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Automotive Clutch and Construction and working of Clutch and Classification of Clutch

Posted by badrut Saturday, September 19, 2020 0 comments

Welcome to Explore Automotive, In this blog we will see the automotive clutch, How automotive clutch works? and How automotive clutch classified? and specially role of clutch in manual transmission vehicles.

In an automobile, the mechanism which is playing major role to transmits the power developed by the engine to the wheels or tracks and accessory equipment is called the power train. In application, a set of gears or a chain and sprocket could perform this task.

Automotive Clutch and Construction and working of Clutch and Classification of Clutch

Image source: needpix.com Clutch Plate

However, automotives and construction equipments are not generally designed for such a simple operating conditions. They are to be designed which provides the pulling power, to move the equipment at high speeds, to travel in a reverse as well as forward direction, and to operate easily on a rough terrain as well as smooth road conditions. To meet these type of varying conditions, vehicle power trains are equipped with a different variety of components. In this blog we discuss the basics of automotive clutches.

Automotive Clutches

An automotive clutch is used generally in all types of automobiles which connect and disconnects the engine and manual transmission or transaxle. The clutch is usually placed in between the back of the engine and the front of the transmission.

With avoiding a few exceptional cases, the clutches common to the Naval Construction Force (NCF) equipments are of the single, double, and multiple-disc types. The clutch that mostly used in automobiles is the single-disc type (Figure 1).

Figure 1: Single Disc Clutch

The double-disc clutch is nearly the same as of the single-disc, except that another driven disc and an intermediate driving plate are to be added. This type of clutch is mostly used in heavy-duty vehicles and in construction equipments. The multiple-disc clutch is used in the automatic transmissions and for the steering clutch used in tracked equipment and in motorcycles also.

Operating principles, different component functions and maintenance requirements are essentially the same for all types of three clutches mentioned. This being the case, the single-disc clutch will be used to understand you with the fundamentals of the clutch.

A) Clutch Construction

The clutch is the first unit in drive train component which is powered by the engine crankshaft. The clutch lets the driver control power flow between the engine and transmission or transaxle. Before understanding the operation of a clutch, we must know about the parts and their functions. This information is very useful when we start learning to diagnose and repair the clutch assembly.

1. Clutch Release Mechanism

A clutch release mechanism allows that the operator to operate the clutch. Generally, this mechanism consists of the clutch pedal assembly, a mechanical linkage, cable, or hydraulic circuit, and the clutch fork. Some of the manufacturers include the release bearing as part a of the clutch release mechanism.

2. Manual Transmission

2.1 Linkage

Clutch linkage mechanism uses different levers and rods to transfer the motion from the clutch pedal to the clutch fork. One of the configurations is shown in Figure 2. When the clutch pedal is pressed by the driver, a pushrod shoves on the bell crank lever and the bell crank reverses the forward movement of the clutch pedal.

Figure 2: Clutch Linkage

And the other end of the bell crank lever is connected to the release rod. Then the release rod transfers the bell crank lever movement to the clutch fork. It also provides adjustment method for the clutch.

2.2 Cable

Clutch cable mechanism uses a steel cable which is covered with flexible housing to transfer the pedal movement to the clutch fork. As shown in Figure 3, the cable is generally fastened at the upper end of the clutch pedal, with the other end of the cable connected to the clutch fork.

Figure 3: Clutch Cable

The cable housing is mounted in a stationary position in such a way that it allows the cable to slide or move inside the housing whenever the clutch pedal is press or release. At one end of the clutch cable housing having a threaded sleeve for the clutch adjustment.

3. Hydraulic

Hydraulic clutch release mechanism working on a simple hydraulic circuit which helps to transfer the clutch pedal action to the clutch fork (Figure 4). It has three basic parts that are master cylinder, hydraulic lines, and a slave cylinder.

Figure 4: Hydraulic Clutch

Movement of the clutch pedal generates hydraulic pressure in the master cylinder, which is actuating the slave cylinder. Then the slave cylinder moves the clutch fork.

3.1 Slave Cylinder with Clutch Master Cylinder

Master cylinder is the main controlling cylinder which develops the hydraulic pressure. The slave cylinder is an operating cylinder that actuated by the hydraulic pressure developed by the master cylinder.

4. Clutch Fork

The clutch fork which is also called as clutch arm or release arm which transfers the motion from the release mechanism to the release bearing and pressure plate. The clutch fork coupled through a square hole in the bell housing and mounted on a pivot.

