A Project Report on
ANALYSIS OF SUBMARINE RADOME Submitted in partial fulfillment of the requirement for award of the degree of BACHELOR OF TECHNOLOGY IN AERONAUTICAL ENGINEERING BY
M.KARTHIKEYA REDDY 08D41A2107 SRIKANTH SINGH 08D41A2111 MD ALIYSHAAN MOHIUDDIN 08D41A2128 K.SUDHEER 08D41A2150
Under the Esteemed Guidance of Ms. P. Nanda Jyothi (Associate Professor)
DEPARTMENT OF AERONAUTICAL ENGINEERING
SRI INDU COLLEGE OF ENGINEERING AND TECHNOLOGY Sheriguda village, Ibrahimpatnam, R.R dist. (Affiliated to Jawaharlal Nehru Technological University) April 2012
1
CERTIFICATE This to certify that this report on “ANALYSIS OF SUBMARINE RADOME” is a bonafide work successfully carried out at Sri Indu College Of Engineering & Technology BY
M.KARTHIKEYA REDDY 08D41A2107 SRIKANTH SINGH 08D41A2111 MD ALIYSHAAN MOHIUDDIN 08D41A2128 K.SUDHEER 08D41A2150 Students of IV year II sem Aeronautical Engineering under our guidance our guidance has submitted in partial fulfillment as the requirement for the award of degree of
BACHELOR OF TECHNOLOGY IN AERONAUTICAL ENGINEERING
Internal Guide
External Guide
Ms.P.Nanda Jyothi (Associate professor)
Head of Department
Mr.M.Srinivasa Rao (HOD)
2
CERTIFICATE This to certify that this report on “ANALYSIS OF SUBMARINE RADOME” is a bonafide work successfully carried out at Sri Indu College Of Engineering & Technology BY
M.KARTHIKEYA REDDY 08D41A2107 SRIKANTH SINGH 08D41A2111 MD ALIYSHAAN MOHIUDDIN 08D41A2128 K.SUDHEER 08D41A2150 Students of IV year II sem Aeronautical Engineering under our guidance our guidance has submitted in partial fulfillment as the requirement for the award of degree of
BACHELOR OF TECHNOLOGY IN AERONAUTICAL ENGINEERING
Internal Guide
External Guide
Ms.P.Nanda Jyothi (Associate professor)
Head of Department
Mr.M.Srinivasa Rao (HOD)
2
ACKNOWLEDGEMENT We would like to express our deep sense of gratitude and sincere thanks to Jagadeesh Reddy P, Principal Principal Director of CADD CADD Institute Institute Hyderabad for giving us the opportunity for the course and project work. We would also like to express our sincere gratitude to our college Principal Mr. P. Mallesham for being a constant source of inspiration and giving us permission to complete our project. We would like to extend our sincere thanks to our HOD Mr. M. Srinivasa Rao and our internal guide Ms. P. Nanda Jyothi , who guided
us throughout the course of the project and made it a grand success. We would like to thank our external guide Mr. Prasad Rao for his expe expert rt guid guidan ance ce and and cont contin inuo uous us enco encour urag agem emen entt and and expr expres esss our our gratitudefor priceless guidance and untiring inspiration during planning and preparation which lead to the successful completion of our project. Fina Finall lly, y, we also also than thank k our our fri friends ends and and staf stafff memb member erss of our our institute Sri Indu College of Engineering and Technology, JNTUH and all others for helping us in all aspects.
M.KARTHIKEYA REDDY
08D41A2107
SRIKANTH SINGH
08D41A2111
MD ALIYSHAAN MOHIUDDIN
08D41A2128
K.SUDHEER
08D41A2150
3
CONTENTS Abstract List of figures 1. INTRODUCTION 1.1. Radome configurations
2
1.2. Structural support
3
1.3. Impact of incident angle
4
1.4. Functions of radome
6
2. LITERATURE SURVEY 2.1 Scope of present study
3. DESIGN OF SUBMARINE RADOME
10
12
3.1 PRO/ENGINEER
13
3.2 Modules of PRO/E
13
3.3 Modeling procedure
14
3.4 Modules of PRO/E
16
3.5 Modeling procedure
16
4. FRP MATERIALS
20
4.1 Materials
20
4.2 Reinforcement
21
4.2.1 Glass fibers
21
4.2.2 Various types of sandwich structures
24
4.2.3 Material section
25
4.3 Matrix
26
4.4 Factors influencing composite fibers
27
4.5 Materials
27
4.6 Functions of the matrix
28
4.7 Advantages of composites
28
4.7.1 High specific stiffness and strength
28
4.8 Limitations of composite materials
28
4.9 Applications of composite materials
30
5. ANALYSIS OF SUBMARINE RADOME
31 4
5.1 Finite element method
31
5.2 Explanation of finite element method
35
5.3 Types of elements
37
5.3.1 Size of elements
37
5.3.2 Location of nodes
37
5.3.3 Number of elements
38
5.3.4 Nodal degree of freedom
38
5.3.5 Coordinate system
38
5.4 Formation of matrices and vectors
39
5.4.1 Direct approach
39
5.4.2 Variation approach
39
5.4.3 Weighted residual approach
39
5.4.4 General application of the method
40
5.5 Limitations of FEM
42 43
6. IMPLEMENTATIONS
6.1 Introduction
43
6.2 Theoretical analysis
43
6.2.1 Load cases
43
6.3 ANSYS
46
6.3.1 Program overview 6.4 Meshing
47 49
6.4.1 Fully automatic mesh generation
49
6.4.2 Mesh generation using 2D element
49
6.4.3 Mesh generation using 3D element
49
6.5 Approach to analysis 6.5.1 Static analysis 6.6 Modal analysis 7. FABRICATION METHODS FOR RADOMES
7.1 Filament winding
51 51 57 62
62
7.2 Vacuum bag molding
62
7.3 Auto clave molding
63
7.4 Matched die molding
64
8. RESULTS & DISCUSSIONS
65
9. CONCLUSIONS & FUTURE WORK
67 5
REFERENCES
ABSTRACT Radomes are the electromagnetic windows that protect microwave sub-systems from the environmental effects. The major requirement of radome is its transparency to microwaves and for most of the cases mechanical properties are also equally important. Radome for underwater applications has to withstand high water pressure of the order of 45 bars. Composite materials owing to their high strength to weight ratio, high stiffness and better corrosion resistance are potential source for under water applications. The concept of 'tailoring' the material properties to suit the radome is obtained by selecting proper reinforcement, resin matrix and their compositions. The mechanical properties of composite material, evaluated by testing specimens as per ASTM standards, are utilized in designing the radome. The modulus properties calculated using classical theories of composite materials and compared with test results. The theoretical values utilized to carry out the Finite Element Analysis of the radome. ANSYS a Finite Element software package used to analyze the problem. As the
cross sectional thickness of radome varies, the complexity in fabrication is overcome by adopting matched die techniques. The radome design and finite element analysis validation concluded by conducting the pressure test on radome. The modal analysis is also carried out on radome to check for the natural frequency of the radome. So that resonance does not occur if the natural frequency of the radome coincides with the excitation frequency of the submarine.
6
List of figures 1. 1.1 Functioning of radome
2
2. 1.2 Submarine radome
7
3. 3.1 Fly out icons
18
4. 3.2 2D drawing
19
5. 3.3 Isometric view of the radome
20
6. 3.4 Side view of the radome
21
7. 3.5 Top view of the radome
22
8. 6.1 Nodal solution def+undeformed
59
9. 6.2 Nodal solution
59
10. 6.3 Pressure
60
11. 6.4 Vonmisses stress
60
12. 6.5 Graph Sxy-displacement
61
13. 6.6 Vector plot predefined
61
14. 6.7 Vonmisses graph
62
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1. INTRODUCTION The basic function of a radome is to form a protective cover between an antenna and the environment with minimal impact to the electrical performance of the antenna. Under ideal conditions, a radome is electrically invisible. How well a radome accomplishes this depends on matching its configuration and materials composition to a particular application and Radio Frequency range. Radomes can be found protecting a wide range of outdoor terrestrial and shipboard communications systems and radar installations as well as airborne avionics system antennas. The proper selection of a radome for a given antenna can actually help improve overall system performance by: 1. Maintaining alignment by eliminating wind loading, Allowing for all-weather operations by protecting the system from rain, snow, hail, sand, salt spray, insects, animals, UV damage, and wide temperature fluctuations. 2. Providing shelter for installation and maintenance personnel 3. Preventing visual observation of system (security) 4. Minimizing downtime, and extending component and system operating life.
Figure 1.1 Functioning of radome
Historically, a variety of materials have been used for constructing radomes, including balsa and plywood in early structures. Modern ground-based and ship-based
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radomes are manufactured using composite materials such as fiberglass, quartz, and aramid fibers held together with polyester, epoxy, and other resins, such as the one shown. Foam and honeycomb cores are often added between inner and outer “skins” of the radome to function as a low-dielectric-constant spacer material providing structural strength and rigidity. 1.1 Radome Configurations:
Several radome configurations are used to minimize RF reflections, including electrically thin, half-wave, A-sandwich, C-sandwich and others. The best configuration for a particular application depends on the mechanical requirements and operating frequency. A radome that is electrically thin (less than 0.1 wavelengths)as shown, will generally deliver good RF performance. This is because signal reflections at the freespace/dielectric boundary are cancelled out by out-of-phase reflections from the dielectric/free space boundary on the other side of the dielectric material. Signal losses are low and the net transmission from an electrically thin dielectric laminate is very high. Unfortunately, electrically thin radomes provide very little thermal insulation and are not suitable for locations with wide temperature extremes and a requirement for controlled temperatures. Another radome approach that works well is a configuration based on the halfwavelength-thick solid laminate shown in Figure 5. It is similar to the electrically thin configuration because the reflections cancel out. The wave travels 180° through the laminate, is reflected with a phase shift of -180°, and travels another 180° on the return trip to achieve the net 180° phase shift required for cancellation. Figure 6 shows the performance of the same laminate described in Figure 4 at higher frequencies (through 35 GHz) where it is 0.5 wavelengths thick. A-sandwich radome configuration consists of low dielectric foam or honeycomb core sandwiched between two thin laminates. Its operation is similar to the half-wavelength-thick solid laminate. However, it is 0.25 wavelengths thick because the reflection coefficients from the skins have the same amplitude and phase. The round trip for the reflection from the second skin is 0.5 wavelengths. The reflections, which are 180°, are out of phase.
