Chapter 1INTRODUCTIONA composite is a structural material that consists of two or more combined constituents that are combined at a macroscopic level and are not soluble in each other. One constituent is called reinforcing phase and the one in which it is embedded is called the matrix. The reinforcing phase material may be in the form of fibers, particles, or flakes. The matrix phase materials are generally continuous. In this form, both fibers and matrix retain their physical and chemical identities, yet they produce a combination of properties that cannot be achieved with either of the constituents acting alone. In general, fibers are the principal load carrying members, while the surrounding matrix keeps them, and protects them from environmental damages due to elevated temperatures and humidity.

The composite materials have the following properties:1. High specific strength

2. High specific stiffness

3. More thermal stability

4. More corrosion and wear resistance

5. High fatigue life

Fibers of FRC

The primary function of the fibers is to carry the loads along their longitudinal directions. Common fiber reinforcing agents include

Aluminum, Aluminum oxide, Aluminum silica

Asbestos

Beryllium, Beryllium carbide, Beryllium oxide

Carbon (Graphite)

Glass (E-glass, S-glass, D-glass)

Molybdenum

Polyamide (Aromatic polyamide, Aramid), e.g., Kevlar 29 and Kevlar 49

Polyester

Quartz (Fused silica)

Steel

Tantalum

Titanium Tungsten Tungsten mono carbideMatrix of FRC (FIBRE REINFORCED COMPOSITES) The primary functions of the matrix are to transfer stresses between the reinforcing fibers (hold fibers together) and protect the fibers from mechanical and/or environmental damages. A basic requirement for a matrix material is that its strain at break must be larger than the fibers it is holding.

Most matrices are made of resins for their wide variation in properties and relatively low cost. Common resin materials include

Resin Matrix

Epoxy

Phenolic

Polyester

Polyurethane

Vinyl Ester

Among these resin materials, polyesters are the most widely used. Epoxies, which have higher adhesion and less shrinkage than polyesters, come in second for their higher costs. Although less common, non-resin matrices (mostly metals) can still be found in applications requiring higher performance at elevated temperatures, especially in the defense industry.

1.1 DESIGN CONSIDERATION FOR FRP:FRP (Fibre Reinforced Plastic) is used in designs because the strength and modulus of elasticity are superior to non-reinforced plastics and also other material choices. Presently polymer composites are becoming competitive to conventional metallic materials in the aspects of properties and economics. Being unidirectional FRP materials are orthotropic in nature the significance of orientation sequence contributes critically in the optimization process. This unique characteristic provides a large scope for the designer to play with the orientation sequence of stacking to arrive at desired elastic properties which intern lead to select best orientation sequences in connection with the specific applications. Orientating the direction of fibers either, unidirectional, 2-dimensionally, or 3-dimensionally during production affects the degree of strength, flexibility, and elasticity of the final product.

Fibers orientate in the direction of forces display greater resistance to distortion from these forces and vice versa, thus areas of a product that must withstand forces will be reinforced with fibers in the same direction, and areas that require flexibility, such as natural hinges, will use fibers in a perpendicular direction to forces. Using more dimensions avoids this either or scenario and creates objects that seek to avoid any specific weak points due to the unidirectional orientation of fibers. The properties of strength, flexibility and elasticity can also be magnified or diminished through the geometric shape and design of the final product. These include such design consideration such as ensuring proper wall thickness and creating multifunctional geometric shapes that can be molding as single pieces, creating shapes that have more material and structural integrity by reducing joints, connections, and hardware. General forms of composites are shown below,

Fig 1.1 distribution of fibres in a matrix

Fig 1.2 : Relation between strength and arrangement of reinforcement

The modern composite material is a system composed of two or more dissimilar materials, differing in forms, and insoluble in each other, physically distinct and chemically inhomogeneous. The resulting products properties are much different from the properties of constituent materials. One of the materials is called reinforcement, in the form of fibre, woven fabric sheets, or particles, embedded in the other materials called matrix. Composites are used because the overall properties of the composite are superior to those of the individual components.1.2. ADVANTAGES OF COMPOSITES High resistance to fatigue and corrosion degradation.

High strength or stiffness to weight ratio. As enumerated above, weight savings are significant ranging from 25-45% of the weight of conventional metallic designs.

Due to greater reliability, there are fewer inspections and structural repairs.

Directional tailoring capabilities to meet the design requirements. The fibre pattern can be laid in a manner that will tailor the structure to efficiently sustain the applied loads.

Fibre to fibre redundant load path.

Improved dent resistance is normally achieved. Composite panels do not sustain damage as easily as thin gauge sheet metals.

It is easier to achieve smooth aerodynamic profiles for drag reduction. Complex double curvature parts with a smooth surface finish can be made in one manufacturing operation.

Composites offer improved torsional stiffness. This implies high whirling speeds, reduced number of intermediate bearings and supporting structural elements. The overall part count and manufacturing & assembly costs are thus reduced.

High resistance to impact damage.

Thermoplastics have rapid process cycles, making them attractive for high volume commercial applications that traditionally have been the domain of sheet metals. Moreover, thermoplastics can also be reformed.

Like metals, thermoplastics have indefinite shelf life.

Composites are dimensionally stable i.e. they have low thermal conductivity and low coefficient of thermal expansion. Composite materials can be tailored to comply with a broad range of thermal expansion design requirements and to minimize thermal stresses.

Manufacture and assembly are simplified because of part integration (joint/fastener reduction) thereby reducing cost.

The improved weather ability of composites in a marine environment as well as their corrosion resistance and durability reduce the down time for maintenance.

Close tolerances can be achieved without machining.

Material is reduced because composite parts and structures are frequently built to shape rather than machined to the required configuration, as is common with metals.

Excellent heat sink properties of composites, especially Carbon-Carbon, combined with their lightweight have extended their use for aircraft brakes.

