review draft

7/31/2019 Review Draft

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Introduction

A Radome is a rigid, weatherproof structural enclosure that protects a microwave

or radar antenna against the natures 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.

Therefore 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 radiation, corrosion.

Hence radomes are required to protect the radars so that the longevity of the radars is

assured with no dip in its performance. Radomes can be 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 analyzed Laminated Structures from Fibre-

Reinforced Composite Material. In the literature there are tremendous number of models and

methods for analyzing laminated structures. 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.


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Vincent Manet 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 Youngs 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 Vaikhanskiys 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 canproduce 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.

P. Davies, P. Chauchot, B. Bigourdan conducted case studies on a wide sandwich

beam loaded under uniform pressure. First, results from different analytical methods and finite

element codes are compared. Then test results are presented for a glass/epoxy facing-PVC foam

core sandwich panel. Finally predicted strains and out-of-plane displacements are correlated with

experimental results.

Boualem Keskes1, Yves Menger, Ahmed Abbadi, Joseph Gilgert, Nourredine

Bouaouadja and Zitouni Azari worked on a fatigue characterization of honeycomb sandwich

panels with defects. In real situations, these panels can be affected by manufacturing defects and

impacts, and it is important to know the effects of these defects and the behaviors of the damaged

panel; it also important to determine the location of the defect. In the investigation these defects

were be simulated by a blind hole in the centre of the lower face sheet. Static and fatigue tests

(four-point bending) with acoustic-emission monitoring were carried out on sandwich panels

with defects. The load/displacement and the S-N fatigue curves are presented and analyzed.

A. T. Nettles, D.G Lance and A. J. Hodgeworked on, An Examination ofImpact

damage in Glass/Phenolic and Aluminum Honeycomb Core Composite Panels. They compared

two different core materials in different experimental conditions, then finally concluded that


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for a given impact energy, the face sheet on the aluminum core samples demonstrated more de-

laminations than the glass/phenolic core. Both glass/phenolic and aluminum core specimens

displayed core buckling as the first damage mode, followed by de-laminations in the lacings,

matrix cracking, core cracking (fiber the glass/phenolic samples), and finally fiber breakage in

the facings.

Nomenclature1. Specific Strength, - is a material's strength (force per unit area at failure) divided by its

density.

2. Specific Modulus, - is defined as the elastic modulus per mass density of a material.

3. Density, - density of a material is defined as its mass per unit volume.4. Dielectric Constant, d - reflects the extent to which it concentrates electrostatic lines of

flux.

5. Loss Tangent, tan - is a parameter of a dielectric material that quantifies its inherent

dissipation of electromagnetic energy.

Criteria for material selectionLow dielectric constant and loss tangent value for constituents are the key

design requirements since the transmission of electromagnetic energy is important.

Reinforcements here are typically E-Glass (Borosilicate), S-Glass (Mg/Al Silicate),

Aramid (Polyp-phenylene-terephthamide), Quartz (Silica) etc.

All the parameters stated above are tabulated for different reinforcing materials.

This will help us select favourable constituent materials in the construction of sandwich

model, required for the Radome Analysis. The most preferred constituent is selected

based on low di-electric constant and low loss tangent value but high mechanical

strength; corresponding to the higher values of specific modulus and specific strength.

But at the same time it is important to select a material based on the ease of

availability, manufacturing technology, fabrication at economic costs. So another table

constituting cost/kg of the materials is available, it should lead us to the selection of a

suitable material with optimum performance considering the above requisite parameters.


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Observing all the properties above, we can see that E-Glass for very cheap cost

serves 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

below:

Mechanical Properties for E-Glass-Epoxy Pre-preg

Serial No Property Value

1 Youngs Modulus, Ex N/mm-2

21700

2 Youngs Modulus, EyN/mm-2 20600

3 Poissons Ratio 0.13

4 Shear Modulus, Gxy N/mm-2

2845

5 Tensile Strength, N/mm-2

415

6 Compressive Strength, N/mm-2 415

7 Shear Strength, N/mm-2 42

The properties of Rohacell are given below:

Mechanical Properties for Rohacell 51 WF

Serial No Property Value

1 Youngs Modulus, N/mm-2

75

2 Density, Kg/m-3

52

3 Tensile Strength, N/mm-2 1.6

4 Compressive Strength, N/mm-2

0.8

5 Shear Strength, N/mm-2 0.8

Detailed 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


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The material construction for sandwich portion is as follows:

Skin: Three layers of E-Glass pre-pregs on either side of core.

Core: 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 is as follows:

Eighteen layers of E-Glass pre-pregs.


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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.

Meshed Radome

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.

Boundary Constraint

Loading

SATCOM radome is analysed for Limit and Ultimate load cases. Using the Loads presentedbelow structural analysis of the SATCOM structure will be performed. 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.


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Pressure distribution for positive limit Load

Pressure distribution for negative limit Load


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Result Summary

When the Limit load is positive the deflection undergone is 6.317

.

Fig: Deflection plot for limit load (positive pressure)

When the Limit load is positive the strain undergone is 0.002

Fig: Strain plot for limit load (positive pressure)


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When the Limit load is positive the value of Tsai Wu Index of 0.044

Fig: Tsai Wu index plot for limit load (positive pressure)

When the Limit load is negative the deflection undergone is 13.988

Deflection plot for limit load (negative pressure)


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When the Limit load is negative the strain undergone is 0.002

Fig: Strain plot for limit load (negative pressure)

When the Limit load is negative the value ofTsai Wu Index is 0.136.

Fig: Tsai Wu index plot for limit load (negative pressure)

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