When the clutch fork is tends to move by using the release mechanism, it pries on the release bearing to disengage the clutch. A rubber boot fitted over the clutch fork which is designed to keep away road dirt, rocks, oil, water, and other debris from entering the clutch housing.

5. Clutch Housing

Clutch housing is also called the bell housing. It bolted to the rear of the engine which encloses the clutch assembly, with the manual transmission bolted to the back of the housing.

The lower front of the housing having a metal cover that can be able to remove for the flywheel ring gear inspection or when the engine must be separated from the clutch assembly. A hole is provided at the side of the housing for the clutch fork. It can be made of aluminum, magnesium, or cast iron.

6. Release Bearing

Release bearing also called as the throw-out bearing which includes a ball bearing and collar assembly. It controls or minimizes the friction between the pressure plate levers and the release fork.

The release bearing is completely sealed unit pack with a lubricant. It slides over a hub sleeve which is extending out from the front of the manual transmission or transaxle and is moved by using either hydraulic or manual pressure.

7. Hydraulic Type

Hydraulic release bearing avoids the stock mechanical release bearing linkage and the slave cylinder. The release bearing mounted on the transmission face or slips over the input shaft of the transmission system.

When the clutch pedal is to be pressed, the bearing face presses against the pressure plate to disengage the clutch of the vehicle.

8. Manual Type

Release bearing taking over the end of the clutch fork. Small spring clips which holds the bearing on the fork. Then the fork movement takes place in either direction slides the release bearing along the transmission hub sleeve.

9. Pressure Plate

Pressure plate is a spring-loaded device that can either engage or disengage the clutch plate disc and the flywheel by applying the pressure when pedal pressed. It is connected to the flywheel.

The clutch disc fits in between the flywheel and the pressure plate. Now, there are two types of pressure plates—the one is coil spring type and another one is that diaphragm type.

9.1 Coil Spring Pressure Plate

Coil spring type pressure plate uses a small coil springs which are similar to the valve springs (Figure 5). The face of the pressure plate is a large, flat ring which comes in contacts with the clutch disc during clutch engagement.

Figure 5: Coil Spring Pressure Plate

The back side of the pressure plate having pockets for the coil springs and brackets which used to hinging the release levers. During the clutch action, the pressure plate moves backward and force inside the clutch cover.

The release levers are hinged inside the pressure plate to pry on and move the pressure plate face outwards from the clutch plate disc and flywheel. Small clip-type springs are fitted around the release levers to keep them rattling when gets fully released.

The pressure plate cover fitted over the springs, the release levers, and the pressure plate face. Its main purpose is to hold the whole assembly together as a unit. Holes which are surrounded by the outer edge of the cover are for bolting the pressure plate to the flywheel.

9.2 Diaphragm Pressure Plate

As shown in (Figure 6), the diaphragm pressure plate uses a single diaphragm spring instead of the coil springs. The diaphragm spring is a large, round disc made up of spring steel.

Figure 6: Diaphragm Pressure Plate

The spring is bent or dished and has pie-shaped segments which is running from the outer edge to the center. The diaphragm spring is mounted in the pressure plate with the outer edge which touching the back of the pressure plate face.

The outer rim of the diaphragm is getting secured to the pressure plate and is pivoted on rings approximately 1 inch from the outer edge off the same.

By the application of pressure at the inner section of the diaphragm will cause the outer rim to move outward from the flywheel and draw the pressure plate away from the clutch disc, disengaging the clutch.

10. Clutch Disc

10.1 Wet Type

A “wet” clutch is always immersed in a cooling lubricating fluid, which also keeps the surfaces to be clean and gives smoother performance enhancements and longer life. Wet clutches tends to lose some energy to the lubricating liquid.

Since, the surfaces of wet clutch can be slippery in condition, stacking with multiple clutch discs which can compensate for the lower coefficient of friction and so eliminate the slippage under the power when fully engaged.

Wet clutches are designed in such a way that which provides a long, service-free life. They often last the entire life of the machine in which they are installed on. For maintenance, we must need to refer the manufacturer’s manual.

10.2 Dry Type

Clutch disc which is also called as friction lining is a “dry” clutch and it consists of a splined hub and a round metal plate which is covered with friction material (lining).

The splines in the center of the clutch disc meshing with the splines which are on the input shaft of the manual transmission. This condition makes the input shaft and disc to be turn together.

However, the disc is free to slide backward and forward on the shaft. Clutch disc torsion springs which are also termed as damping springs, absorbs some of the vibration and shock which produced by the clutch engagement.