Improved friction and wear properties.

The ability to tailor the basic material properties of a Laminate has allowed new approaches to the design of aero elastic flight structures.

The role of engineering materials in the development of modern technology need not be emphasized. It is the materials through which a designer puts forward his ideas into practice. We use a variety of materials for our needs and comfort and have been developing new materials for meeting our technological requirement. As the levels of technology have become more and more sophisticated, the materials used also have to be correspondingly made more efficient and effective. Several performance characteristics are expected from these materials. They are:

The materials to be used for sophisticated applications like aircraft and space applications should have higher performance, efficiency and reliability

Materials have to be of light-weight for many applications so that the resulting products can be efficient and cost effective.

Materials must have combinations of properties for specific uses since present day products of modern technological origins operate in environment that are special or extreme like very high temperature (of order of 2500 0 K), cryogenic condition, vacuum (as in space), high hydrostatic pressure (as in deep sea).

The conventional material may not always be capable of meeting the demand of such environments. Hence new materials being created for meeting these performance requirements and composite materials form one class of such materials developed.

1.3 Failure modes:

Structural failure can occur in FRP materials when: Tensile forces stretch the matrix more then the fibers, causing the material to shear at the interface between matrix and fibers.

Tensile forces near the end of the fibers exceed the tolerances of the matrix, separating the fibers from the matrix.

Tensile forces can also exceed the tolerances of the fibers causing the fibers themselves to fracture leading to material failure1.4 Reasons Why Sandwich (Polyurethane Foam) is Selected For Certain Applications:

Sandwich structures have been widely used for many years in applications such as aircraft panels, marine-craft hulls, racing car bodies and spacecraft solar arrays. The combination of high strength-to-weight and high stiffness-to-weight ratios, which are extremely important design parameters in many applications, makes the use of sandwich structures a highly competitive design option that provides for very high structural performance, which is often paired with multifunctional capabilities. Examples of areas where sandwich structures are being used for high-performance purposes include aerospace, automotive, marine, wind turbine and many other industries.

Substituting parts of the original cores by core materials of a higher density (or other suitable materials) usually solves the problem of local reinforcement of sandwich structures for rigging, joining or supporting purposes. However, this at the same time introduces new interfaces (boundaries) between materials of rather different elastic properties in the same sandwich core.

Fig 1.3 : Polyurethane Foam

A major purpose of the core insert is to facilitate the transmission of the direct and shear stresses through the thickness of the sandwich element without impairment the structural integrity of the whole sandwich. However, any inclusions in a sandwich panel bring about material discontinuities, which results inevitably in local effects in the vicinity of the discontinuities. These local effects manifest themselves in a local rise of stresses in all components of the sandwich panel, and these local stresses may exceed the allowable stresses in the core, insert and faces. The flexibility of composite sandwich construction allows innovative structural developments from this material. This composite material can also be combined with traditional construction materials or be shaped and formed to carry loads that cannot be carried by the individual sandwich structure. In addition, sandwich structure can be designed with the desired stiffness and strength with no additional weight to suit various structural applications.The evolution of a composite sandwich structure with lightweight, high strength core material and with good holding capacity for mechanical connections provides an opportunity to develop this material for structural beam applications. The concept of sandwich panels provides an efficient structural system suitable for a variety of applications, including floor and roof panels, pedestrian bridge decks and cladding walls for buildings. Typically, a low density core, which could also have excellent insulation characteristics, is sandwiched between high strength and stiffness thin skins bonded to the core. The system provides very high strength- and stiffness-to-weight ratios. This concept was introduced in the early 20th century in aircraft, automobile and ship industries. Sandwich panels with metal, and then FRP, skins have been investigated experimentally and analytically by several researchers over the years among many others.1.5 Reasons Why Composites Are Selected For Certain Applications:

High strength to weight ratio (low density, high tensile strength)

High creep resistance

High tensile strength at elevated temperatures

High toughness

Good rating for fatigue

The increased use of advanced structural materials may have significant impacts on basic manufacturing industries. The automotive industry provides an excellent example, since it is widely viewed as being the industry in which the greatest volume of advanced composite materials, particularly polymer matrix composites (PMCs), will be used in the future. Motivations for using PMCs include weight reduction for better fuel efficiency, improved ride quality, and corrosion resistance. Extensive use of composites in automobile body structures would have important impacts on methods of fabrication. The application of advanced materials to automotive structures will require:

1) Clear evidence of the performance capabilities of the PMC structures, including long-term effects;

2) The development of high-speed, reliable manufacturing and assembly processes with associated quality control;

3) Evidence of economic incentives (which will be sensitively dependent on the manufacturing processes).

The three performance criteria applicable to a new material for use in automotive structural applications are fatigue (durability), energy absorption (in a crash), and ride quality in terms of noise, vibration, and harshness (generally related to material stiffness). There is emerging evidence from both fundamental research data and field experience that glass fiber-reinforced PMCs can be designed to fulfill these criteria. At present, the successful application of PMCs to automobile body structures is more dependent on quick, low-cost processing methods and materials than it is on performance characteristics.

CARBON FIBER-REINFORCED SIC MATRIX COMPOSITES

In the last 20 years a great development has occurred in impact machines. The development of reliable mini-transducers has allowed researchers to apply to pendulums and drop weight instruments the same concepts widely applied in the past to static loading machines. With oscilloscopes in the past and by personal computers these days it is possible to measure the force transferred by a striker to a specimen even at high loading rates.

This revolution has provided researchers, industries with a new way to characterize material properties during impact. The minimum knowledge required about a material to characterize fracture properties comes from a force-time (or force-displacement) diagram.

When performing a test with an instrumented falling weight, it is possible to record the force acting on the specimen throughout the impact.

