International Journal of Engineering Materials and Manufacture (2023) 8(1) 13-20 https://doi.org/10.26776/ijemm.08.01.2023.02 S. A. R Sharifah Nadhirah, J. S. Mohamed Ali , M. S. I Shaik Dawood, R. Adib Hamdani Department of Mechanical and Aerospace Engineering, Kulliyyah of Engineering, International Islamic University Malaysia. Email: jaffar@iium.edu.my Reference: Sharifah Nadhirah, S. A. R., Mohamed Ali, J. S., Shaik Dawood, M. S. I., Adib Hamdani, R. (2023) Influence of Composite Skin on the Energy Absorption Characteristics of an Aircraft Fuselage. International Journal of Engineering Materials and Manufacture, 8(1), 13-20. Influence of Composite Skin on the Energy Absorption Characteristics of an Aircraft Fuselage S. A. R. Sharifah Nadhirah, J. S. Mohamed Ali, M. S. I. Shaik Dawood, R. Adib Hamdani Received: 08 January 2023 Accepted: 12 January 2023 Published: 20 January 2023 Publisher: Deer Hill Publications © 2023 The Author(s) Creative Commons: CC BY 4.0 ABSTRACT In aviation history, accidents during landing are more frequent compared to other phases of flight and in most cases the aircraft is forced to make an emergency belly landing when landing gear malfunctions. This study aims to analyse the energy absorption of a typical composite aircraft fuselage section under such crash belly landing. A finite element model of this composite fuselage section was created and analyzed using LS-Dyna finite element software and the effects of different composite materials on the energy absorption of fuselage section were studied. Four different composite materials that are commonly used in aerospace application were selected for the purpose of the present study namely glass/epoxy, graphite/epoxy, Kevlar/epoxy, and boron/epoxy. An impact velocity of 7 m/s against rigid ground was considered. Results showed that frames and skin contribute the most in energy absorption while the passenger floor and strut contribute the least. Moreover, it was found that fuselage skin made of glass/epoxy absorbed more energy compared to other materials and it had the highest specific energy absorption. Keywords: crashworthiness, belly landing, energy absorption, composite materials, LS-Dyna 1 INTRODUCTION In the aircraft design, crashworthiness is a major concern, especially for civil aircraft. Crashworthiness is the ability of the aircraft structure to protect passengers from injury during a crash. In aviation history, belly landing crashes have been reported as one of the most frequent accidents. Belly landing mainly occurs due to landing gear failure or when pilots forget to deploy the landing gear. There are many risks in performing belly landing as the impact energy can be transferred to the passenger cabin and this can cause injuries or even fatalities. Crash survivability of these types of events can be significantly improved by designing the aircraft structures for crashworthiness. Since it is expensive to do a full-scale drop test, Finite Element Analysis (FEA) is used to perform crash analysis. However, a real fuselage is a complex structure as it consists of many parts, so to ease analysis process, the fuselage model can be simplified according to the simplified principles proposed by Adams and Lankarini [1]. The most common materials used for fuselage is aluminium alloys while laminated composites are used in military aircraft [2]. The usage of composite materials has dramatically increased in commercial aircraft since 2005 [3]. Laminated composites such as glass/epoxy, graphite/epoxy, Kevlar/epoxy, and boron/epoxy are commonly used in aerospace applications. The current state of the art regarding the relationships between failure mode and energy absorption, the primary material, geometrical and physical parameters relevant to crashworthiness design, and methods for evaluating the energy absorption capacity of polymer composites have been reviewed by Carrythers et al. [4]. A study on the relationship between material property variables, geometrical variables and testing variables and energy absorbing characteristics of composite material was conducted in 1998 [5]. Another study was conducted to investigate the effect of lamination scheme and angle variations to the displacements and failure behavior of two most common composite laminated plates, which are glass/epoxy and graphite/epoxy laminates [6]. A study on the effects of high strain rate (HSR) loadings on the glass fiber failure process in epoxy matrix during tensile loading was carried out in 2019 [7]. Abdewi et al. examined the effect of corrugation geometry on the crushing behavior, energy absorption, failure mechanism and failure mode of woven roving glass fibre/epoxy laminated composite tube and found that there is no effect of corrugation geometry observed for lateral crushing [8]. In 2014, Xue et al. studied the crashworthiness characteristics of aluminium fuselage [9]. Mou Haolei et al. studied the influences of composite skin of a fuselage section and the results showed that the crashworthiness can be Influence of Composite Skin on the Energy Absorption Characteristics of an Aircraft Fuselage 14 effectively improved by selecting