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A C-sandwich radome consists of three skin layers and two foam layers, as shown in Figure 9. The thickness of each foam layer, and possibly the skins, can be tuned for optimal RF performance in the bands of interest. This can lead to many potential construction combinations that can provide good RF performance and high mechanical strength. C-sandwich constructions provide better performance than Asandwich radomes; however, the added complexity increases material and labor costs.
1.2Structural Support: Although radomes are used extensively on airframes and missiles, this section focuses specifically on support structures for terrestrial and shipboard systems. Ground and shipboard radomes can range in size from very small antenna covers to massivestructures. Self-supporting radomes are usually based on an A-sandwich configuration. They are made of rigid sections that are bolted or latched together. If phase delay and insertion loss through the seam is matched to the rest of the radome, the seam becomes largely invisible to the electromagnetic wave front. Unlike other radome types mentioned in this article, A-sandwich radomes require no air blowers to maintain pressure and are not dependant on electrical power to maintain their electro-magnetic or structural performance. A-sandwich radomes generally have lower overall operation and maintenance costs. Inflatable radomes are made of electrically thin dielectric cloth. By being electrically thin, they are capable of achieving very low loss over wide bandwidths. The tradeoff for high performance, however, is that they require a constant supply of air. Inflatable radomes must be supported by internally generated air pressure, which is supplied by air blowers or air compressors. In order to maintain adequate air pressure, inflatable radomes must be equipped with airlocks at all doors and a standby power supply to operate the blowers at all times and under all environmental conditions. Should the membrane suffer damage or if power is interrupted, it’s possible for the radome to deflate and collapse. Operating and maintenance costs for this type of radome usually exceed those all other radome types. Metal space frame radomes support the window portion of the radome consisting of the electrically thin, half-wave, or A-sandwich configuration, often in the shape of a geodesic dome. The window portion typically has very low loss. However,
10
signal blockage from the frame reduces system gain and reflects noise back into the system. Because the frame reflects and refracts the RF wave front, it increases side lobe levels. A method used to prevent large side lobes is the use of a quasi-random frame pattern. The quasi-random pattern is also used to minimize side lobes for the other support structure types. In contrast to metal space frame radomes, dielectric space frame radomes are supported by dielectric members who are somewhat electrically transparent. However, the wave front is phase delayed as it passes through the dielectric support, alternating between in and out of phase, depending on frequency. If the delay is 180° out of phase with the phase of the incident signal, the energy that passes through the frame subtracts from the gain. This leads to a frequency dependant sinusoidal ripple in the insertion loss and the lost energy goes into the side lobes. This makes dielectric space frame radomes best suited to systems that operate at less than 1 GHz. Both types of space frame radomes usually require the use of air blowers or compressors in order to maintain and enhance the structural integrity of their thin membrane coverings during windy conditions. Failure to maintain positive pressure can result in membrane damage and failure. 1.3 IMPACT OF INCIDENT ANGLE:
All of the plots and explanations thus far show reflections at normal incidence. Typically, an electromagnetic wave hits the radome surface at an oblique angle, or in the case of a spherical radome, a continuous range of oblique angles. The transmission characteristics of the radome change with the wave incidence angle and polarization. Electric fields that are parallel to the plane of incidence have much higher transmission than fields that are perpendicular to the plane of incidence. Aerodynamic radomes used on aircraft and missiles often see high incidence angles. This can result in large amounts of axial ratio degradation for circularly polarized antennas and higher insertion loss. Electromagnetic wave fronts from parabolic antennas located inside spherically shaped radomes see low incident angles at the center of the wave front. Out on the edges, however, the incident angle becomes higher. If the antenna illumination pattern is symmetric, and the antenna is placed at the center of the spherical radome, the symmetric shape of the radome cancels out axial ratio degradation from the oblique incidence angles seen by the antenna.
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Composites are gaining wider acceptance for use on board warships and submarines due to number of advantages viz. high strength to weight ratio, ability to be moulded into complex shapes, better EMI performance, absence of corrosion palliatives which otherwise are source for electronic and magnetic signature. Composite materials made from E-Glass fibers and epoxy resins have become very popular as a radome material due to its outstanding transparency to microwaves and having good mechanical properties. The increasing popularity of the material for underwater application are posing great difficulties to the designer to select right combination of composition & shape of radome due to the complex nature of the structure and the loading conditions for the useful operation life. Mechanical properties of composite materials are influenced by several factors like reinforcement, fiber orientation, adhesion, composition, manufacturing process etc. Conducting the tests on standard specimens and evaluating mechanical properties is the most important aspect in design of composite material applications. The ASTM guidelines followed in testing and preparation of standard test specimens. The micromechanics and failure mechanism of composite material is very complex compared to the conventional isotropic materials. Depending on the reinforcement, composition content & its percentage, appropriate theory & failure mechanism can be considered for designing the radome. Finite Element Analysis of radome design is carried-out using (Analysis System) ANSYS a FEA software package. Geometrical model of radome is generated as per radome sketch. Suitable elements are selected and optimum size of mesh is generated. Material properties, evaluated from tests, are assigned. Boundary conditions, load cases are applied to complete the preprocessing stage. The post results obtained after FE analysis compared with design requirements. The main objective of this project is to develop composite radome which protects the electronic equipment from high water pressure and transparent to electromagnetic waves.
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Figure 1.2 Submarine radome
The geometric shape of the radome is a cylindrical barrel covered with a hemi-spherical dome at the top. It has a circular plate at the bottom end of the cylinder having M6 size holes which acts as a flange. The radome is secured to the submarine structure with M6 bolts on its flange. Radome is made of sandwiched construction with glass reinforced plastic (GRP) as sheet material and syntactic foam as core. E glass woven fabric & Epoxy resin is used. 1.4 FUNCTIONS OF THE RADOME:
The Functions of the radome are as follows: 1. The Radome protects the installation from the deteriorating effects of environment and extends the durability of antenna and other equipment. 2. The overall performance of the antenna will be increased with the use of radome 3. FRP radome helps to have overall economy and weight reduction. 4. A radome permits the air borne antenna to function with good efficiency under high head of the water over the submarine.
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2. LITERATURE SURVEY Although the name of the finite element method was given recently the concept has been used several centuries back. For example, Ancient mathematicians found the circumference of a circle by approximating it as a polygon. In terms of the present day notation each side of the polygon can be called a finite element, by considering the approximating polygon inscribed or circumscribed, one can obtain a lower bound or an upper bound for the true circumference. Further, as the number of sides of the polygon is increased the approximate values coverage to the true value, these characteristics will hold true in any general finite element application. In recent times an approach similar to the finite element method, involving the use of piece wise continuous functions defined over triangular regions, was first suggested by R. Courant in 1943 in the literature of applied mathematics. The finite element method as known today has been presented in 1956 by M.J. Turner, R.W. Clough, and H.C. Martain & L.J. Toop. This paper presents applications of simple finite elements (pin-joined bar & Triangular plates with in plane loads) for the analysis of aircraft structure and is considered as the key contributions in the development of the finite element method. The digital computer provides a rapid means of performing the many calculations involved in the finite element analysis and made the method practically viable, along with the development of high speed digital computers the application of the finite element method progressed at a very impressive rate. The book by Przemieniecki, and Zienkievicz and Hoslister presented the finite element method as the applied to the solutions of the stress analysis problems. The book by Zienkievicz’s and Cheug” the finite element method in structural and continuum mechanics” (Mc. graw hill, London, 1971) presented the broad interpretation of the finite element method and its applicability to any general field problems. With this broad interpretation of the finite element method it has been found that the finite element equation also be derived by using a weighted residual method such as Galerkin method or the least squares method.
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This lead to widespread interest among applied mathematicians in applying the finite element method for the solution of the linear and non-linear differential equations. Over the years several papers, conference proceedings and books have been published on this subject with all this progress today the finite element method is considered as one of the well established and convenient analysis tool by engineers and applied scientists. The history of submarines subsequent to the first truly operational vessel, Holland, launched in 1899, showed two significant advances as opposed to steady incremental developments. These resulted from full scientific studies of all the problems. The first of these advances was made by the Germans at the end of World War II, when they produced the Type 21 which had major improvements in range and battery time while their underwater speed increased to 18 knots compared to 5 knots on previous vessels. Design diving depth was increased dramatically. They could operate below the Allies Submarine defense weapon systems. The second advance was made by US designers who produced Albacore in 1953 with a shape suited to full underwater operation. Its length-to beam ratio was only 7.7 and top underwater speed was 33 knots. The drag coefficient was only 0.1 compared to 0.35 on previous submersible designs. It is clear that scientific studies should be a starting point for any future submarine design. A review of the literature covers priorities in design and shows how enhancement of one feature interacts with other features and may even result in an overall loss of performance despite the perceived advantage of the enhanced feature. Hydrodynamic aspects are then discussed starting with the shape and reasons why a length-to-beam ratio of about 7.5 gives the minimum resistance. All features affecting the resistance are discussed including the boundary layer, laminar flow, transition, turbulence separation and how the flow over the principle passive sonar should be as quiet and smooth as possible. Added resistance from sails, masts, snorkels and appendages need careful streamlining and attention in design. A proposed profile of a new submarine is presented which has the passive sonar far forward in the streamlined nose with the torpedo tubes positioned further aft. It should be a quieter vessel with more effective sonar. The profile requires shortening to reduce the displacement and then the internals need rearranging. The design process then begins, which is iterative. In order to proceed with such concepts it is vital to have a database. Our current submarine, the Collins class, should be the base from which all changes and proposals 15
are measured. It is suggested detailed wind tunnel studies should be undertaken concurrently with computational fluid dynamic (CFD) evaluations. The results should then be compared with full scale trials to establish propeller efficiencies and roughness factors as well as the contributions for each feature, hull, sail, control fins, masts and snorkels, flood openings and others. This database will allow more precise comparisons for any improvements which may be considered in a future design. Glass fiber recycled poly(ethylene terephthalate) composites: mechanical and thermal properties by A.L.F. de M. Giraldi, Department of Polymer Technology, College of Chemical Engineering, State University of Campinas, SP, Brazil. Their Investigations of thermal and mechanical properties of recycled poly(ethylene terephthalate) (PET) reinforced with glass fiber have been carried out, focusing on the influence of two variables involved in the extrusion process: screw speed and torque. A Factorial Experimental Design of the processing conditions during extrusion (screw speed and torque) was done to get the best
thermo
mechanical properties
versus
processing
conditions.