The force-deformation curve on ceramic materials at a certain loading rate provides an explanation of several physical phenomena. The Fractovis instrumented impact tester manufactured by CEAST S.p.A, Italy, which is shown in Fig:1.4 is designed to perform tests on specimens of different materials (ceramic composites) under a wide range of variable conditions: energy, temperature, impact velocity.

The Fractovis impact testing machine provides full details of the impact event, from the initial contact to the final failure of the specimen, and enables monitoring the relative force-deformation curve on its data acquisition system DAS 8000 WIN.

Fig: 1.4 Factories Instrumented Impact Tester

The new DAS 8000 WIN is CEASTs innovative Data Acquisition System for instrumented impact tests. The DAS 8000 is designed to record data during rapid events such as impact tests and to control some instruments prior to and during test execution. Silicon carbide matrix based composites exhibit promising thermal / mechanical properties at high temperatures and offer very good oxidation and thermal shock resistance.They are finding increasing applications in aerospace, defence and industries. Carbon fiber-reinforced SiC matrix composites are preferred to C-C composites for oxidizing and highly erosive environment. C-SiC composites are used up to15000 C for long durations and up to 20000C for short durations.The main applications of C-SiC composites are: nose tips of reusable space vehicles, leading edges of hypersonic vehicles, erosion resistant jet vanes for thrust vectoring of rocket motors and wear resistant brake materials for high speed automobiles. The mechanical and thermal properties of the fiber-reinforced composites can be tailored by adjusting fiber volume fraction and fiber orientation to meet the needs of the application.C-SiC composites retain mechanical strength up to 17000C and have a high thermal conductivity and low thermal expansion and hence excellent thermal shock resistance. There are several methods to fabricate C-SiC composites, such as chemical vapor infiltration (CVI), slurry infiltration combined with hot pressing, polymer-infiltration-pyrolysis (PIP), etc.

Among these methods, Liquid Silicon Infiltration (LSI) process offers many potential advantages such as single step process, low processing temperature, and near-net-shape processing. Continuous fiber reinforced ceramic matrix composites (CFCCs) are very interesting structural materials because of their higher performance and higher fracture toughness.

For this reason, CFCCs are considered as the most potential to be used in advanced aero-engines. Among these CFCCs, carbon fiber reinforced silicon carbide composites (C-SiC) are most promising and Have been receiving considerable interest. Many investigations have been conducted on two dimensional woven C-SiC composite materials.

Recently, attention has been focused on three dimensional woven or braided ceramic matrix composite materials in order to meet mechanical and thermal properties requirements along the thickness of the composites. Despite the attractiveness of fiber-reinforced composites as engineering components, they are not currently being applied to the extent that they could be.

Even when they have been employed, relatively low stress applications and large safety factors are usually considered. The main reason is the difficulty and uncertainty that exists in determining their failure strength, fracture toughness, operating lifetime in severe conditions, because the nature of the deformation and failure behavior of the composites are very complicated.

Continuous carbon fiber reinforced silicon carbide ceramic matrix composites (C-SiC) are promising candidates for many applications, particularly as aerospace and aircraft thermo structural components. Nevertheless, a critical drawback of the C-SiC is the poor oxidation resistance which limits long term applications of this composite in high temperature oxidizing environments.

The quality control and assurance of C-SiC composites has always been a challenge due to their anisotropy and in homogeneity. Although Ceramic Matrix Composites (CMCs) are more damage tolerant than monolithic ceramics, they are sensible to defects such as delaminations, cracks or material conglomerations within the ceramic matrix.

When manufacturing CMCs by the LSI process, it can locally occur that the carbon matrix does not react to form SiC during the infiltration of the liquid silicon. Such defects can occur uniquely or concurrently, demanding intensive non-destructive analysis methods. Any of the mentioned defects can produce a critical flaw distribution, resulting in the failure of the component. Therefore, these defects need to be detected as early as possible during the fabrication process as well as during service.

Fig: 1.5 Heterogeneous structure of C-SiC

These defects are often complex and distributed within the material.For a consistent quality control, an analysis during each stage is required.C-SiC composites by means of liquid silicon infiltration have also been developed for large space optics. The liquid silicon infiltration process is a simple, fast and low-cost manufacturing process for structural components. Low shrinkage during the silicon infiltration into C-C substrates allows machining of C-C substrates to the final component shape.The mechanical and thermo mechanical behavior and joining technology of C-SiC composite have been sufficient to fabricate a large scaled optical mirror whose aperture diameter is more than 3 meters. C-SiC composites address the brittleness, difficulty of manufacture and poor handlability of neat SiC ceramics.

The features of C-SiC composites are:

Low density

High stiffness

High bending strength

Low coefficient of thermal expansion

High coefficient of thermal conductivity

No porosity

Simple manufacturing process, and fast and low cost machining

Easy milling of C-C green body

Short manufacturing time

High fracture toughness, good handling ability

Considerable flexibility in structural design

Ultra-lightweight capability (small wall thickness) Fig: 1.5 shows a typical photomicrograph of a conventional C-SiC composite. The black area shows carbon fiber, white area shows free Silicon and gray area shows SiC. The carbon fibers exhibit significant reaction with hot liquid silicon. In the micrograph, only a small amount of residual carbon fiber is observed.

Considerable free silicon is also observed. Reduced numbers of reinforcing fibers and an abundance of free silicon is expected to decrease the strength of the composite. When sufficient silicon carbide

matrix is obtained and the amount of the free silicon is minimized, the composites should have more strength and toughness.

Fig: 1.6 Micrograph of a conventional C-SiC composite

In this study, a new C-C green body is introduced to control reaction in the silicon infiltration process. The process conditions including carbonization, graphitization, and liquid silicon infiltration (or siliconization) are also optimized to improve composite properties.

Fig: 1.6 shows the fabrication process of C-SiC composites.

The process is composed of 4 steps:

1) Preparation of carbon fiber preform: Carbon fiber preform is composed of pitch-based milled fibers and binder. Carbon fiber and binder are mixed uniformly then loaded into a metal mold and pressurized. Carbon fibers are oriented randomly in the preform.