appropriate composite ply numbers and ply angles [10]. Yidris et. al performed a quasi-static crash analysis of a woven c-glass/epoxy fuselage section of an unmanned aerial vehicle (UAV) to investigate energy absorbing capability [11]. With a wide-bodied aircraft made primarily of composites, Yu and Xue simulate drop tests and compared the responses of T800/QY8911 composite and GLARE [12]. In 2017, Crash analysis of a metallic fuselage on land and water was investigated [13]. In a recent study, Riccio et al. investigated the influence of material toughness on the capability of a composite fuselage barrel to tolerate an impact on a rigid surface [14]. A comparative study by experimental, numerical, and analytical methods on oblique lateral crashworthy characteristics of thin-walled circular tubes with aluminium, GFRP and CFRP materials was carried out [15]. From the literature survey, no work has been reported on the effects of different composite materials on the energy absorption characteristics of a fuselage. Thus, this paper aims to investigate the effects of different composite materials on energy absorption of a fuselage section using LS-Dyna. 2 FINITE ELEMENT MODELLING 2.1 Fuselage Modelling In this study, LS-Prepost was used to model a typical fuselage section while the finite element analysis was done using LS-Dyna explicit finite element code. The geometry of the fuselage was kept simple which consists of five parts; skin, Z-shaped frames, U-shaped stringers, passenger floor and U-shaped struts. The length and the diameter of the fuselage are 1 m and 2 m, respectively. The thickness of the skin is 0.5 cm and the thickness for the rest of the fuselage components is 0.2 cm. All the fuselage components were meshed using shell elements. Based on the conducted convergence study, the finite element model consists of 42945 nodes and 39736 elements. The impact velocity used is 7 m/s and the rigid ground was modelled using RIGIDWALL_PLANAR. The total simulation time was set to 150 ms. Figure 1a: Fuselage components Figure 1b: Fuselage dimension Figure 1: Fuselage components with dimension Figure 2: FE model of the fuselage and ground Sharifah Nadirah et al. (2023): International Journal of Engineering Materials and Manufacture, 8(1), 13-20 15 2.2 Materials Selection All the five parts were modelled with aluminium, and for the parametric study, only the skin material was changed to composite. In modelling the parts made of aluminium, MAT_098 (Simplified Johnson Cook) were used while in modelling the part made of composite materials – which is the skin, MAT_054 (Enhance Composite Damage) was used. The skin when replaced by composite laminates, consists of 40 layers with each lamina having a thickness of 0.125mm. The laminate fibre angle is measured with respect to the axis of the fuselage. Tables 1 and 2 show the material properties of the aluminium and composite respectively used in the analysis. Table 1: Aluminium material properties [16] Density [kg/m 3 ] Elastic Modulus [GPa] Poisson’s ratio AL 7075 2796 71 0.34 Table 2: Composite materials properties [17, 18] Properties Glass/Epoxy Graphite/Epoxy Kevlar/Epoxy Boron/Epoxy Density [kg/m 3 ] 1900 1600 1380 2000 Young’s Modulus (longitudinal), E1 [GPa] 38.6 181 80 204 Young’s Modulus (transverse), E2 [GPa] 8.27 10.3 5.5 18.5 Poisson’s ratio, µ12 0.26 0.28 0.34 0.23 Shear Modulus of elasticity, G12 [GPa] 4.14 7.17 2.2 5.59 Longitudinal Tensile Strength, Xt [GPa] 1.062 1.5 1.4 1.26 Longitudinal Compressive Strength, Xc [GPa] 0.61 1.5 0.335 2.5 Transverse Tensile Strength, Yt [GPa] 0.031 0.04 0.03 0.061 Transverse Compressive Strength, Yc [GPa] 0.118 0.246 0.158 0.202 Shear Strength, S12 [GPa] 0.072 0.068 0.049 0.067 3 RESULTS AND DISCUSSIONS 3.1 Validation on Aluminium Fuselage Crash Test Since experimental benchmark results are not available in the literature, only a qualitative validation was carried out. For the validation, the work of Xue et al. [9] was used in which an aluminium fuselage crash on ground at an impact velocity of 9 m/s was studied and the energy absorption of each element of a fuselage is reported. Comparing the internal energy graph of the present study (Fig 3.) with that of [9] shown in Fig 4., it is observed that the trend is similar where frames absorbed the most of impact energy followed by the skin, struts, stringers, and the passenger floor. Figure 3: Internal energy absorbed by each component of aluminium fuselage Influence of Composite Skin on the Energy Absorption Characteristics of an Aircraft Fuselage 16 Figure 4: Energy vs time curve of each component (P. Xue et al., 2014) 9 3.2 Energy Absorption of Different Composite Materials The structural behaviour of each part during impact is depicted in the Figure 6-10 below. It can be seen in Figure 6- 10 that upon impact, lower part of the fuselage sustained damage. In all the cases studied, the frames absorbed more energy than other elements. Comparing the damage pattern of various materials, aluminium fuselage has less damage compared to other materials as aluminium is less brittle than composite. It is known that aluminium material absorb energy in plastic