Mechanical
properties such as Young's Modulus and Impact Resistance increased after the addition of glass fiber in recycled PET matrix. Inter laminar fracture of commingled-fabric-based GF/PET composites. L. Ye and K. Friedrich Department of Mechanical and Mechatronic Engineering at the University of Sydney, NSW 2006, Australia, Institute for Composite Materials Ltd, University of Kaiserslautern, Germany. A 45:55 weight% mixture of commingled glass/polyethylene terephthalate(PET) fabric was selected to study the relationships between material micro structure, Mode I and Mode II inter laminar fracture toughness and failure mechanisms. Composite laminates subjected to different cooling histories were manufactured with in a steel mould using a laboratory heat press. Mode I and Mode II inter laminar fracture tests were performed using double cantilever beam and end-notched flexure specimens.PET matrix morphology appeared to be sensitive to the thermal histories, although this occurred on a subspherulitic
scale (in contrast to
observations made with polypropylene-based composites). The spherulitic textures were generally very fine and no evidence of inter spherulitic fracture paths could be identified. When the composites were subjected to low cooling rates or an isothermal crystallization process, many small matrix cracks developed between fibers within the reinforcing bundles. The lower the cooling rate, the higher the density of matrix cracks per unit volume of material. The Hybrid Composite inter laminar fracture toughness in
16
the laminates with slow cooling rates was much lower than in the case where a quasiquenched condition was applied. Characterization of thermoplastic poly(ethyleneterephthalate)-glass fiber composites, crystallization study By Catherine Gauthier , Laboratoire d 'Etudes des Matriaux Plastiques et des Biomatériaux, Université Claude Bernard, France. They investigated the influence of glass fibers on crystallization kinetics and on matrix morphology for poly (ethylene terephthalate) (PET)/glass fiber composites. The following parameters are also considered: fusion-crystallization conditions, thermal stability and the addition of nucleating agents in the matrix (talc or sodium benzoate). It clearly appears that the influence of those additives on the crystallization of PET is predominant compared to the effect of stiffening fibers. Moreover, the application of shear stresses at the PET/glass fiber interface promotes the growth of a different crystalline superstructure.
2.1 SCOPE OF PRESENT STUDY: A probe by accident into the field of thermosetting polymers has brought about a quantum growth in its basic as well as technological aspects. The synthetic thermosetting polymers with the combinational properties of the existing conventional
high strength polymers and
glass fibers
with a
variety of
work presented here reveals that large effort has
gone into
filler materials have altogether offered a new field of research. The review
of
the understanding of the mechanical, thermal and physical properties of thermo sets. A thorough literature search reveals that there are no systematic studies on mechanical properties of thermosetting composites. There is ample scope for fabrication of newer composites with different weight fractions of glass fiber and PET in polymers and there characterization for physical, mechanical and thermal properties. With a variety of filler Materials have altogether offered a new field of research.
Hybrid Composites-the understanding of the mechanical and thermal properties of thermo sets:
A thorough literature search reveals that there are no systematic studies on mechanical
properties
of thermosetting
composites.
There
is
ample
scope
for fabrication of newer composites with different weight fractions of glass fiber and
17
fillers
in polymers and
their
characterization for
physical,
mechanical
and
thermal properties. In this thesis, a wealth of data on mechanical properties of polymer glass filler composites has been generated. These data are useful for material technologists, mechanical engineers and defense engineering, who can make use of this database for the generation of new materials for specific application. In that respect it has been used GF and virgin PET fibers in the form of woven mat and epoxy as matrix. Laminates are obtained from vacuum bag molding technique. Tests carried out to evaluate PhysicMechanical and thermal properties according to ASTM standards.
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3. DESIGN OF SUBMARINE RADOME Design as a noun informally refers to a plan or convention for the construction
of
an object or
a
system
(as
in blueprints,
engineering
drawing,
circuit
diagrams and sewing patterns) while "to design" refers to making this plan. No generally-accepted definition of "design" exists and the term has different connotations in different fields. However, one can also design by directly constructing an object (as in pottery, engineering, management, and graphic design). More formally design has been defined as follows. A specification of an object, manifested by an agent, intended to accomplish goals, in a particular environment, using a set of primitive components, satisfying a set of requirements, subject to constraints; to create a design, in an environment. Another definition for design is a roadmap or a strategic approach for someone to achieve a unique expectation. It defines the specifications, plans, parameters, costs, activities, processes and how and what to do within legal, political, social, environmental, safety and economic constraints in achieving that objective . Here, a "specification" can be manifested as either a plan or a finished product, and "primitives" are the elements from which the design object is composed. With such a broad denotation, there is no universal language or unifying institution for designers of all disciplines. This allows for many differing philosophies and approaches towards the subject. The person designing is called a designer , which is also a term used for people who work professionally in one of the various design areas, usually also specifying which area is being dealt with (such as a fashion designer , concept designer or web designer ). A designer's sequence of activities is called a design process. The scientific study of design is called DESIGN. Designing often necessitates considering the aesthetic, functional, economic and sociopolitical dimensions of both the design object and design process. It may involve considerable research, thought, modeling, interactive adjustment, and re-design. Meanwhile, diverse kinds of objects may be designed, including clothing, skyscrapers, corporate, skyscrapers, corporate identities, business processes and even methods of designing.
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3.1PRO/E: Pro-E Design, LLC was founded in Macedonia, Ohio by Brian T. Hengle. A graduate of Cleveland State University, Brian holds a bachelor of Civil Engineering with a concentration in Structures. He is registered in the state of Ohio as a Professional Engineer. Pro/ENGINEER is a parametric, integrated 3D CAD/CAM/CAE solution created by Parametric Technology Corporation (PTC). It was the first to market with parametric, feature-based, associative modeling software. The application runs on Microsoft-Windows platform, and provides modeling, assembly and drafting, finite element analysis, and NC and tooling functionality for mechanical engineers. The Pro/ENGINEER name was changed to Cero element/Pro on October 28, 2010, coinciding with PTC’s announcement of Cero, new design software. Pro/ENGINEER (Pro/E for short) is a commercial CAD/CAM package that is widely used in industry for CAD/CAM applications. It is one of the new generations of systems that not only offer a full 3-D solid modeler, in contrast to purely 2-D and surface modelers, but also parametric functionality and full associatively. This means that explicit relationships can be established between design variables and changes can be made at any point in the modeling process and the whole model is updated. The method of constructing a model of an object is very similar to that followed in the production of a physical component. For example the manufacture of the shaped block in Figure 1 would start with the choice of construction environment, the selection of a piece of stock material followed by a series of manufacturing processes, e.g. milling, drilling, welding/sticking. Pro/E has direct analogues for most of these operations as various types of FEATURES which can be combined to generate a complete representation of a PART, Pro/E's terminology for a single component. Features fall into three main categories, Construction, Sketched and Pick/Placed. Pro/E is mainly used for CAD.pro/E is generally defined as feature based, associative, parametric.
3.2 FEATURE BASED: When you want to create any solid model, you have to create it using number of features hence it is known as feature base . Pro/ENGINEER is feature-based. Geometry 20
is composed of a series of easy to understand features. A feature is the smallest building block in a part model. Things to remember:
• Pro/ENGINEER allows building a model incrementally, adding individual features one at a time.
• This means, as you construct your model feature by feature you choose your building blocks as well as the order you create them in, thus capturing your design intent.
• Design intent is the motive, the all-driving force, behind every feature creation.
• Simple features make your individual parts as well as the overall model flexible and reliable.
The fly-out icons will appears automatically on the right side screen when you enter the sketcher mode.
These icons are logically grouped together, based on capability.
3.3 ASSOCIATIVE : Pro/ ENGINEER models are often combinations of various parts, assemblies, drawings, and other objects. Pro/ENGINEER makes all these entities fully associative. That means if you make changes at a certain level those changes propagate to all the levels. For example if you change dimensions on a drawing the change will be reflected in the associated part. Any modification made in any module will automatically create modification in the other module. So this type of connection from module to module is known as associative.
•File: Contains commands for manipulating files
•Edit: Contains action commands
•View: Contains commands for controlling model display and display performance.
•Datum: Creates datum features
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•Analysis:
Provides access to options for model, surface, curve and motion analysis, as well as sensitivity and optimization studies .
• Info:
Contains commands for performing queries and generating reports.
•Applications: Provides access to various Pro/ENGINEER modules,
•Utilities: Contains commands for customizing your working environment.
•Windows: Contains commands for managing various Pro/ENGNEER windows.
figure3.1 Fly-out icons
With fly-out icons, you can access the most frequently used sketching tools with Single click, without having to go to pull down menus. 22
3.4 Modules of PRO/E 1 .SKETCHER 2. PART DESIGN 3. ASSEMBLY 4. DRAWING
3.5 MODELLING PROCEDURE: FOLLWING PROCEDURE IS ADOPTED IN MODELING OF THE SUBMARINE RADOME •
Initially 2D drawings were created using sketcher toolbar; tools in profile tool bar such As line, circle, rectangle, rectangle, point, reference lines etc … and sketch sketch references like grid, vertex, and dimensions are used.