2) C-C Process: The carbon fiber preform is impregnated with the coal tar pitch, which is a carbon matrix precursor. Then the preform is carbonized and graphitized in an inert atmosphere.3) Milling: In this process, the C-C green body is machined into end product geometry with a NC machine. 4) Liquid silicon infiltration: Liquid silicon is infiltrated into the C-C substrate and reacted with the carbon matrix to form silicon carbide. In this study, the volume fraction of carbon fiber in the preform is more than 30%. The carbonization temperature in an inert atmosphere is not less than 2000 degrees Centigrade. Liquid silicon is infiltrated under vacuum at not less than 1600 degrees Centigrade.

Fig: 1.7 Fabrication process of C SiC composites

However, these ceramics are brittle because of covalent and ionic bonding that is characteristic of this class of materials. No or very little yield can occur as a result of strong bonding which results in large stress concentrations to develop at a crack tip and cause the crack tip to propagate with little expended energy. This results in material with low fracture toughness.1.2 LSI CompositesLiquid silicon infiltrated (LSI) carbon-carbon composites provide a potentially attractive construction material for high-temperature heat exchangers, piping, pumps, and vessels for nuclear applications, due to their ability to maintain nearly full mechanical strength to high temperatures (up to 14000C), the simplicity of their fabrication, their low residual porosity, and their low cost.

LSI composites are fabricated from low-modulus carbon fiber that can be purchased in bulk at lower costs. LSI C-SiC materials have desirable hightemperature properties.

Low specific density (2.6 2.7 g/cm3)

Tunable stiffness (240-260 GPa) and strength (50-210 MPa)

Low coefficient of thermal expansion (200C-10000C:1.8-4.1x10-6 K-1)

High thermal conductivity and diffusion (~20 135 W/mK)

Better properties, particularly fracture toughness, can be obtained with carbon

SiC fiber composites with chemical vapor infiltration and other methods of fabrication. LSI composites also are not expected to perform well under neutron irradiation.However, the low cost and simple fabrication and joining methods that can be performed with LSI C-SiC composites, combined with the excellent high temperature properties, make them interesting candidates for heat exchanger, centrifugal pump, and other flow-loop components.Silicon carbide ceramics have wide applications in various industrial fields because of their excellent high-temperature strength and modulus, low density, good oxidation resistance and high hardness etc. However, like all ceramic materials, they are generally notch-sensitive and low in toughness; hence they are unreliable as structural materials.

Continuous fiber reinforced SiC composites have been demonstrated to be the most effective way for improving toughness. Generally, there are several methods to fabricate fiber reinforced SiC composites, such as chemical vapor infiltration (CVI), slurry infiltration combined with hot-pressing and polymer-impregnation etc. The notable feature of CVI is that the process can be conducted at about 11000C, much lower than the sintering temperature of SiC.

Moreover, it can be applied to complex shapes with near-net-shape feature. However, the CVI process takes a very long time for densification of the composites. Fiber reinforced SiC composites can also be fabricated through slurry infiltration. Fibers have been infiltrated with a slurry of SiC and then hot-pressed at high temperature.

This process is limited to simple shapes, and the samples have to be hot-pressed at very high temperatures near 19000C, which causes degradation of reinforcements. One other method to prepare continuous fiber reinforced ceramic composites is to use a preceramic polymer, such as polycarbosilane, which is frequently used to fabricate ceramic fiber or film.

This method is gaining increasing attention in recent years because of its low processing temperature and good shaping features. The preceramic polymer liquid with low viscosity, which can be obtained by dissolving in organic solvent or melting at elevated temperature, can be easily infiltrated into the fiber preform by using vacuum impregnation or pressure impregnation.

It is well demonstrated that the interface between fiber and matrix plays a key role in translating the mechanical properties of reinforcements to the mechanical properties of the ceramic matrix composites. Weak interfaces are preferred in order to obtain high-performance fiber reinforced ceramic composites which have high toughness and high strength.

Possible interface bonding in composites includes interdiffusion, chemical bonding, reaction bonding and mechanical bonding, etc. which strongly depend on processing conditions and the structure

and properties of matrix and fibers, especially the surface structures of fibers.

In designing fiber reinforced ceramic materials, fibers, matrix and processing conditions should be carefully considered to diminish or avoid chemical bonding, reaction bonding and interdiffusion bonding. However, the mechanical bonding, which depends on the surface roughness of fibers and residual stresses in composites, cannot be avoided.

In many cases, in order to reduce the interfacial bonding strength in ceramic matrix composites, carbon or BN is frequently coated on the surface of fibers. In carbon fiber reinforced SiC composites (C-SiC), it is possible to tailor the interface by controlling the structure and surface structure of carbon fibers.There are a variety of commercially available carbon fibers from PAN-based high strength carbon fibers to pitch-based high modulus carbon fibers which show very different structures. However, these carbon fibers are produced mainly for resin matrix composites in which strong interfacial bonding is preferred.

Therefore, it can be expected that these carbon fibers may show different behavior in ceramic matrix composites. There are still relatively few reports about the effects of different types of carbon fibers on the mechanical properties and fracture behavior of C-SiC prepared from preceramic polymers.

Carbon fiber reinforced silicon carbide composite (C-SiC) is a kind of promising thermal structural composite for use in applications requiring high strength, low density, and high fracture toughness at elevated temperatures in aero-engines and aerospace.

However, a severe problem is that the carbon fiber and interfacial layer of the composite are easy to be attacked by oxygen in air at the temperatures as low as 3700C. The main reason is that there are many micro cracks on the silicon carbide matrix resulted from the mismatch of thermal expansion coefficients between carbon fiber and silicon carbide matrix. The oxidation has a significant effect on the weakening and mechanical properties of the materials.1.2. INTRODUCTION TO FEA

Engineers today face increasingly difficult challenges to contend in rapidly changing global market-to-market products in better quality at lowest cost possible, so that the product has a good market in competition. To achieve these goals, one of the powerful tools available for the designer is computer aided finite element analysis.