Damping springs are small coil springs which located in between the clutch disc splined hub and the friction disc assembly. When the clutch is getting engaged, the pressure plate jams the stationary disc against the spinning flywheel.

The torsion springs get compress and soften as the disc first begins to turn with the flywheel. Clutch disc facing springs which are also called as the cushioning springs.

These are flat metal springs located under the friction lining of the disc. These springs have a slight wave or curve, which allowing the lining to flex inward slightly during the initial engagement and also allows for smooth engagement.

The clutch disc friction material, also called as disc lining or facing, which is made up of heat resistant asbestos, cotton fibers, and copper wires woven or molded together. Grooves are cut into the friction material to provide aid cooling and release of the clutch disc. Rivets are used to bond the friction material on to the both sides of the metal body of the disc.

11. Flywheel

The flywheel is the main mounting surface for the clutch (Figure 7). The pressure plate bolted to the flywheel face. The clutch disc is clamped and held against the flywheel by using the spring action of the pressure plate.

Figure 7: Flywheel and Pilot Bearing

Face of the flywheel is precision machined which is a smooth surface. The face of the flywheel that touches the clutch disc which is made up of iron. Even if the flywheel were aluminum, the face is made of iron because it wears well and helps to dissipates heat better.

12. Pilot Bearing

The pilot bearing or bushing is to be pressed into the end of the crankshaft to support the end of the transmission input shaft (Figure 7). The pilot bearing is a solid bronze bushing, but it can also be a roller or a ball bearing.

The end of the transmission input shaft has a small journal which is machined on its end. This journal slides at the inner side of the pilot bearing. Pilot bearing prevents the transmission shaft and clutch disc from the effect of wobbling up and down when the clutch is released. It can also assist the input shaft at center of the disc on the flywheel.

B. Clutch Operation

When the operator or driver of the vehicle presses the clutch pedal, the clutch release mechanism pulls or pushes on the clutch release lever or clutch fork (Figure 8).

Figure 8: Clutch Operation

The fork moves the release bearing into the center of the pressure plate, which causing the pressure plate to pull outward direction from the clutch disc which releasing the disc from the flywheel. The engine crankshaft can be turn without turning the clutch disc and transmission input shaft.

When the operator or driver of the vehicle releases the clutch pedal, spring pressure inside the pressure plate pushes forward on the clutch disc. This action get locks the flywheel, the clutch disc, the pressure plate, and the transmission input shaft together.

The engine again starts to rotate the transmission input shaft, the transmission gears, the drive train, and the wheels of the vehicle.

C. Clutch Start Switch

Many of the new vehicles coming in the markets incorporate a clutch start switch into the starting system. The clutch start switch is mounted on to the clutch pedal assembly.

The clutch start switch helps to prevent the engine from cranking unless the clutch pedal is depressed fully. This switch act as a safety device that keeps the engine from possibly starting while in geared condition.

Unless the switch is to be closed (clutch pedal depressed), that switch also prevents current from reaching the starter solenoid. With the transmission in neutral condition, the clutch start switch is bypassed so the engine will crank and get start.

D. Clutch Adjustments

Clutch adjustments are to be made to compensate the wear of the clutch disc lining and linkage between the clutch pedal and the clutch release lever. This adjustment involves setting the correct amount of free play in the release mechanism.

Too much free play can causes the clutch to drag during clutch disengagement condition. And too little free play can also cause the clutch slippage. So, It is important for us to know that how to adjust the three types of clutch release mechanisms.

1. Clutch Linkage Adjustment

Mechanical clutch linkage is to be adjusted at the release rod which going to the release fork shown in (Figure 2). At one end of the release rod is to be threaded.

The effective length of the release rod can beincreased to raise height of the clutch pedal (decrease free travel). It can also be decrease to lower height of the clutch pedal (increase free travel).

To change the clutch adjustment, simply loosen the release rod nuts placed on it. Turn the release rod nuts on to the threaded rod until wereached the desired free pedal travel.

2. Pressure Plate Adjustment

When a new pressure plate is to be installed, always remember that to check the pressure plate for the proper adjustments. These adjustments will ensure that proper operation of the pressure plate.

The first adjustment ensures that proper movement of the pressure plate is to be done in relation to the cover. As shown in Figure 9, with the use of a straight edge and a scale, begin turning the adjusting screws until we obtain the proper clearance between the straight-edge and the plate as shown.