Figure3.2 2D drawing •
The The create created d draw drawing ingss were were then then comp comple letel tely y cons constra traine ined d using using the tool tool in constraint tool bar like constraint and auto constraint.
•
Then 2D drawings were converted into 3D using sketch based features tools such as extrude, swept blend, blend.
•
3D objects are modified as required using engineering feature tool bar, tools such as edge fillet, chamfer are used.
23
Figure3.3 isometric view of the radome
24
25
Figure3.4 side view of the radome
26
Figure3.5 top view of the radome
27
4. FRP MATERIALS 4.1MATERIAL: Since the performance of a radome depends on the materials used, the materials play an important role in the design of the radome. In the beginning plywood was used for fabrication of radomes. But due to its moisture absorption tendency now it is not used. Metals cannot be used as radome materials because they are conductors of electricity which will absorb the transmitted electromagnetic waves by the antenna. To avoid moisture absorption resin impregnated glass fabric was applied as thin layer on the outer surface of the plywood radome. But the performance of this layer is very much limited. After that polyester foam phenolic resin impregnated cotton canvas was used to protect moisture absorption even though this exhibits good strength, its resistance to heavy physical stresses was poor. To increase the wall strength, the thickness of the radome wall has to be increased which is not suitable to radar wave length. At this critical stage composite materials fiber reinforced plastics came into existence. Composite material is the combination of two or more materials with different properties and characteristics of the parent material. A composite is a mixture of materials with adequate bond between the constituents; the constituents retain their physical identity even after several years. 28
The composite material mainly contains two components namely reinforcement and the matrix.
4.2 REINFORCEMENT : The main function of the reinforcement is to improve the overall mechanical properties of the composite. In general this reinforcement material will have higher tensile strength and young modulus than that of matrix material. These are used in the form of fibers. Many materials like glass carbon fiber, graphite, boron fiber, asbestos, whiskers, Kevlar etc, can be used as reinforcement material, out of these glass fiber is most versatile. For very high performance applications advanced composites made of either carbon fibers or boron fibers are used. But in India fibers are used to reduce the cost of production. 4.2.1 GLASS FIBRES:
These fibers are graded as E,A,C,S,Z,M & D.
•
‘E’ glass is electrical grade which is having high bulk electrical resistivity and high surface resistivity.
•
‘A’ glass in one which is having high alkali content and of very limited use.
•
‘C’ glass is a chemical quality and used for corrosion resistance to acids.
•
‘S’ glass has higher strength and elastic module that E glass.
29
•
‘Z’ glass is used to reinforce the cement products.
•
‘M ’glass has high value of young’s modulus but specific strength is low.
•
‘D’ glass has low dielectric loss value and hence is specifically suited for high performance electronic applications radomes etc.,
The various forms of glass fiber reinforcement are as continuous strand and roving, chopped strands, yarns, mats etc., Table 1. Comparison of typical properties for some common fibers:
Materials
Density (g/cm3)
TensileStrength (MPa)
Young modulus (GPa)
E-Glass
2.55
2000
80
S-Glass
2.49
4750
89
Alumina (Saffil)
3.28
1950
297
Carbon
2.00
2900
525
Kevlar 29
1.44
2860
64
Kevlar 49
1.44
3750
136
CARBON FIBRES: These are used for higher temperature applications compared to
‘E’ glass. BORON FIBRES:
These are used for light weight aerospace composite structures. The density of boron fiber is only 2.6 x 10 Micro Kg/Culm. Boron fibers are extremely hard and have very high tensile strength & Module. WHISKERS:
Whiskers are strong and the best properties of glass and boron are present in whiskers. They have the elongation of glass fibers (3 to 4 %) & the modulus of boron is 410 KN/mm.
30
MATRIX SYSTEMS:
The plastic phase which holds together the reinforcing fibers is called the matrix. The plastic material acts as the medium through which load is transferred from one fiber to the other fiber also the matrix protects the reinforcing fibers from weather and provides shape and finish to the composite material. There are a variety of matrix materials available, some additives are added to the matrix to get some desired improved properties like hardness, temperature, resistance etc., and these additives include curing agents, fillers and stabilizers. The matrix materials/plastic resins which undergo deformation and can be reworked when heated are known as thermoplastics & the plastics which will not undergo deformation and cannot be reworked are called thermo setting plastics. THERMO PLASTICS:
Ex: Polyethylene, Polystyrene, Teflon, PVC, Acrylic etc., THERMOSETTING PLASTICS:
Polyster, Epoxy, Phenolics, Silicones, Urea formaldehyde etc., Polymers in a stage of incomplete polymerization are called resins. A resin must be subjected to further polymerization during processing by heat or addition of catalyst or harder. The thermosetting resins are as follows. POLYSTER:
Polyesters are made in two stages. In the first stage unsaturated moiety is made, in the second stage the unsaturated base resin is dissolved in a suitable unsaturated monomer. The cross linking of polymers is called curing and it is achieved by adding catalyst/ initiator and an accelerator / promoter at room temperature and at elevated temperature first adding a catalyst suitably the monomers. EPOXIES:
31
Epoxy resins in the uncured state are liquids of low melting solids which set to a solid infusible mass on reacting with a curing agent or hardener. The most widely used type of epoxy resin in the world is that derived from epichlorohydrin and Biphenyl ‘A’ Diluents are added to epoxy resins primarily to lower viscosity and thus improve handling characteristics. Epoxy resins are preferred because of its good electrical properties, excellent chemical properties, outstanding toughness and better adhesive properties.
PHENOLIC RESINS:
These are used where high strength and high temperatures (up to300 deg. centigrade) are used. Higher pressures are necessary to cure phenol mouldings. SILICON RESINS:
Silicons are the first of the inorganic polymers they are the combination of silicon - oxygen linkages. The outstanding electrical properties of silicon fiber glass laminates coupled with the retention of mechanical properties at elevated temperatures have made this type of composite a standard for radomes of supersonic vehicles. Glass fibers are bonded to a low density cellular polystyrene material and used a radome material. For attaining good mechanical properties and better adhesion between the fabric and resin system epoxide resins are developed. Later on a wide variety of matrix systems were developed like phenolics, silicon etc., in reinforcement also different types of fabric are developed to attain desired specific properties. They are like Kevlar, Carbon Boron etc., for under water applications the Radom has to withstand high hydrostatic pressures, so sufficient thickness of wall should be provided. If the thickness of the wall is increased solidly, the transmission losses will be more because of the high dielectric constants of the wall materials. To overcome the problem of transmission losses alternate layer of low dielectric constant and high dielectric constant materials are used. This type of structure is called sandwich wall and the radome is known as sandwich radome. 32
4.2.2 VARIOUS TYPES OF SANDWICH STRUCTURES:
The simplest form of laminated construction is two ply sandwiches; this consists of high dielectric constant thick skin supported internally by a porous low dielectric material. This suits for low frequencies. A sandwich wall consists of an outer and inner skin of high dielectric material and a core of low dielectric material. The optimum skin spacing or core thickness is approximately 1/4th wave length. In this construction the strength to weight ratio is greater than of a solid wall radome and it also has broad band capabilities. The problem in this is sandwich construction. 4.2.3 MATERIAL SELECTION:
The metals have been the most preferred engineering materials because of their mechanical properties.
Composite materials with high specific modulus and
specific strength are fast becoming the choice of materials for engineering applications where weight is a crucial factor. In case of radomes light weight facilitates easy handling and rotation in required direction which allows power saving, cost saving and greater safety. The materials must be such that it should not interfere with RF signals. Compared to metals, composite materials provide better solution to the requirements of masts. 4.2.4 HOW DO COMPOSITES DIFFER FROM METALS:
Composite materials have many characteristics that are different from conventional engineering materials. Most engineering materials are homogeneous and isotropic. In contrast composite materials are heterogeneous and orthotropic or more generally anisotropic. HOMOGENEOUS:
A homogeneous body has uniform properties throughout i.e. properties are not a function of position in the body. ISOTROPIC:
33
An isotropic body has material properties that are the same in every direction at a point in the body. i.e. the properties are not a function of orientation at a point in the body. HETEROGENOUS:
A heterogeneous body has non-uniform properties over the body i.e. the properties are a function of position in the body. ORTHOTROPIC:
An orthotropic body has material properties that are different in three mutually perpendicular directions at a point in the body.
The properties are a function of
orientation at a point in the body. ANISOTROPIC:
An anisotropic body has material properties that are different in all directions at a point in the body. There are no planes of material and property symmetry. Again, the properties are a function of orientation at a point in the body. FIBER:
Any material in an elongated form such that it has very high length to diameter ratio is called a fiber. Fibers are much stiffer and stronger than the same material in bulk form. Materials have actual strengths which are several magnitudes lower than the theoretical strengths. This difference is due to the inherent flaws in the material. As the fibers become smaller in diameter, it reduces the chances of an inherent flaw in the material; thereby the strength is increased.
4.3 MATRIX: A bonding material which adheres to and contains the fibers is called matrix. Metals, thermoplastics, thermosetting resins, ceramics can be used as matrix materials. Epoxy resins are the most commonly used matrix materials. 34
LAMINA:
A lamina is a flat (sometimes curved as in a shell) arrangement of unidirectional fibers or woven fibers in a matrix.
LAMINATE:
A Laminate is a stack of laminae with various orientations of principal material directions in the laminae. 4.4 FIBER FACTORS INFLUENCING COMPOSITE PROPERTIES:
The fibers in a composite material are the major load carrying agents. The fiber factors which influence the performance of a composite are length, orientation, shape and material. LENGTH:
The fibers can be either long or short. Long continuous fibers are easy to orient and process and have improved surface finish and dimensional stability. The short fibers cannot be controlled fully for proper orientation however short fibers provide low cost and have few flaws and therefore have higher strength. ORIENTATION:
Fibers oriented in one direction give very high stiffness and strength in that direction. If the fibers are oriented in more than one direction, for the same volume of fibers per unit volume of the composite, it cannot match stiffness and strength of unidirectional composites. SHAPE:
The most common shape of fibers is circular because handling and manufacturing them is easy. Hexagon and square shaped fibers are possible but their advantage of 35
strength and high packing factors do not outweigh the difficulty in handling and processing them.