Finite element Analysis is a powerful numerical technique for analysis .FEA is used for stress analysis in that area of solid mechanics. The basic concept of finite element method is that a body / structure may be divided in to smaller elements called finite elements. The properties of the element are formulated and combined to obtain the solution for the entire body or structure. For a given practical design problem the engineer has to idealize the physical system into a FE model with proper boundary conditions and loads that are acting on the system. Then the discretization of a given body or structure into cells of finite elements is performed and the mathematical model is analyzed for every element and then for complete structure. The various unknown parameters are computed by using known parameters.

1.2.1 FEM in Retrospect:With the advancement of computer technology and CAD systems, complex problems can be modeled with relative ease. Several alternative configurations can be tried out before fabricating the initial prototype. By using the FEM an approximate behavior of the continuum can be determined which will significantly facilitate for superior design.

FEM originated as a method of stress analysis, but today it is used to solve problems of heat transfer, fluid flow, lubrication, magnetic and electric field, fractures mechanics and in many other engineering fields. Abundant software packages based on FEM and FEA have been developed such as SAP, ANSYS, STADD and STRUDAL. The analysis package ANSYS is like an encyclopedia of finite element packages. ANSYS has got almost all types of elements with many fields of mechanics.

1.2.2 Finite Element Procedure:

The FE procedure can be broadly classified into

Pre processing

Processing (solution)

Post processing

1.2.3 Pre-Processing:

Pre processing consists of model generation and discretization into finite elements. These steps involved are appended below.

Select a suitable element for analysis.

Define material properties such as Youngs Modulus, Poissons ratio etc.

Prepare the sketch of continuum to be analyzed.

Mesh generation i.e. dividing the geometry into a number of suitable fine elements, which are interconnected at the nodes.

1.2.4 Processing (solution):After the model is built in pre processing phase, the solution to the analysis is obtained in the processing phase. The analysis type indicates to the processor the governing equation to be used to solve the problems; the general categories available include structural, thermal, electromagnetic, computational fluid dynamics etc. Each category can include several specific analysis types, such as static or dynamic analysis. Processing requires no user interface. All analysis types are based on classical engineering concepts. These concepts can be formulated into matrix forms that are suitable for analysis using FEM. It calculates transformation matrices, maps elements into global system, and assembles the elements. Boundary conditions are introduced and solution procedures are performed.Most of the mechanical engineers are familiar with the structural static analysis. It is used to determine the displacements, stresses, strains and forces that occur in the continuum as a result of applied loads.

The governing equation for static analysis is:

[K] [Q] = [F]

Where,

[K] = Structural stiffness matrix

[Q] = Nodal displacement vector

[F] = Loads applied include concentric, thermal etc.

In the processing phase we really end up with governing equations for each element. By solving each equation at each node we obtain the degrees of freedom, which would give the approximate behavior of complete model.

1.2.5 Post Processing:

Post processing deals with the results such as deformed configuration, shapes, and stress distribution, temperature.

Any post processor displays graphically the results in the following modes

Displacement of shapes deformed and undeformed mesh is displayed.

Contours A display of scalars like temperature distribution.

Animation Time dependent or harmonic results are portrayed vividly.

Auto generation-results are presented as charts, tables graphs etc.

The major job of post processor is to present results in an easy way to understand. Pictorial representation interactive graphics is the best. This aids in determining basic trends and then concentrates on critical areas.

1.2.6 Advantages of FEM:

This method can be efficiently applied to cater irregular geometry.

It can take care of any type of boundary conditions.

Material anisotropy and in homogeneity can be treated without much difficulty.

Optimization of design can be done with ease.

1.2.7 Disadvantages of FEM:

To solve problem the approximations used do not provide accurate results.

Stress value may vary from fine mesh to average mesh analysis.

1.3 MESHING AND ANALYSIS OF SPOILER:

1.3.1 Mesh Generation

For meshing component, element selection is important. The element has specifications of nodes having degrees of freedom at each node, 1.3.2 Quality criteria :

Warpage

If one node of the plane element deviates from the plane then the angle between node and the plane is called warpage angle

Twisting of either 2D or 3D element is also called warpage angle

This angle should not be more than 5 degrees Warpage angle

Aspect ratio

It is defined as the ratio of maximum length of the element to the minimum length of the element

It should not be more than 5

Skew angle

It is defined as the angle between two altitudes of the vertices in a triangle element

Should not more than 60 degrees

Chordal deviation

It defined as the deviation of the finite element model from actual geometric model

Should not be more than 0.1

Internal angles

Perfect square element is an ideal element

Minimum angle should not be less than 45 deg

Maximum angle should not be greater than 135 deg

Length

Length is defined as the distance between two nodes in the element

Minimum length should not be less than 65 % of the element size

Jacobian

It is a matrix; it is defined as the partial derivatives of natural coordinates w.r.t the global coordinates. Perfect square element has a jacobian value 1.

1.3.3 STEPS IN ANALYSIS

Processor:

In this type of analysis to be performed is specified. The componenet is designed with specific material need to be defined. In order to define conventional material Youngs modulus, Density and poisons ratio must be feede. If material is composite again it can be orthotropic or anisotropic have to be defined with minimum of 9 to 16 parameters. Such as youngs modulus in X,Y,Z directions, shear modulus in different directions

Solution:

In this type and magnitude of force or pressure is defined. The displacement condition of component has to be defined such as translator or rotary conditions.

