm (as in length (direction x)× width (direction z)× thickness (direction (y)).    2 The low velocity drop weight impact test machine was used with 16 kg mass and a 12 mm diameter hemispherical projectiles which was used in the previous research [7, 19]. The projectile is shown in Figure 1.    Figure 1. Projectile used in the low velocity drop weight impact tests.  The samples were clamped by using the test rig depicted in Error! Reference source not found.2. The low velocity drop weight impact was performed by applying 40, 50, 60, 70 and 80 J in accordance with ASTM D7766 .The applied energy was calculated using 𝐸=𝑚 ×  𝑔 ×  ℎ                   (Equation 1:     Figure 2. The assembly picture of the test rig.  𝐸 = 𝑚 ×  𝑔 ×  ℎ                   (Equation 1) where 𝐸= potential energy, J 𝑚= mass, kg 𝑔= gravitational constant, m/s2  ℎ= height, m   After the test, the delaminated area of the back surface of the samples was measured. The measurement process is explained in the previous research [7, 19]. The process was applied for the study.        3 3. Results In Table 3, the comparison of the damaged areas of the back surface of the samples tested different level of impact load are depicted. Balsa wood cored sandwich composite structures damaged at all applied energies. The first damage appeared on the distal face of the PVC and PET cored sandwich composite structures at an energy of 50 J. The perforation impact energy was 80 J for both types of sample. Debonding damage was observed in all cases for the balsa wood cored sandwich composite structures. The propagation of the failure on the back surface of the sandwich composite structures are different as seen in Table 2. The first failure of PVC and PET cored sandwich composite structures was observed and propagated horizontal through axis of the sample, however; the failure of balsa wood cored sandwich composite structure propagated circular on the back surface of the structures as seen the back surface picture of the sample in Table 2. TABLE 2. THE DAMAGED AREA OF THE PVC CORED SANDWICH COMPOSITE STRUCTURE    Applied energy 40 J 50 J 60 J 70 J 80 J PVC      Damaged  area:  0 % Damaged  Area:  1.4 % Damaged Area:  6.5 % Damaged Area:  10.7% Damaged Area:  13.1% PET      Damaged Area:  0 % Damaged  Area:  1.2 % Damaged Area:  4.3 % Damaged Area: 8.6 % Damaged Area:  11.6 % Balsa Wood      Damaged Area:  5.2 % Damaged  Area:  16.1 % Damaged Area:  26.6 % Damaged Area:  41.9 % Damaged Area:  54.2 %      4 4. Discussion In the research, three different core materials for sandwich composite structures have been investigated in terms of the effect on the impact behaviour, by performing low velocity drop weight impact tests and measuring the damaged area at the back face of the samples.   The effect of different core materials on the mechanical behaviour of the structures is crucial. Balsa wood cored sandwich composite structures behaved in a very brittle manner as can be seen in Figure 4. Therefore, balsa wood cored sandwich composite structure had damage on the distal face of the samples and failure was observed as debonding for all applied impact energies. At an impact energy of 80 J, the percentage of debonding on the distal face of the balsa wood-plain cored sandwich composites is around 60 % and the percentage of debonding on the distal face of PET and PVC cored sandwich composite structures is nearly 10 %. It can be reported that the microstructure of the core materials plays an important role, because even the density of the balsa wood is greater than the density of the PET and PVC, the structures which have PVC and PET as core materials have less damage than the structures which have balsa wood as a core material    Figure 4. Comparison of the damaged area of the PVC-plain, PET-Plain and balsa wood-plain cored sandwich composite structures.       5 5. Conclusion In this study, the effect of the core materials on the impact behaviour of the sandwich composite structures has been investigated. The main following conclusions can be drawn:   The microstructure of the core material influences the impact behaviour of the structure independent of the density of the core materials. Therefore, ductility of the core material has great effect on the impact behaviour of the structures.   The density of the core material is a very important property for predicting the impact behaviour of the structure whose core has same micro structure. Thus, the impact behaviour of the structure becomes better with an increase of the strength and density of the core considering the polymer cores.  6. References [1] R. Nasirzadeh and A. R. Sabet, "Study of foam density variations in composite sandwich panels under high velocity impact loading," International Journal of Impact Engineering, vol. 63, pp. 129-139, 2014. [2] A. Rajaneesh, I. Sridhar, and S. Rajendran, "Relative performance of metal and polymeric foam sandwich plates under low velocity impact," International Journal of Impact Engineering, vol. 65, pp. 126-136, 2014. [3] A. Rajaneesh, I. Sridhar, and S. Rajendran, "Failure mode maps for circular composites sandwich plates under bending," International Journal of Mechanical Sciences, vol. 83, pp. 184-195, 6// 2014. [4] T. D. Ashwill and J. A. Paquette, "Composite materials for innovative wind turbine blades," Wind Energy Technology Department, Sandia National Laboratories, Albuquerque, NM, vol. 87185, 2008. [5] P. S. Veers, T. D. Ashwill, H. J. Sutherland, D. L. Laird, D. W. Lobitz, D. A. Griffin, et al., "Trends in the Design, Manufacture and Evaluation of Wind Turbine Blades," Wind Energy, vol. 6, pp. 245-259, 2003. [6] L. Mishnaevsky Jr and O. Favorsky, "Composite materials in wind energy technology," Thermal to Mechanical Energy Conversion: Engines and Requirements, EOLSS Publishers: Oxford, UK, 2011. [7] C. Kaboglu, S. 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Steeves, "Optimizing sandwich beams for strength and stiffness," Journal of Sandwich Structures and Materials, vol. 14, pp. 573-595, September 1, 2012 2012. [12] C. A. Steeves and N. A. Fleck, "Collapse mechanisms of sandwich beams with composite faces and a foam core, loaded in three-point bending. Part I: analytical models and minimum weight design," International Journal of Mechanical Sciences, vol. 46, pp. 561-583, 4// 2004. [13] M. Battley and M. Burman, "Characterization of Ductile Core Materials," Journal of Sandwich Structures and Materials, vol. 12, pp. 237-252, March 1, 2010 2010. [14] R. Juntikka and S. Hallstrom, "Shear Characterization of Sandwich Core Materials Using Four-point Bending," Journal of Sandwich Structures and Materials, vol. 9, pp. 67-94, January 1, 2007 2007. [15] C. Kaboglu, Pimenta, S., Morris, A., Dear, J.P., "The effect of different types of core material on th flexural behaviour of sandwich composites for wind turbine blades," Journal of Thermal Engineering, vol. 3, pp. 1102-1109, 2017. [16] (2014). AirexBaltekBanova. Available: http://www.3accorematerials.com/products/airex/ [17] (2011). Data Sheet AIREX C70 Universal Structural Foam. Available: http://www.3accorematerials.com/products/airex/airexreg-c70.html [18] (2014). Gurit Product Catalogue. Available: http://www.gurit.com/files/documents/product-catalogue-2014-emeawebpdf.pdf [19] C. Kaboglu, "Composite Materials for Wind Energy Applications," Imperial College London, London, United Kingdom2017.  

The Influence of different types of core materials on the impact behaviour of sandwich structures

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