Finite Element Analysis of Radome for Airborne Satellite Communication Radar

Transcription

1 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 101 Finite Element Analysis of Radome for Airborne Satellite Communication Radar Niranjan Murthy, V. Anusha, Saema Ahmed and Virat Yadav Abstract--- A Radome ( stands for radar dome) is a rigid, weatherproof structural enclosure that protects a microwave or radar antenna against the nature s forces. It is constructed from materials that minimally attenuates the signal transmission from the radar and protects the radar by efficient mechanical strength. In this work, a Radome for an airborne SAT-COM radar, mounted on the fuselage of an aircraft is to be designed based on two requirements. First, structurally to protect the antenna from the environment; Secondly, to maintain good permissibility for conduction of electromagnetic radiation by the Antenna. Radome has to withstand the Aerodynamic Loads under the environmental conditions as well as it should not interfere with the signal transmission process of the Radar. Based on the study of Mechanical and Electro-Magnetic properties of Composites Materials, a suitable material is chosen for Analysis of the Radome Design. The analytical solution to the problem is complicated process because of the materials used, geometry and operating condition of the structure. Finite Element Analysis is used to analyze the Radome for aerodynamic loads experienced by it during the flight. Ansys platform is used to find the displacement, stress and Tsai-Wu index of the Radome under aerodynamic loading condition. In this work, a Sandwich construction of Radome is done and implemented for analysis. The deformation due to the loads; Tsai-Wu indices for various layers were calculated. The results from the analysis are taken into consideration and if the design meets the requirements it is allowed to be mounted on the fuselage and operated on the aircraft in the flight. Keywords--- Radome, Airborne A I. INTRODUCTION Radome is a rigid, weatherproof structural enclosure that protects a microwave or radar antenna against the nature s forces; at the same time it should not be a hindrance to the electromagnetic waves propagating to and from the radar antenna. The word radome is a portmanteau of radar and dome. It is very important to select a suitable material to design a radome as the electromagnetic operation is pivotal to the performance of the antenna and the chosen material should minimally attenuate with signal transmission of the radar, but then again the material needs to be strong enough to bear other factors which may physically damage the radar. Radars are themselves capable of various operations and house various electronic equipment which contribute to its optimum functioning. However all radars are prone to environmental factors and this may take a toll on its performance if not well equipped to confront the forces of nature such as rain, wind, snow, ultra-violet radiations and corrosion. Hence radomes are required to protect the radars so that the longevity of the radars is assured with no dip in its performance. They are constructed in several shapes depending upon the particular application using various construction materials (fiberglass, PTFE-coated fabric, etc.). When used on UAVs or other aircraft, in addition to such protection, the radome also streamlines the antenna system, thus reducing drag. K. Rohwer, S. Friedrichs, C. Wehmeyer (1) analyzed Laminated Structures from Fibre-Reinforced Composite Material. In the literature there are tremendous number of models and methods for analyzing laminated structures. Niranjan Murthy, Assistant Professor, Mechanical Engineering Department, M.S.Ramaiah Institute of Technology, Bangalore nrn_smit@yahoo.com V. Anusha, Student, M.E, M.S.R.I.T, Bangalore-54. Saema Ahmed, Student, M.E, M.S.R.I.T, Bangalore-54. Virat Yadav, Student, M.E, M.S.R.I.T, Bangalore-54. PAPER ID: MED19

2 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 102 With respect to the assumptions across the laminate thickness, theories with various mathematical functions are to be distinguished from layer- wise approaches, whereas for the latter the functional degrees of freedom can be dependent or independent of the number of layers. Vincent Manet (2) has used different models to compute displacements and stresses of a simply supported sandwich beam subjected to a uniform pressure. 8-node quadrilateral elements (Plane 82), multi-layered 8-node quadrilateral shell elements (Shell 91) and multi-layered 20- node cubic elements (Solid 46) are used. The influences of mesh refinement and of the ratio of Young s moduli of the layers are studied. Finally, a local Reissner method is presented and assessed, which permits an improvement in the accuracy of interface stresses for a high ratio of Young's moduli of the layers with Plane 82 elements. Steven R. Nutt, H. Shen, and Lev Vaikhanskiy s (3) work at USC has focused on strategies to enhance the toughness and overall mechanical performance of polymer foams for use in lightweight sandwich structures. Both mechanical and chemical approaches have been employed with reasonable success. Fiber reinforcement, though difficult from a processing perspective, can lead to substantial enhancements in toughness and strength, while reducing friability. Chemical modifications are also challenging from a processing perspective, but can produce similar enhancements in performance. Efforts to enhance performance of phenolic foam and PVC foam through fiber reinforcement and chemical modification are described, along with the resulting enhancements in performance. Nomenclature µ - Specific Strength, E Young s Modulus σ Tensile Strength γ - Specific Modulus ρ - Density d - Dielectric Constant tan α - Loss Tangent II. CRITERIA FOR MATERIAL SELECTION Low dielectric constant and loss tangent values and high mechanical strength are the main parameters in designing a radome. Reinforcements here are typically E-Glass (Borosilicate), S-Glass (Mg/Al Silicate), Aramid (Polyp-phenylene-terephthamide), Quartz (Silica) etc. The parameters stated are tabulated for different reinforcing materials. This will help in selecting favorable constituent materials in the construction of sandwich model, required for the Radome Analysis. It is important to select a material based on the ease of availability, manufacturing technology, fabrication at economic costs. Cost/kg of the materials is found and tabulated which leads to the selection of a suitable material with optimum performance considering the above requisite parameters. The values for Dielectric constant, Loss tangent and Mechanical properties for different constituent materials are tabulated. It is found that E-Glass for its low cost can serve as the best fibre in terms of good mechanical strength and has a less value of loss tangent. Similarly Epoxy matrix can be chosen as a suitable material for the construction of Radome. Laminas are made of E-Glass-Epoxy; the properties of the lamina are given Table 1: Table 1: Mechanical Properties for E-Glass-Epoxy Pre-preg Serial No Property Value 1 Young s Modulus, E x N/mm Young s Modulus, E y N/mm Poisson s Ratio Shear Modulus, G xy N/mm Tensile Strength, N/mm Compressive Strength, N/mm Shear Strength, N/mm -2 42