Postprocessor:

In post processor the required results are obtained

Resultant deformations

Stress distribution, von-misses stresses

Deformation in static and dynamic load directionChapter 2LITERATURE REVIEWBecause of the higher performance when compared with super-alloys at higher temperatures, and larger fracture toughness compared with monolithic ceramics, continuous fiber reinforced ceramic matrix composites (CFCCs) find many applications and are considered to be very interesting structural materials. Due to this reason, CFCCs are considered as the most potential to be used in advanced aero-engines.

The carbon fiber reinforced silicon carbide composites (C-SiC) are most promising and have been receiving considerable interest among the CFCCs. It is reported that many investigations have been conducted on two dimensional woven C-SiC composite materials. In order to meet mechanical and thermal property requirements along the thickness of the composites, an attention has been focused recently on three dimensional woven or braided ceramic matrix composite materials.

The fiber reinforced composite materials are not presently being used to the extent that they could be, despite the attractiveness of these materials as engineering components. Relatively low stress applications and large safety factors are usually considered even when they have been employed. The difficulty and uncertainty is said to be the main reason that exists in determining their failure strength, fracture toughness, operating lifetime in severe conditions, because the nature of the deformation and failure behavior of the composites are very complicated

The 3D textile C-SiC composite materials produced by chemical vapor infiltration are examined by Yongdong Xu et al. Their work emphasizes mainly on: (1) To develop the understanding of the effects of architecture on the mechanical properties and the damage behavior of the composites(2) To expand the experimental knowledge for the design of the 3D textile composite materials.The fabrication of three-dimensional carbon-silicon carbide composites is explained by the authors Yongdong Xu et al. in their paper by the process of chemical vapor infiltration, and the microstructure and mechanical properties are investigated. It is also observed that the composites (C-SiC) with no pyrolytic carbon interfacial layer, the mechanical properties (flexural strength, flexural elastic modulus, shear strength and fracture toughness) are increased with density of the composites.

Because of strong bonding between the fiber and matrix, high density (=2.1 gcm-3) C-SiC composites exhibit high fracture toughness but with brittle fracture behavior. A non-catastrophic failure mode with bundle pull-out is shown by low density composites. Better mechanical properties are exhibited by the composites (C/PyC/SiC) with pyrolytic carbon interfacial layer and a typical failure behavior involving fiber pull-out and brittle fracture of sub-bundle. Because of its heat stability, superior strength at elevated temperatures and low density, silicon carbide has excellent characteristics as a structural material for use at high temperatures for mechanical parts. However, brittleness is the key issue hindering its wider application.

To improve the fracture toughness, continuous carbon fiber reinforced silicon carbide composites (C-SiC) are a promising way. C-SiC composites exhibit higher mechanical properties and excellent oxidation resistance properties when compared with carbon-carbon composites.

For applications in space and aero-engine turbines, it is expected that C-SiC can withstand exposure to a service environment up to 1650C. Several processes have been developed so far, for fabrication of the composites, which include: slurry infiltration and hot-pressing, polymer conversion, liquid silicon infiltration, and chemical vapor infiltration.

The formation of a silicon carbide ceramic matrix inside the carbon fiber preform by slurry impregnation is allowed by the slurry infiltration and hot-pressing This method has a short processing time and is inexpensive in practice, and has been used most effectively with glass and glass-ceramic matrix systems.

It is less effective in refractory matrix systems (e.g. SiC), because of the absence of viscous flow. The general limitation of this process is the ability to produce only one or two-dimensional reinforced composites. Great composition homogeneity, the potential for forming unique multiphase matrix, and ease of infiltration of forming are said to be the advantages of polymer conversion.

This technique has the principal disadvantage of high shrinkage and low yield of polymer during pyrolysis. The liquid silicon infiltration method is based on a carbon-polymer pyrolysis and subsequently followed by an infiltration of liquid silicon. Chemical vapor infiltration (CVI) has developed as an extension of the well established chemical vapor deposition and has already been in commercial production.The primary advantages of this method are uniform coating of tailored compositions for interfacial properties which include interfacial bonding and interfacial mechanical properties. With this method, near-net-shaped components can be fabricated at relatively lower temperatures (900-l1000C).

It is reported that C-SiC composites with uni-directional(1-D) and two-dimensional (2-D) fabric preform have been investigated extensively The composites exhibit strong anisotropy because the arrangement of fibers in the perform is anisotropic. Recently, an attention has been directed to the three dimensional composites in order to improve their mechanical and thermal properties.

A chemical vapor infiltration combined with silicon melt infiltration method is developed by Yong dong Xu et al. for fabricating composites, in order to reduce processing costs and improve the thermal stability of three-dimensional carbon fiber-reinforced silicon carbide composites.

Chemical vapor infiltration (CVI) and silicon melt infiltration (SMI) are mainly used according to the size of the pores in the preform, to infiltrate small pores between fibers in a bundle and large pores between bundles, respectively. A pyrolytic carbon interfacial layer and a silicon carbide barrier layer are deposited on the surface of the carbon fiber in this chemical vapor infiltration process.The pre-coated perform is infiltrated with the pitch which is pyrolysed to form a porous carbon matrix in the pores. The perform, finally, is infiltrated with silicon melt to obtain composites. The optimum thickness of the pyrolytic carbon layer is obtained by studying the influence of the interface thickness on the mechanical properties and thus the failure behavior of the composites is investigated.

It is observed from experimental results that CVI+SMI composites exhibited good thermal stability of the mechanical properties and failure behavior after the composites are annealed at high temperatures. Because of their superior strength, fracture toughness, and abrasive properties, continuous carbon fiber-reinforced silicon carbide matrix composites (C-SiC) have been receiving considerable attentions for high-temperature structural applications.