4.5 MATERIAL: The material of the fiber directly influences the mechanical performance of a composite. Fibers are generally expected to have high elastic modulus and strength. This expectation and cost have been key factors in graphite, aramids and glass dominating the fiber market for composites.
4.6 FUNCTIONS OF THE MATRIX: 1. The matrix binds the fibers together, holding them aligned in the important stressed direction. Loads applied to the composite are then transferred into the fibers, the principal load bearing component, through the matrix, enabling the composite to withstand compression, flexural and shear forces as well as tensile loads. 2. The matrix isolates the fibers from each other so that they can act as separate entities and the failure of one fiber does not result in immediate failure of composite. 3. The matrix should protect the reinforcing fibers from mechanical damage and from environmental attack.
A ductile matrix will provide a means of slowing down or
stopping cracks that might have originated at broken fibers while a brittle matrix may depend upon the fibers to act as matrix crack stopper. 4. Through the quality of its grip on the fibers the interfacial bond strength the matrix can also be an important means of increasing the toughness of the composite.
4.7 ADVANTAGES OF COMPOSITES: Fibrous composites are often the material of choice of designers for variety of reasons including low weight, high stiffness, high strength, electrical conductivity ( or non conductivity), low thermal expansion,
low or high rate of heat transfer and
corrosion resistance. 4.7.1 HIGH SPECIFIC STIFFNESS AND STRENGTH:
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Undoubtedly the most often cited advantage of fibrous composites is their high specific stiffness and high specific strength as compared with traditional engineering materials. These properties lead to improved performance and reduced energy consumption, both vitally important in the design of almost all engineering structures. Unlike isotropic materials, composites are directionally dependent. Thus composites can be designed to have the desired properties in specified directions without over designing in other directions. Tailored Design: Composites can be engineered to meet the specific demands of each particular application. Available design options include. i.
The choice of materials (fiber and matrix)
ii.
The volume fraction of fiber and matrix
iii.
Fabrication method
iv.
Number of layers in a given direction
v.
Thickness of individual layers
vi.
Type of layer (unidirectional or fabric)
vii.
Layer stacking sequence (symmetric or anti-symmetric) This vast array of design variables for composites contrasts sharply with more
traditional engineering materials, where the choices are much more limited.
The
availability of a wide array of structural materials means that more efficient structures can be fabricated with less material waste. The matrix is of considerably lower density, stiffness and strength than the fibers. However, the combination of fibers and matrix can have very high strength and stiffness, yet have low density.
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4.8 LIMITATIONS OF COMPOSITE MATERIALS: There are some drawbacks and limitations in use of composites and these include:i.
High cost of fabrication of composites is a critical issue improvement in processing and manufacturing will lower these costs in the future.
ii.
Because of anisotropy of composites, the structural analysis (computational, experimental) is more complicated and intensive.
4.9 APPLICATIONS OF COMPOSITE MATERIALS: Composite materials have been successfully applied in a wide variety of fields. AEROSPACE:-
In aircrafts, spacecrafts & helicopters, the composites have been used successfully. High specific modules and strength and dimensional stability during large changes in temperature in space make composites the material of choice in space applications. SPORTING GOODS:-
Composites are used in athletic equipment to improve composites through lighter weight and improved tailoring composites have been used for tennis racket, boat hulls, speed boats, hockey sticks etc. MILITARY:-
Helmets, bullet proof vests, lighter weapons, portable bridges are a few examples of military applications.
Apart from these, composites have been used
successfully in automotive industry, medical and electronic applications.
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5. ANLASYS OF SUBMARINE RADOME 5.1. GENERAL DESCRIPTION OF THE FINITE ELEMENT METHOD: 5.1.1 FINITE ELEMENT METHOD:
The finite element method is a numerical technique, well suited to digital computers, which can be applied to solve problems in solid mechanics, fluid mechanics, heat transfer and vibrations. The procedures to solve problems in each of these fields are similar; however this discussion will address the application of finite element methods to solid mechanics problems. In all finite element models the domain (the solid in solid mechanics problems) is divided into a finite number of elements. These elements are connected at points called nodes. In solids models, displacements in each element are directly related to the nodal displacements. The nodal displacements are then related to the strains and the stresses in the elements. The finite element method tries to choose the nodal displacements so that the stresses are in equilibrium (approximately) with the applied loads. The nodal displacements must also be consistent with any constraints on the motion of the structure. The finite element method converts the conditions of equilibrium into a set of linear algebraic equations for the nodal displacements. Once the equations are solved, one can find the actual strains and stresses in all the elements. By breaking the structure into a larger number of smaller elements, the stresses become closer to achieving equilibrium with the applied loads. Therefore an important concept in the use of finite element methods is that, in general, a finite element model approaches the true solution
39
to the problem only as the element density is increased (see the discussion on Limitations of Finite Element Methods) There are a number of steps in the solution procedure using finite element methods. All finite element packages require the user to go through these steps in one form or another. 1) Specifying Geometry - First the geometry of the structure to be analyzed is defined. This can be done either by entering the geometric information in the finite element package through the keyboard or mouse, or by importing the model from a solid modeler like Mechanical Desk Top. 2) Specify Element Type and Material Properties - Next, the material properties are defined. In an elastic analysis of an isotropic solid these consist of the Young’s modulus and the Poisson’s ratio of the material. 3) Mesh the Object - Then, the structure is broken (or meshed) into small elements. This involves defining the types of elements into which the structure will be broken, as well as specifying how the structure will be subdivided into elements (how it will be meshed). This subdivision into elements can either be input by the user or, with some finite element programs (or add-ons) can be chosen automatically by the computer based on the geometry of the structure (this is called automeshing). 4) Apply Boundary Conditions and External Loads - Next, the boundary conditions (e.g. location of supports) and the external loads are specified. 5) Generate a Solution - Then the solution is generated based on the previously input parameters. 6) Post processing - Based on the initial conditions and applied loads, data is returned after a solution is processed. This data can be viewed in a variety of graphs and displays. 7) Refine the Mesh - Finite element methods are approximate methods and, in general, the accuracy of the approximation increases with the number of elements used. The 40
number of elements needed for an accurate model depends on the problem and the specific results to be extracted from it. Thus, in order to judge the accuracy of results from a single finite element run, you need to increase the number of elements in the object and see if or how the results change. 8) Interpreting Results - This step is perhaps the most critical step in the entire analysis because it requires that the modeler use his or her fundamental knowledge of mechanics to interpret and understand the output of the model. This is critical for applying correct results to solve real engineering problems and in identifying when modeling mistakes have been made (which can easily occur). The eight steps mentioned above have to be carried out before any meaningful information can be obtained regardless of the size and complexity of the problem to be solved. However, the specific commands and procedures that must be used for each of the steps will vary from one finite element package to another. The solution procedure for ANSYS is described in this tutor. Note that ANSYS (like any other FEM package) has numerous capabilities out of which only a few would be used in simple Static Analysis problems. In the finite element method, the actual continuum or body of matter like solid, liquid or gas is represented as some assemblage of sub divisions called finite elements. These elements are considered to be interconnected as specified joints which are called nodes or nodal points. The nodes usually lay on the element boundaries where adjacent elements are considered to be connected, since the actual variation of the field variable (like displacement, stress, temperature, pressure & velocity) inside the continuum is not known. We assume that the variation of the field variable inside a finite element can be approximated by a simple function. The approximating functions (also called interpolation models) are defined in terms of the values at the nodes. When the field equations (like equilibrium equations) for the while continuum are written the new unknown will be the nodal values of the field variable, by solving field equations, which are generally in the form of matrix equations. The nodal values of the field variable will be known once these are known; the approximating function defines the field variable throughout the assemblage of elements.
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The solution of a general continuum by the finite element method always follows an orderly step by step process; the step by step procedure for static structural problem can be stated as follows: Step1. Discritization of structural domain: The first step in the finite element method is to divide the structure or solution region into sub divisions or elements. Step2. Selection of a proper interpolation model: Since the displacement (field variable) solution of a complex structure under any specified load condition cannot be predicted exactly, we assume some suitable solution within an element to approximate the unknown solution. The assumed solution must be simple from computation point of view and it should satisfy certain convergence requirements. Step3. Derivations of element stiffness matrices (Characteristic matrices) and load vectors: From the assumed displacement model the stiffness matrix [K(e)] and the load vector P(e) of element ‘e’ are to be derived by using either equilibrium conditions or a suitable variation principle. Step4. Assemblage of element equations to obtain the overall equilibrium equation: Since the structure is composed of several finite elements, the individual element stiffness matrices and load vectors are to be assembled in a suitable manner and the overall equilibrium equation has to be formulated [k]_=P Where [k] is called assembled stiffness matrix, _ is called vector of nodal displacements and P is the vector of nodal forces for the complete structure. Step 5: Solution of system equations to find nodal values of the displacements (field variable)
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The Overall equilibrium equations have to be modified to account for the boundary conditions of the problem. After the incorporation of the boundary conditions, the equilibrium equations can be expressed as [K]_ = P. For the linear problems, the vector ‘-’ can be solved very easily but for non-linear problems, the solution has to be obtained in a sequence of steps, each step involving the modification of the stiffness matrix [K] and for the load vector ‘P’. Step 6: Computation of element strains & stresses from the known model displacements: If required, the element strains & stresses can be computed by using the necessary equations of solid or structural mechanics. In the steps the words indicated in brackets implements for the general FEM step by step procedure.