and elastic region while composite materials mostly absorb energy in elastic deformation, making it more brittle than aluminium. Hence, the damages on composite fuselage are greater than the aluminium ones. Figure 5: Before the impact Figure 6: Aluminium fuselage after impact Sharifah Nadirah et al. (2023): International Journal of Engineering Materials and Manufacture, 8(1), 13-20 17 Figure 7: Glass/Epoxy skin fuselage Figure 8: Graphite/Epoxy skin fuselage Figure 9: Kevlar/Epoxy skin fuselage Figure 10: Boron/Epoxy skin fuselage Figure 11 shows the internal energy of different fuselage components made of aluminium at an impact velocity of 7 m/s. The trend of the graphs is the same as shown in Figure 3. Figure 11: Internal energy plot for Aluminium fuselage at 7 m/s Influence of Composite Skin on the Energy Absorption Characteristics of an Aircraft Fuselage 18 Figure 12 shows the comparison of energy absorption of the skin using different materials. At 150 ms, the energy absorbed by graphite/epoxy is 1.35 kJ, glass/epoxy (1.77 kJ), Kevlar/epoxy (1.25 kJ) boron/epoxy (1.19 kJ), and aluminium (0.27 kJ). Glass/epoxy absorbed higher energy compared to other materials while aluminium absorbed the least. Figure 13 shows the overall internal energy absorbed by the whole structure and it can be seen that glass/epoxy absorbs higher amount of total energy followed by boron/epoxy, graphite/epoxy, Kevlar/epoxy and lastly, aluminium. Figure 12: Energy absorption of the skin of different materials Figure 13: Total internal energy of fuselage made of different composite materials with unidirectional laminate 3.3 Energy Absorption of quasi-isotropic laminate For quasi-isotropic laminate, a layup of [905/455/05/-455]s was used. Comparing Figure 13 and Figure 14, when the skin is made with quasi-isotropic laminate, the energy absorbed reduced as compared to unidirectional laminate. This 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 20 40 60 80 100 120 140 In te rn a l E n e rg y ( J) Time (ms) Energy Absorption of the Skin of Different Materials Aluminium Glass/Epoxy Graphite/Epoxy Kevlar/Epoxy Boron/Epoxy 0 1000 2000 3000 4000 5000 6000 7000 0 20 40 60 80 100 120 140 E n e rg y ( J) Time (ms) Total Internal Energy of Different Materials Aluminium Glass/Epoxy Graphite/Epoxy Kevlar/Epoxy Boron/Epoxy Sharifah Nadirah et al. (2023): International Journal of Engineering Materials and Manufacture, 8(1), 13-20 19 is because with quasi-isotropic laminate skin, the fuselage becomes stiffer as quasi-isotropic is known to have a laminate with 0°, ±45° and 90°. Thus, it absorbed less energy and have lesser damage. However, in this case glass/epoxy absorbed the highest energy followed by boron/epoxy, Kevlar/epoxy and lastly, graphite/epoxy. Figure 14: Total internal energy of fuselage made of different composite materials with quasi-isotropic laminate Table 3: Specific energy for different materials Materials Mass of the structure (kg) Specific energy (J/kg) (Unidirectional) Specific energy (J/kg) (Quasi-isotropic) Aluminium 282.791 17.692 17.692 Glass/Epoxy 257.679 22.704 20.891 Graphite/Epoxy 245.182 22.130 19.202 Kevlar/Epoxy 238.363 22.046 20.143 Boron/Epoxy 257.816 21.716 19.149 As the advantage of composites is on weight savings, it is important to compare the specific energy absorption. The mass of the structure can be computed directly from LS-Dyna and it is included in the Table 3 above for comparison. It can be noted from Table 3 that fuselage made of unidirectional glass-epoxy laminate proved to be the best in absorbing energy with the least mass. Similarly, fuselage made of Kevlar/epoxy with least mass, has higher specific energy as compared to boron/epoxy. The specific energy absorbed by aluminium fuselage is also included here for comparison. Fuselage made completely of Aluminium has the lowest specific energy as compared to any other composite laminates. 4 CONCLUSIONS This study simulates fuselage crash tests on rigid ground using LS-Dyna. The aluminium fuselage skin was replaced with different materials to study the effect of different materials on energy absorption characteristics of a fuselage. It was noticed that the lower part of fuselage deformed the most due to the impact and the results showed that frames and skin contribute the most in energy absorption while passenger floor and strut contribute the least, regardless of any material. As for the effect of different materials on energy absorbing capability, it is found that glass/epoxy absorbs more energy than any other composite materials as well as aluminium. Finally, it was found that fuselage made of glass-epoxy laminate with highest specific energy proved to be the best laminate in absorbing energy with the least mass. 0 1000 2000 3000 4000 5000 6000 7000 0 20 40 60 80 100 120 140 E n e rg y ( J) Time (ms) Internal Energy of Different Materials for quasi-isotropic laminate Glass/Epoxy Graphite/Epoxy Kevlar/Epoxy Boron/Epoxy Influence of Composite Skin on the Energy Absorption Characteristics of an Aircraft Fuselage 20 REFERENCES 1. Adams, A., & Lankarani, H. M. (2003). 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