C-SiC composites compared with carbon-carbon composites, exhibit higher mechanical properties and oxidation resistance. For fabrication of the composites, so far, four kinds of processing techniques have been developed which include: slurry infiltration and hot-pressing, polymer conversion, silicon melt infiltration, and chemical vapor infiltration.The formation of a ceramic matrix inside the carbon fiber perform is allowed by the technique of slurry infiltration and hot-pressing by the process of slurry impregnation, followed by hot-pressing [29]. In practice, this method has a short processing time and is inexpensive, and has been used most effectively for glass and glassceramic matrix systems.It is less effective in refractory matrix systems (e.g. SiC), because of the absence of viscous flow. The arising problem is fiber degradation or damage resulting from mechanical contact with the refractory particles. The general limitation is the ability to produce only 1- or 2-dimensional fiber-reinforced composites.

Great composition homogeneity, the potential for forming unique multiphase matrix, and ease of infiltration of forming are the advantages of polymer conversion. High shrinkage and low yield of polymer during pyrolysis are the principal disadvantages of this technique.

The conversion from the liquid precursor state to the solid ceramic state is accompanied by the volume shrinkage of the infiltrant, thus multiple re-infiltrations and conversions are required to obtain high-density composites. The method of silicon melt infiltration (SMI) is based on a polymer pyrolysis and subsequently followed by the infiltration of silicon melt.

Chapter 3SIMULATIONS OF ON 3D C-SiC COMPOSITE SPECIMENSImpact test:The configuration used in analysis is shown in Fig: 3.1 where the striker of nose radius r, traveling at a velocity v, impacts the specimen from a height h, at a distance of 22 mm from V- notch along the length of the specimen. A V-notch of root radius 0.25 mm with included angle of 450 having a depth of 2.54 mm is provided following the ASTM D 256 standards for the Izod impact test. The configuration is described conveniently in terms of rectangular coordinates.

Fig 3.1 Impact test specimen with loading and boundary conditions

The geometric model is created using ANSYS LS-DYNA software. A 3D structural solid 164 element is used to mesh the geometry of the striker as well as the specimen. Solid 164 element is having 8 nodes with the following degrees of freedom at each node: translations, velocities, and accelerations in the nodal x, y and z directions.

3D solid modeling is used to generate the specimen which then is meshed with solid 164 element shown in Fig: 3.2. A refined mesh is obtained after convergence check with a total number of 880 elements having 1260 nodes as shown in Fig: 3.3.

87

56

y

4x3

z

12

Fig: 3.2 solid 164 element

Fig. 3.3 Finite Element model of the notched specimen.

The impact process is simulated on the computer by following a transient analysis. The objective of this work is to demonstrate the explicit dynamics using ANSYS LS-DYNA for complex contact dynamics situation which simulates Izod impact test of experimentation.During the free fall stage, the striker is simply accelerating due to gravity. The analysis is started when the striker is 0.5 m above the specimen in order to save CPU time. The initial velocity of 3 ms-1 is applied to simulate the process. This velocity is an approximation obtained by using V= (2gh)1/2 where g is acceleration due to gravity and h is displacement. Air friction is assumed to be neglected.

The striker is constrained to have translations in y direction (1dof). It is restrained to have translations in x, z directions and rotations about x, y, z directions. The deformation obtained from ANSYS is 3.92 mm as shown in Fig: 3.4 The stress distribution is shown in Fig: 3.5. The maximum stress obtained in y-direction is 13.95 MPa. The impact strength obtained for the sample specimen by FEA analysis is 27.36 kJ/m2

Fig: 3.4 Deformation of the specimen in mm

Fig. 3.5 Stress distribution in MPa.

Flexural test (ASTM C 1341 standards)This test method is used for material development, quality control, and material flexural specifications. Although flexural test methods are commonly used to determine design strengths of monolithic advanced ceramics, the use of flexure test data for determining tensile or compressive properties of CFCC materials is strongly discouraged. The non uniform stress distributions in the flexure test specimen, the dissimilar mechanical behavior in tension and compression for CFCCs, low shear strengths of CFCCs, and anisotropy in fiber architecture all lead to ambiguity in using flexure results for CFCC material design data). Rather, uniaxial-forced tensile and compressive tests are recommended for developing CFCC material design data based on a uniformly stressed test condition.In this test method, the flexure stress is computed from elastic beam theory with the simplifying assumptions that the material is homogeneous and linearly elastic. This is valid for composites where the principal fiber direction is coincident/transverse with the axis of the beam. These assumptions are necessary to calculate a flexural strength value, but limit the application to comparative type testing such as used for material development, quality control, and flexure specifications. Such comparative testing requires consistent and standardized test conditions, that is, test specimen geometry/thickness, strain rates, and atmospheric/test conditions.This test method covers the determination of flexural properties of continuous fiber-reinforced ceramic composites in the form of rectangular bars formed directly or cut from sheets, plates, or molded shapes. Three test geometries are described as follows:

Test Geometry IA three-point loading system utilizing center point force application on a simply supported beam.

Test Geometry IIAA four-point loading system utilizing two force application points equally spaced from their adjacent support points with a distance between force application points of one half of the support span.

Test Geometry IIBA four-point loading system utilizing two force application points equally spaced from their adjacent support points with a distance between force application points of one third of the support span.

This test method applies primarily to all advanced ceramic matrix composites with continuous fiber reinforcement: uni-directional (1-D), bi-directional (2-D), tri-directional (3-D), and other continuous fiber architectures. In addition, this test method may also be used with glass (amorphous) matrix composites with continuous fiber reinforcement. However, flexural strength cannot be determined for those materials that do not break or fail by tension or compression in the outer fibers. This test method does not directly address discontinuous fiber-reinforced, whisker-reinforced, or particulate-reinforced ceramics. Those types of ceramic matrix composites are better tested in flexure using Test Methods.

The specimen is held in the testing machine as a simply supported beam and load is gradually applied at the centre. When the applied load reaches ultimate value the specimen breaks and subsequently the load falls to zero.