5.2 EXPLANATION OF FEM STEP BY STEP PROCEDURE: The steps involved in finite element analysis are stated earlier, general explanation of each step of the step by step procedure of FEM is given below. This description provides general outlook on bird’s eye view of FEM. DISCRETIZATION OF DOMAIN:
The discretization of domain of solution region into sub regions (finite elements) is the first step in the finite element. This is equivalent to replacing the domain having an infinite number of degrees of freedom by a system having finite number of degrees of freedom. The process of discretization is essentially an exercise of engineering judgment. The shapes, size number & configuration of the elements have to be chosen carefully such that the original body or domain is simulated as closely as possible without increasing the computation effort needed for the solution. BASIC ELEMENT SHAPES:
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For any given physical body we have to use engineering judgment in selecting appropriate elements for discretization. Mostly the choice of the type of the element is dictated by the geometry of the body and the number of independent spatial coordinates necessary to describe the system. Some of the popularly used are one, two & three dimensional elements. When the geometry, material, properties & parameters (like stress, displacement, pressure & temperature) can be described in terms of only one spatial coordinate, we can use one dimensional element. Although this element has a cross sectional area, it is generally schematically as a five segment. Using this type of elements the cross sectional area along the length may be varied. When the configuration and the details of the problem can be described in terms of two independent spatial co-ordinates, we can use the two dimensional elements. The basic element useful for two dimensional analyses is the triangular element. Although a quadrilateral for its special forms, rectangle & parallelogram elements can be obtained by assembling two or four triangular elements, in some cases the use of quadrilateral elements prove to be advantageous. If the geometry, material properties and other parameters of the body can be described by three spatial coordinates. We can idealize the body by using three dimensional elements. The basic three dimensional, analogous to the triangular elements in the case of two dimensional problem is the tetrahedron element. Some problems which are actually three dimensional can be described by only one or two independent coordinates. Such problems can be idealized by using an axissymmetric or ring type elements. The problem that posses axial symmetry like pistons, storage tanks, Valves, rocket nozzles & re-entry vehicle shield fall into this category. The present problem inner casing also comes under the same category. So in this problem the assume element for discritization is axisymmetric quadrilateral two dimensional element. For discritization of problems involving curved geometry, finite elements with curved side are useful. The ability to model curved boundaries has been made possible by the additional of middle nodes. Finite elements with straight lines are
44
known as linear elements, while those with curved sides are called higher order elements.
5.3 TYPE OF ELEMENTS: Often the type of elements to be used is evident from the physical problem itself for example if the problem involves the analysis of a truss structure under a given set of load conditions the type of elements to be used idealization is obviously the bar or line elements. However in some cases the type of elements to be used of idealization may not be apparent and in such cases one has to choose the type of elements judicially. In certain problems the given body cannot be represented as an assemblage of only one type of elements. In such cases, we may have to use two or more types of elements of idealization. 5.3.1 SIZE OF ELEMENTS:
The size of the elements influences the convergence of the solution directly & hence it has to be chosen with care. If the size of the element is small, the final solution is expected to be more accurate. However, we have to remember that the use of the elements of smaller size will also mean more computational time. Sometimes we may have to use elements of different sizes in the same body. The size concentration is expected compared to faraway places. In general, use a finer mesh in that region, another characteristic related element solution is the aspect ratio of the elements. The aspect ratio describes the shape of the elements in the assemblage of elements, for twodimensional elements aspect ratio is taken as the ratio of the largest dimension of the element to the smallest dimension elements with as aspect ratio of nearly unity generally yield best results. 5.3.2 LOCATION OF NODES:
If the body has no abrupt changes in geometry, material properties and external conditions (like load, temperature etc.,) the body can be divided into equal sub divisions and hence the spacing of the nodes can be uniform. On the other hand, if there are any
45
discontinuities in the problem nodes have to be introduced obviously at these discontinuities. 5.3.3 NUMBER OF ELEMENTS:
The number of elements to be chosen for idealization is related to the accuracy desired, size of elements and the number of degrees of freedom involved although an increase in number of elements generally mean more accurate results, for any given problem there will be certain number of elements reaches the point shown in the figure so significant improvement will be found. Moreover, since the use of larger number of elements involves larger number of degrees of freedom, we may not be able to store the resulting matrices in the available computer memory. 5.3.4 NODAL DEGREES OF FREEDOM:
The basic idea of FEA is to consider a body as composed of several elements which are connected at specified node points. The unknown solution or the field variable (like displacement, pressure and temperature) inside any finite element is assumed to be given by a simple function in terms of the nodal values of the element. The nodal displacement rotations necessary to specify the deformation of the finite element are the degrees of freedom of element. The nodal values of the solution, also known as nodal degree of freedom, are treated as unknowns in formulating the systems of overall equations, the solution of the system equation (like force equilibrium equations) gives the values of the unknown nodal degree of freedom. Once, the nodal degree of freedom are known, the solution with in the any element (and hence with in complete body) will also be known to us. For having the results in terms of nodal degrees of freedom the interpolation function must be derived in terms of nodal degree of freedom. 5.3.5 COORDINATE SYSTEM:
A local coordinate system is not that is defined for a particular element and necessary for the entire body of structure, the coordinate system for entire body is called as the “Global Coordinate system”. A natural coordinate system is a local coordinate system which permits a specification of a point within the element by a set 46
of dimension less numbers whose magnitude never exceeds unity. The derivation of element characteristic matrices and vectors involves the integration of the shape functions or their derivative or both over the element. These integrals can be evaluated easily if the interpolation functions are written in terms of local coordinate system. 5.4 FORMATION OF ELEMENT CHARACTERISTIC MATRICES AND VECTORS :
The characteristic matrices and characteristic vectors (also termed as vectors of nodal actions) of finite elements can be derived by using any of the following approaches: 5.4.1 DIRECT APPROACH:
In this method, direct physical reasoning is used to establish the element properties (Characteristic matrices and vectors) in terms of pertinent variables. 5.4.2 VARIATION APPROACH:
In this method, the finite element analysis interpolated as an approximate means for solving variation problems. Since physical and engineering problems can be readily applied for finding their appropriate solutions. The variational approach has been most widely used in the literature in formulating finite element equations. A major limitation in the method is that it requires the physical or engineering problem to be stated in variational form which may not be possible in all cases. 5.4.3 WEIGHTED RESIDUAL APPROACH:
In this method, the element matrices and vectors are directly form the governing differential equations of the problem without reliance on the variation statement of the problem, this method offers the most general procedure for deriving finite element equations and can be applied to almost all practical difference procedures can be used. 47
They are, a)
Collocation method.
b)
Sub domain collocation method.
c)
Galerkin method.
d)
Least squares method.
Assembly of Element Matrices and vectors and derivation of system equations 5.4.4 GENERAL APPLICATION OF THE METHOD:
Although the method has been extensively in use in the field of structural mechanics, it has been successfully applied to solve several other types of engineering problems like heat conduction, fluid dynamics, see page flow and electric and magnetic fields. The general applicability of the method prompted mathematicians to use these techniques for the solution of complicated boundary value and other problems. The general applicability of the finite element methods can be seen by observing the strong similarities that exist between various types of engineering problems. For illustration, let us consider the following phenomena 1.
One dimensional heat transfer
2.
One dimensional fluid flow
HIGHER ORDER AND ISOPARAMETRIC ELEMENT FORMULATIONS:
Whenever the interpolations polynomial is assumed to be of order two or more, the element is known as a “higher order element”. Thus a higher order element can be either a complex or a multiplex element. In higher order elements, some secondary (mid size and / or interior) nodes are introduced in addition to the primary (Corner) nodes in order to match the number of nodal degrees of freedom with the number of generalized coordinates in the interpolation polynomial.
48
In general, fewer higher order elements are needful to achieve the same degree of accuracy in the final results. Although it does not reduce the computational time, the reduction in the number of elements generally reduces the effort needed in the preparation of data cards and hence the chances of error in the input data. The higher order elements are especially useful in those cases where the gradient of the filed variable is expected to vary rapidly, In these cases the simplex elements, which approximate the gradient by a set of constant values, do not yield good results. The combinations of greater accuracy and a reduction in the data preparation effort have resulted in the widespread uses of higher order elements in practical applications. We shall consider some of the popularly used higher order elements, in the chapter, some special interpolation formulae were developed for specific applications. Problems involving curved boundaries cannot be modeled satisfactorily by using straight sided elements. The family of elements known as “iso parametric elements” has been developed for this purpose. The basic idea underlying the isoparametric elements is to use the same interpolation functions to define the element shape or geometry as well as the field variable within the element. To derive the isoparametric element equations; we first introduce a local a natural coordinate system for each element shape. Then the interpolation or shape functions will have to be expressed in terms of the natural co-ordinates. The representation of geometry in terms of (non linear) shape functions can be considered as a mapping procedure which transforms a regular shape like a straight sided triangle or rectangle in local coordinates system in to a distorted shape like a curved sided triangle or rectangle in the global Cartesian coordinate system. This concept can be used in representing in the problem with curved sided isoparametric elements. Today isoparametric elements are extensively used in three dimensional and shell analysis problems. In the later part of this chapter, we shall discuss the formulation of isoparametric elements. The aspects of numerical integration are essential for computation with isoparametric elements is also discussed towards the end of the chapter.
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5.5 LIMITATIONS OF FINITE ELEMENT METHODS: Finite element methods are extremely versatile and powerful and can enable designers to obtain information about the behavior of complicated structures with almost arbitrary loading. In spite of the significant advances that have been made in developing finite element packages, the results obtained must be carefully examined before they can be used. This point cannot be overemphasized . The most significant limitation of finite element methods is that the accuracy of the obtained solution is usually a function of the mesh resolution. Any regions of highly concentrated stress, such as around loading points and supports, must be carefully analyzed with the use of a sufficiently refined mesh. In addition, there are some problems which are inherently singular (the stresses are theoretically infinite). Special efforts must be made to analyze such problems. An additional concern for any user is that because current packages can solve so many sophisticated problems, there is a strong temptation to “solve” problems without doing the hard work of thinking through them and understanding the underlying mechanics and physical applications. Modern finite element packages are powerful tools that have become increasingly indispensable to mechanical design and analysis. However, they also make it easy for users to make big mistakes. Obtaining solutions with finite element methods often requires substantial amounts of computer and user time. Nevertheless, finite element packages have become increasingly indispensable to mechanical design and analysis.