3 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 103 Table 2: Mechanical Properties for Rohacell 51 WF Serial No Property Value 1 Young s Modulus, N/mm Density, Kg/m Tensile Strength, N/mm Compressive Strength, N/mm Shear Strength, N/mm III. DESIGN OF RADOME CAD Model Based on the specifications and requirements the SATCOM Radome is conceptualized as below in order to fully cater to the requirement of a Satellite Communication Antenna. The size of SATCOM Radome is 2702 mm X 703 mm X 563mm.Orthographic and Isometric views of the SATCOM Radome given in Fig 1. Figure 1: CAD Model of Radome For constructing the sandwich portion, the skin, three layers of E-Glass pre-pregs are on either side of core. The Core consists of Rohacell Foam (PMI)-51WF of 5 mm thick. The stiffness to the Sandwich portion is provided by the layers of E-Glass on both sides of the Rohacell Core, whereas the core maintains a good permittivity for the signal transmission by the Antenna which the radome protects. The material construction for monolithic portion consists of eighteen layers of E-Glass pre-pregs.. Figure 2: Construction Drawing of the Radome The shaded portion in the above diagram shows the sandwich portion of the radome, the un-shaded area represents the monolithic part. Aluminum alloy strip of 0.8 mm thick and 40 mm width is bonded on the outer surface of the radome at the attachment region. The SATCOM radome will be fixed to the interface structure by 69 counter sunk bolts of stainless steel. IV. PRE-PROCESSING OF RADOME Pre-processing, involves the preparation of data, such as nodal co-ordinates, connectivity, boundary conditions, loading conditions and material information. The purpose of mesh generation is to generate element connectivity data and nodal co-ordinate data by reading other input data to the mesh. It is the most time consuming process in the whole of analysis since it involves optimizing and verification of several parameters before doing the processing of

4 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 104 the given geometry. The following operations are performed in the pre-processing of radome: Firstly the CAD geometry is imported and repaired to remove errors if any. Then meshing of the geometry surface is done. The meshed area is optimized for Quality Index.Loads and boundary constraints are applied. Material constants are assigned before creating the Pre-preg Layers. The files are saved for Processing/Solving. Boundary Conditions: A Node is created at the centre of each attachment hole. This node is connected the circumferential nodes of the holes using RBE3 element. The centre node of all holes is constrained in all degrees of freedom. Figure 3: Meshed Radome without Boundary Constraint Figure 4: Meshed Radome with Boundary Constraint. Loading: SATCOM radome is analysed for Limit and Ultimate Pressure load cases. The radome is divided into 32 sections to apply the pressure as provided in the report. The positive pressure values act from outside towards inside the radome and negative pressures act in the opposite direction corresponding to suction. V. RESULTS AND DISCUSSIONS Figure 5: Pressure Distribution for Positive Limit Load When positive limit load for pressure is applied on the radome, maximum deformation is experienced at the nose and minimum deformation occurs at the middle as shown in fig 5. Figure 6: Pressure Distribution for Negative Limit Load When negative limit load for pressure is applied on the radome, maximum deformation is experienced towards

5 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 105 the middle and minimum deformation occurs in the tail as shown in fig 6. When the Limit load is positive, the deflection undergone is and maximum deflection occurs at the centre while minimum occurs at the boundaries as shown in Figure 7. Figure 7: Deflection Plot for Limit Load (Positive Pressure). When the Limit load is positive, the strain undergone is which is very negligible and occurs at the boundaries as shown in fig 8. Figure 8: Strain Plot for Limit Load (Positive Pressure) When the Limit load is positive, the value of Tsai Wu Index of is experienced by the whole radome as shown in fig 9. When the Limit load is negative, the deflection undergone is at the centre and it is least along the boundaries as shown in fig 10 Figure 9: Tsai Wu Index Plot for Limit Load (Positive Pressure)

6 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 106 Figure 10: Deflection Plot for Limit Load (Negative Pressure) When the Limit load is negative, the strain undergone is that occurs at the centre and is very negligible along the boundaries as shown in fig11. When the Limit load is negative, the value of Tsai Wu Index is which is very negligible and occurs throughout the radome as shown in fig 12. Figure 11: Strain Plot for Limit Load (Negative Pressure) Figure 12: Tsai Wu Index Plot for Limit Load (Negative Pressure) REFERENCES [1] K. Rohwer, S. Friedrichs, C. Wehmeyer - Analyzing Laminated Structures from Fibre Reinforced Composite Material An Assessment Bernoulli (2005) 59-77

7 International Conference on Challenges and Opportunities in Mechanical Engineering, Industrial Engineering and Management Studies 107 [2] Vincent Manet - On the computation of interface stresses by finite elements for sandwich materials Composite Structures, Volume 44, Issue 4 (1999) [3] Steven R. Nutt, H. Shen, and Lev Vaikhanskiy s - Experimental and Analytical Study of Nonlinear Bending Response of Sandwich Beams Composite Structures, 60 [2] (2003) [4] ASM Handbook of Composites - Volume 21 [5] William D. Callister - Materials Science and Engineering [6] Altair HypermeshWorks Inc. [7] [8]