Fig. 3.6 Flexural test specimen with loading and boundary conditions

The finite element model is generated using ANSYS software. A 3D solid 45 brick element with 8 nodes is used to mesh the geometry of the specimen. Solid 45 element has 3 dofs. A refined mesh is obtained with 1560 elements and 2376 nodes which is shown in Fig: 3.6.

3.7 Finite Element model of the test specimen

The computer simulations of flexural test are performed by choosing the ultimate loads recorded in the test. The deformation obtained by computer simulation of flexural test is 2.58 mm as shown in Fig: 3.7.

The flexural stress distribution is shown in Fig: 3.10. The maximum flexural strength obtained by computer analysis is 240.17 MPa.

3.8 Deformation of the flexural specimen in mm

3.9 Flexural stress distribution in MPa

The shear stress distribution is shown in Fig: 3.8.

The maximum shear stress obtained from this analysis is observed to be 31.73 MPa

3.10 Shear stress distribution in MPa

Tensile test (ASTM C 1275 standards)This test method covers the determination of tensile behavior including tensile strength and stress-strain response under monotonic uniaxial loading of continuous fiber-reinforced advanced ceramics at ambient temperature. This test method addresses, but is not restricted to, various suggested test specimen geometries as listed in the appendix. In addition, test specimen fabrication methods, testing modes (force, displacement, or strain control), testing rates (force rate, stress rate, displacement rate, or strain rate), allowable bending, and data collection and reporting procedures are addressed. Note that tensile strength as used in this test method refers to the tensile strength obtained under monotonic uniaxial loading where monotonic refers to a continuous nonstop test rate with no reversals from test initiation to final fracture.

This test method applies primarily to all advanced ceramic matrix composites with continuous fiber reinforcement: uni-directional (1-D), bi-directional (2-D), and tri-directional (3-D). In addition, this test method may also be used with glass (amorphous) matrix composites with 1-D, 2-D, and 3-D continuous fiber reinforcement. This test method does not address directly discontinuous fiber-reinforced, whisker-reinforced or particulate-reinforced ceramics, although the test methods detailed here may be equally applicable to these composites. Values expressed in this test method are in accordance with the International System of Units (SI) and this standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.Specific hazard statements are given in Section 7 and 8.2.5.2.The loading and boundary conditions are shown in Fig: 3.13. The specimen is fixed in the testing machine and the movable jaw is adjusted for the gauge length of 25 mm. The tensile load is gradually applied till the specimen is broken at the average max. values of 1.26-1.62 kN. The load then falls to zero.

Fig 3.11 Tensile test specimen with loading and boundary conditions

The Finite Element model is created by using ANSYS 11.0 software. The specimen is meshed with the 3D solid 45 brick element which has 3 dofs. Fig: 3.11 shows the finite element model of the tensile test specimen obtained with 9000 elements and 11466 nodes.

Fig. 3. 12 Finite Element model of the tensile test specimenThe test process is simulated on the computer by running ANSYS program. One end of the specimen is fully restrained and the other end is constrained to have translations along the principal material direction. The computer simulations are performed by applying a gradual load. The analysis is completed when the load reaches a maximum value of 1.26-1.62 kN. The deformation obtained in ANSYS is 0.23 mm.

3.13 Deformation of tensile test specimen in mm

The tensile strength distribution for the specimen is shown in Fig: 3.13. The tensile strength obtained for specimen from this analysis is 71.735 MPa.

Fig. 3.14 Tensile strength distribution in MPa

Chapter 4RESULTS AND DISCUSSIONSFirst of all, when the striker touches the specimen the impact point is immediately accelerated from zero velocity to the initial velocity of the striker.

This instantaneous acceleration, for the Newtons second law, causes a first peak of force named inertial peak (because of the inertial nature of this phenomenon). After this, strong oscillation force increases linearly. At low displacements, in fact any material can be considered elastic so that force is proportional to displacement (and therefore to time, if impact energy is high).

The absorbed energy is a measure of material strength and the ductility can be graphically represented as the area beneath the load-displacement curve.

It is obvious that 3D C-SiC composite materials exhibit an excellent impact damage tolerance because of Z-direction fibers. The measured properties of 3D C SiC composites are compared with the properties of 2D C-SiC (available from literature).

Fig 4.1 Variation of Fracture Toughness with Fiber Volume Fraction

Fig 4.2 Variation of Impact Strength with Fiber Volume Fraction

Values from literature

Properties

Unit2D C SiC3D C SiC (LSI)

(LSI)

FiberVol. %40 4240

Volume

Densityg/cc2.42.2 2.4

FlexuralMPa180200210 230

Strength

TensileMPa809070 90

Strength

YoungsGPa253032 35

Modulus

Strain to%0.250.350.20 0.28

failure

ImpactkJ / m2202126 27

strength

Comparison of mechanical properties of 2D CSiC composites

(from literature) with FE results of 3D C-SiC specimen

The average result of flexural strength is observed to be 220 MPa. The variation of failure behavior of composites is caused by alteration of the interfacial bonding between fiber and matrix. The tensile stress within the interfacial phase along the fiber radial direction is generated after the composite material is cooled down from the infiltration temperature to room temperature.

CONCLUSIONSThe important conclusions drawn from the present work are:

1. The SEM micrographs reveal the uniformity of siliconisation and relatively less amount of un reacted silicon and carbon in the final composites.

2. In 3D geometric modeling, it is explained how the fiber

volume fraction is increasing when crimp angle is increased gradually.

3. By using the three dimensional geometric analysis, the orientation of any yarn in space in a unit cell; and the three dimensional geometry of the reinforcing fiber for a woven fiber composite have been completely determined.

4. The mechanical properties of 3D CSiC composites are determined. When fiber volume fraction is increased, fracture toughness and impact strength are increased.

5. The maximum shear stress obtained from Finite Element analysis is observed to be 31.73 MPa.

6. The tensile strength obtained from the FE analysis is 71.735 MPa.

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