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6. IMPLEMENTATIONS 6.1 INTRODUCTION: The finite element method is based on representation of a body by an assemblage of sub divisions called ‘Finite Elements’.
These elements are considered inter
connected at the joint which are called ‘Nodes’.
In order to approximate the
distribution of the actual displacements over each of finite elements, simple functions are chosen. Such assumed functions are called displacement functions. The unknown magnitudes of these displacement functions are the displacements at the nodes. The displacement model/functions can be expressed in various simple forms such as polynomials and trigonometric functions since polynomials offer ease in mathematical manipulations, they have been employed commonly in finite element applications.
6.2 THEORETICAL ANALYSIS 6.2.1 LOAD CASES:
The following different load cases considered for designing radome: Case (i) Water head pressure acting on Radome (due to under water)
Water head Pressure acting on radome (p) = ρgh ρ
= Density of sea water at average temperature of 3.880C with Salinity* of
34.78% = 10270 kg/m 3 h
= Water head (depth at which object is immersed)
= 300m (operational)
= 450m (design) p
= 1,027 x 9.81 x 450m = 45.337 bar
Case (ii) Pressure acting due to Radome platform traveling under water at speed of 25 knots(12.866m/s)
Pressure acting on radome (p) = C d ρ V2/2 Where Cd = 0.5 (for the given radome shape) ρ
= 1,027 Kg/m3 (water density)
V = 12.866 m/sec 51
p = 42500 kgf/m2 = 4.2 bar
Case (iii) Pressure acting due to wind speed @ 240 kmph (when object exposed to wind)
= Cd ρ V2/2
Wind Pressure acting on radome (p)
Where Cd = 0.5 (for the given radome shape) ρ
= 1.225 Kg/m3 (air density)
V = 66.66 m/sec p = 1360.83 kgf/m 2 = 0.1335 bar From the above three load cases; water head pressure acting on radome (due to under water) is predominant. Hence radome is designed to withstand static water head pressure of 45.337 bars. ( * The salt content of seawater is termed as its ‘salinity’ and measure of total quantity of all the dissolved substances in a sample of seawater.)
FE ANALYSIS: ANSYS Ver. XII is used for finite element Analysis of the radome. The geometric model is generated as per the drawing. The model is shown at Appendix. The radome is modeled as composite shell element (SHELL 91) suitable fixed constraints are applied. The FE model is shown at Appendix. The Water Pressure acting on the surface of the radome is calculated by the emperical formula and is applied on the whole surface of the radome. 16 layers of 0.75 mm are taken. Following assumptions are made to analyze the model.
1. Water pressure acting on the periphery of the radome. 2. Material properties taken for E. glass / Epoxy fiber reinforced plastic with fiber -orientation of O and 900 deg.
3. Mounting flange of radome of assumed rigid body.
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The following material properties are extracted from literature & applied to the model
Materials
Density (g/cm3)
Tensile Strength (MPa)
Young modulus (GPa)
E-Glass
2.55
2000
80
N (Poisson’s Ratio)
0.29
6.3 ANSYS: The ANSYS computer software is a large-scale multipurpose finite element method program that may be used for solving several classes of engineering problems. The analysis capabilities of ANSYS include the ability to solve static and dynamic structural analyses , steady state and transient problems, mode frequency and buckling Eigen value problems, static or time varying magnetic analyses and various types of field and coupled applications. The program contains many special features which allow non liberties or secondary effects tube included in the solution such as, plasticity, large strain, hyper elasticity, creep; swelling, large deflection contact stress stiffening temperature dependency, material anisotropy and radiation. As ANSYS was developed, other special capabilities such as, surface structuring, sub modeling, random vibration, piezo-electrics, coupled field analysis and design optimization was added to the program. These capabilities contribute further to make ANSYS a multipurpose analysis tool for varied engineering discipline. The ANSYS program has been in commercial use since 1970 and it is used extensively in the aerospace, automotive, construction, electronics, energy, service, manufacturing, nuclear , oil and steel industries. In addition, many consulting firms and hundreds of universities use ANSYS for analysis, research and educational use. 6.3.1 PROGRAM OVERVIEW:
The ANSYS element library contains more than 60 elements for static and dynamic analysis. Over 20 for the heat transform analysis and include numerous magnetic field and special purpose elements. These varieties of elements are analyzed in the ANSYS
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program as 3-D shells and non linear problems including contact (interfaces) and cables. Analysis of anything in ANSYS has to go through three main steps. They are Preprocessor Solution Postprocessor The inputs for an ANSYS analysis in prepared using preprocessor .The generation preprocessor contains powerful solid modeling and mesh generation capabilities, and is also used to define all other analysis data (geometry) properties like real constant , material properties , constraints ,load manipulation of analysis data. Parametric input, used files, macros and extension on line documentation and graphics capability are available throughout the ANSYS program including isoperimetric. Perspective section, edge a hidden line displays of 3-D structures. X-y graphic of input quantities and result and contour displays of solution results. A graphical user interface to guide new users through the learn 701-15 A static analysis calculation the effects of loads on the structure while ignoring the inertia and damping effects such as those caused by time varying loads, but it can accomplish steady inertia load and static equivalent loads. Static analysis is used to dart ermine. The displacements, stresses, strains and forces in the structures or component due to loads that do not induce significant inertia and damping effects steady loading and response conditions are assumed. The kinds of loading that can be applied in a static analysis include: •
Externally applied pressures and forces.
•
Steady state internal forces (such a gravitational or rotational velocity\Imposed (non-zero) displacement.
•
•
Temperature (for thermal strain) Fluencies (for nuclear swelling)
A static analysis can be either linear or non-linear. In our present work we are going to consider linear static analysis. The procedure for static analysis consists of three main steps: •
Building the model obtaining the solution 54
What is a working plane ?
Although your cursor appears as a point on your screen it can be represents a time through space, normal to the screen. In order to be able to pick a point with your cursor, you first need to define to imaginary plane that when intersected by normal line of your cursor, will yield a crippling point in space. This imaginary plane is called a working plane. Another work to think of the intersection between your cursor and your working plane is, picture your cursor as a point moves around on your working plane. Working plane, then acts as a “tablet” on which you write with your cursor. MODEL GENERATION:
The ultimate purpose of a F.E.A is to recreate mathematically the behavior of an actual engineering system. In other words, the analysis must be accurate mathematical model of a physical prototype. In the bookend this model comprises all the modes, elements, material properties, real constants, boundary conditions and other feature that are used to respective the physical system. The ANSYS program offers you the following approaches to model •
Creating a solid model
•
Using direct generation
•
Importing a model created in a computer aided design (CAD) system.
6.4MESHING: ADVANCED MESHING TECHNIQUES: Mesh generation refers to the generation of nodes and elemental connectivity. It also includes the automation numbering of nodes and elements based on a minimal amount of user supplied data. Mesh generation may be classified into •
Semi - Automatic
•
Fully – Automatic
Semi – automatic:-
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The models are sometimes referred to a “interactive mesh generation methods” to emphasize properly that they require the analysis interaction with the mesh generator to create the mesh. It can be divided into 2 groups: •
Wire – frame and surface based groups.
•
Solid modelling based group.
6.4.1 FULLY AUTOMATIC MESH GENERATION:-
The methods are primarily designed based on the solid modeling theory achieve full Automation and operate on solid models only a full automatic mesh generation can be invoked at the users level by using a command such as “mesh solid attitude. Where mesh solid is the mesh attributes and “d” is a digitize that identifies the solid to be meshed this implies at mesh automation limits user interaction to defining the solid and specifying mesh density parameters. 6.4.2 MESH GENERATION USING 2D ELEMENTS:
Majority of the available mesh generation techniques for 2D can be broadly classified into 6 distinct categories: 1. Topology decomposition approach: The geometry is first defined terms of
vertices and edges. It is decomposed into triangular elements by connecting the vertices. In the approach, there is no control on the element size and shape as it is decided by the geometry itself. 2. Node connection approach: The boundary of the geometry is defined and then
nodes are added on the boundary at suitable intervals. The interior nodes are generated to satisfy mesh penalty requirements. The nodes are then connected to form the element. Critical comparison of the available mesh generation methods and choosier the best out of them is a difficult task. Several mesh generation methods.
6.4.3 MESH GENERATION USING 3-D ELEMENTS:
A generated mesh M must satisfy to following requirements: Mesh should be topologically and geometrically correct: There should be no
intersecting elements, and the elements should be topologically correct.
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The quality of mesh should be as high as possible: M should contain as fel badly
element as pliable. The boundary nodes of M should be positioned exactly on the edges and faces of the model. Mesh should be boundary conforming: There should be no element intersecting the
boundary of the object, and there should be no holes in M in the limit of mesh refinement, M should match the geometric model exactly. With reference to the second requirement, The quality of a mesh is measured by how will the results of the analysis agree with an exact, analytical solution (assuming that the FE solution coverage to the exact solution). The Quality of the mesh depends upon its density and the shape the elements. For instance, it is well known that, for triangular elements. Obtuse angles degrade the accuracy of results. Thus, as few triangles possible with obtuse angles should be created. Further, the mesh density should be higher where the gradient of the function being approximated in the geometry is not simplified when a mesh is derived from a model. It has been shown has been shown that even minor simplification of the geometry can lead to large error, and therefore this is unacceptable. Mesh – generation methods for 3D models usually derive a mesh from a B-rep of the model. Most of these methods are either based on tetrahedrization algorithms for point sets, or on the cutting of elements from a B-rep
6.4.4 MESHING YOUR SOLID MODEL:
How to mesh your solid model:The procedure for generating a mesh of nodes and elements consist of 3 main steps
1. Set the element attributes 2. Set mesh controls (optional), Ansys offers a large no. of mesh controls, which you can choose from to suit your needs. 3. Generating the mesh. The second step, setting mesh controls, is not always necessary,
because the
default mesh controls are appropriate for many models. If no controls are used, the program will use the default settings to produce a free mesh as an alternative, we can 57
use the smart size feature to produce a better quality for mesh before meshing the model, and even before building the model. It is important to think about whether a free mapped mesh appropriate feature analysis. Free mesh (Automatic):
Coordinate In this type of generation the user does not have to specify in each node point and element consecutively i.e. no restrictions in terms of element shapes, are no specified pattern applied to it. This method of generation helps us Faso Fe meshing and needs only enclosed areas to generate elements. Mapped mesh (user defined):
The user manually controls the mesh generation like shape, size etc i.e. restricted on terms of the elements shape it contains and the pattern of the mesh. A mapped area contains either only contains either only quadrilateral or only triangular elements. While mapped volume only contains hexahedron elements. In addition a mapped mesh typically has a regular pattern, with obvious row of elements. If this type of mesh is desired, the user must build the geometry that has a series of fairly regular volumes and/or area.
6.5 APPROACH TO ANALYSIS: 6.5.1 STATIC ANALYSIS: ANSYS has been used for the finite element analysis of the radome. Linear static analysis is carried out to find out the structural response of the model.
Procedure of stress analysis of submarine radome in ANSYS: The following steps are taken in the analysis Pre processor:
Fileimportselect Radome IGES fileOK Preferences→ StructuralOK Element typeadd/edit/del add 1) Shell elastic 4node63OK
Real constantAdd/edit/delete add Shell thickness 3mm 58
Material propertiesmaterial modelstructurallinear elasticorthotropic
E1 (Young’s Modules in x-dir)
80Gpa
E2 (Young’s Modules in y-dir)
75Gpa
E3 (Young’s Modules in z-dir)
80Gpa
G x y (Shear modules in xy plane)
5Gpa
G x z (Shear modules in xz plane)
5Gpa
G yz (Shear modules in yz plane)
5Gpa
Nu x y (Poissons Ratio)
0.30
Nu y z (Poissons Ratio)
0.30
Nu z x (Poissons Ratio)
0.30
Density =2540ok Meshing
Mesh toolok Linessetselect all lineselement edge length=2ok Change mesh options to lines select linesclick meshok Setshellareasselect areasmeshok
Loads →Displacement →Apply → on nodes→ select the nodes →all DOF →click ok
Pressure On nodesselect the nodeok P=45.337x10^5 Pa Solution:
SolveCurrent LSok Solution is done General post processor:
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Plot results
counter
plot
Nodal solution
Deformation
USUM
deformed+ undeformed shape OK OK
Figure 6.1 Nodal solution def+ undeformed
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Figure 6.2 Nodal solution
Figure 6.3 Pressure
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Figure 6.4 Von misses stress
Figure 6.5 Graph SXY- displacement
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Figure 6.6 Vector plot predefined
Figure 6.7 Von misses graph
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6.6MODAL ANALYSIS: Modal Analysis is done to find the natural frequency of the radome. The natural frequency of the radome should be at least four times the excitation frequency so that resonance does not occur.
Procedure of Modal Analysis of Fuselage in ANSYS : The following steps are taken in the modal analysis: Pre processor:
Fileimportselect Radome IGES fileOK Preferences: structural Element type: 1) Shell elastic-4 node63 Real constant Add/edit/delete Shell thickness 3 mm
Material properties
material modelsstructural orthotropic
E1 (Young’s Modules in x-dir)
80Gpa
E2 (Young’s Modules in y-dir)
75Gpa
E3 (Young’s Modules in z-dir)
80Gpa
G x y (Shear modules in xy plane)
5Gpa
G x z (Shear modules in xz plane)
5Gpa
G yz (Shear modules in yz plane)
5Gpa
Nu x y (Poissons Ratio)
0.30
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Nu y z (Poissons Ratio)
0.30
Nu z x (Poissons Ratio)
0.30
Select Density =2540kg/m3 Sections:
Meshing Mesh attributes Select SHELLselect areasOK, Constraining the model Load DisplacementApplyOn nodesall DOF on radome base Solution:
Analysis Type –Modal Analysis optionsselect Block lanczosok Frequency-0-10000 No. of modes to extract=5 ok Solution SolveCurrent LSok Solution is done General post processor:
Read results by pick select modeok Plot resultsCounter plot Nodal solutionDeformation in USUMOK
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Figure6.8 Modal 1
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Figure6.8 Modal 2
Figure6.9 Modal 3
67
Figure6.10 Modal 4
Figure6.11 Modal 5
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Figure6.12 Modal analysis
7.FABRICATION METHODS FOR RADOMES The selection of a manufacturing method for a given Radome design may be based on a number of factors including the Radome performance requirements and the materials of construction. For example selection of a fabrication method for a Radome often starts by the consideration of Vacuum bag or Autoclave molding using glass fabric reinforcement. Frequency requirements for maintaining uniform electrical properties in the Radome wall might eliminate the less expensive fabrication methods and dictate a filament winding approach whereby this control is more readily accomplished.
7.1 FILAMENT WINDING: A major advantage of the filament winding process is that it lends itself to automated equipment. Even more important advantage is that it allows very close control of the resin to glass ratio, which results in a uniform dielectric constant throughout the radome. The ability to produce on a repeatable basis a radome wall of 69
known dielectric constant makes it possible to machine or grind the radome wall to a given physical dimension thereby eliminating in many cases, the necessity for measurement of electrical wall thickness during the final grinding operation. Also the electrical testing and correction time required for the Radome is reduced when a uniform electrical wall is present. In addition, the filament winding process allows the orientation of he fibers in the primary directions of load, thereby providing structural design flexibility not possible with fabric reinforcements. The glass reinforcement plastics normally exhibit dielectric constants of the order of 3.5 to 4.5 at X-band frequency.
7.2 VACUUM BAG MOULDING: Vacuum bag molding “wet lay-up” of glass reinforced plastic radomes is one of the earliest techniques employed. This technique involves laying down dry glass fabric, which is wet with the liquid resin during the lay-up operation. After the desired thickness has been obtained, a plastic film bag is placed over the lay-up, sealed to the mould and connected to the vacuum source, which evacuates the air between the plastic bag and the lay-up. The major advantages of this fabrication process are its relatively low cost and high quality laminate, which can be produced by skilled workers. The removal of excessive resin and air from the aly-up is performed by squeezing or wiping operation using a rubber soft plastic tool. This squeeze operation not the vacuum bag pressure, determines the final thickness and resin content at laminate.
7.3 AUTOCLAVE MOULDING : The Autoclave molding is similar to vacuum bag molding in that the layup is sealed in plastic bag, which is evacuated by a vacuum pump prior to application of the autoclave pressure. Autoclave molding of Radome is normally used with pre-preg materials, which do not allow squeezing to remove entrapped air, and with resin systems, which generate reaction products during cure. Unlike the vacuum bag process, the pre-preg lay-up is normally followed with a perforated plastic film or a glass fabric which been treated to prevent adhesion of the resin. This apparatus is followed by a lay70
up dry bleeder material such as glass or other type fabrics which absorb the excess resin or reaction products or both which are eliminated from the part during the cure. Most Autoclave used in the fiberglass plastic industry have operating pressures between 100 and 200 psi and temperature capabilities upward to 500 oF resin systems such as diallylphthalate and most epoxies may be adequately. Systems such as silicones, phenolics, polyamides and polybenzidazoles are frequently molded at pressures of the order of 200 psi. The higher pressure normally yields superior composites, provided a more reliable manufacturing process and assures greater reproducibility from part to part.
7.4 MATCHED DIE MOULDING: The matched die molding involves the use of male and female dies and offers the advantage of yielding a part having near finished dimension. While this method has been used for producing reinforced plastics radomes, its use for the production of the large manned aircraft radomes has been limited. The major factor which limits the use of this fabrication method is large size normally associated with aircraft radomes and the resulting cost involved in building precision dies and the large high capacity presses required to mould a part of this size. The fabrication technique involves fitting knitted glass socks over a male mould. As many as 25 glass socks may be used to achieve fiberglass content of the finished Radome. After the socks have been fixed in place, the female mould is lowered onto the male mould, located and fixed in place. The cavity between the male and female mould containing the glass socks is evacuated by use of vacuum pumps to a vacuum pressure of approximately 25" of mercury and epoxy resin is pumped into the cavity under pressure of approximately 40 psi. The resin filling operation is reported to take approximately 4 hrs. After the cavity has been completely filled with resin, the mould temperature is raised to 150 oF for 16 hrs to affect the cure. The mould is then coded and the part is removed for the post cure.
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8. RESULTS & DISCUSSIONS 1.
Displacement contours and deformed shape of the radome enclosed in figure6.1. The maximum resultant deformation of 0.2547mm is observed at the Node No. 409, which is the top most point of the radome.
2.
Stress contours S xy enclosed in figure6.5. From Stress contours found the maximum Sxy value is 70.398 MPa(Compressive) for layer no.1. The stresses induced due to water head pressure are within the safe limits.
3.
The modal analysis values are taken from the analysis at sub step 5 we have a frequency of 5.88 cycles / sec and a deflection of 1.763 mm.
4.
The natural frequency of the submarine is 5-33 cycles /sec. Hence the obtained value is about 4 times more than natural frequency of the submarine hence it is safe.
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9. CONCLUSIONS & FUTURE WORK 1.
The experimental test results and theoretical values are in close agreement with each other.
2.
Deformation and stress values obtained from FE analysis are within the safe limits.
3.
Conducting pressure test on radome verified the design aspects and validated the FE analysis.
4.
To improve the electrical performance of the radome without compromising the mechanical properties Hybrid composites to be considered in futuristic radome development.
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REFERENCES Dr. Gates PJ & Lynn NM “ Ships, Submarines & the Sea” Vol.2, Brassey’s (UK), 1990. Bryan Harris “Engineering Composite Materials” 2nd edition, 1999. Sun
CT
“Strength Analysis of Unidirectional
Composite Laminates”
Comprehensive Composite Materials, Vol.1, Elsevier, 2000. Robert M Jones “Mechanics of Composite Materials” Mc Graw-Hill Book Company, 1975. Timoshenko “Theory of Plates & Shells”.
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