Figure 9: Pressure Plate Adjustment

For exact measurements, please refer the manufacturer’s service manual of particular vehicle. For the second adjustment which is important for proper operation in which the release lever allows the release bearing to contact the levers simultaneously while maintaining the adequate clearance of the levers and disc or the pressure plate cover. This adjustment is called as a finger height.

To do the adjustments for the pressure plate, place the assembly on a flat surface and measure the height of the levers, as shown in Figure 10.

Figure 10 Pressure Plate Release Lever Adjustment

And adjust it by loosening the locknut and turning, after the proper height has been set, make sure that the locknuts are to be locked and staked properly with a punch to keep it from getting loose during the operations. Exact release lever height can be taken into consideration from the manufacturer’s service manual.

3. Clutch Cable Adjustment

Like the mechanical linkage, a clutch cable adjustment is to be required to maintain the correct pedal height and free travel according to the operator. Typically the clutch cable having an adjusting nut.

When the nut is to be turned, the length of the cable housing get increases or decreases as per movement. To increase the clutch pedal free travel, turn the clutch cable housing nut to shorten the cable housing, and, to decrease clutch pedal free travel, turn the nut to lengthen the cable housing.

E. Hydraulic Clutch

Hydraulically operated clutch is to be adjusted by changing the length of the slave cylinder pushrod. To adjust the hydraulic clutch, simply turn the nut or nuts which are placed on the pushrod as per requirement.

Composite Materials: Definition and How its Classified?

Posted by badrut Monday, September 14, 2020 0 comments

Welcome to Explore Automotive, In this blog we will see the composite materials in details like what is Composite Materials? How Composite Materials classified? And many more problems which arises in our mind regarding composite materials, here I try to solve and answer the questions in this topic.

Composite Materials: Definition and How its Classified?

(Image source :LamboCARS)

The development of composite materials as well as the related design and manufacturing technologies is one of the most important advances in the history of materials.

Composites are multifunctional materials having unprecedented mechanical and physical properties which can be tailored to meet the requirements of a particular application.

Many composites also exhibit great resistance to wear, corrosion, and high-temperature exposure. These unique characteristics which are helping to provide the mechanical engineer with the design opportunities which is not possible with the conventional monolithic (unreinforced) materials.

Composites technology can also makes the possible use of an entire class of the solid materials, ceramics, in an application for which monolithic versions are not suitable because of their great strength scatter and poor resistance to mechanical and thermal shocks.

Further, many manufacturing processes for composites are well adapted to the fabrication of large, complex structures, which allows consolidation of parts, reducing manufacturing costs.

Composites are important materials which are now used widely, not only in the aerospace industry, but also in a large and increasing number of commercial mechanical engineering applications, such as internalcombustion engines; machine components; thermal management and electronic packaging; automobile, train, and aircraft structures and mechanical components, such as brakes, drive shafts, flywheels, tanks, and pressure vessels; dimensionally stable components; process industries equipment requiring resistance to high-temperature corrosion, oxidation, and wear; offshore and onshore oil exploration and production; marine structures; sports and leisure equipment; ships and boats; and biomedical devices.

(Image Source: Wikimedia commons)

It should be noted that biological structural materials occurring in nature are typically some type of composite. Common examples are wood, bamboo, bone, teeth, and shell.

Further, use of an artificial composite material is not new thing. Straw-reinforced mud bricks were employed in biblical times. Using modern terminology, discussed later, this material would be classified as an organic fiber-reinforced ceramic matrix composite.

Classes and Characteristics of Composite Materials

Solid materials can be divided as into four categories given as—polymers, metals, ceramics, and carbon, which we consider as a separate class because of its unique characteristics.

We found both the reinforcements and matrix materials in those all four categories. This gives us the ability to create a limitless number of new material systems which have unique properties that cannot be obtained with any single monolithic material.

Table 1 Types of Composite Materials

Reinforcement	Matrix
	Polymer Metal Ceramic Carbon
Polymer	x x x x
Metal	x x x x
Ceramic	x x x x
Carbon	x x x

Table 1 shows the types of material combinations which are now in use. Composite materials are usually classified by the type of material used for the matrix.

The four primary categories of composites are polymer matrix composites (PMCs), metal matrix composites (MMCs), ceramic matrix composites (CMCs), and carbon matrix composites (CAMCs). Carbon–carbon composites (CCCs) are the most important subclass of CAMCs.

At this time, PMCs are by far the most widely used type of composites. However, there are important applications of the other types of composites which are indicative of their great potential in mechanical engineering applications.

Figure 1. Types of Reinforcements

In Figure 1 which showing the main types of reinforcements used in composite materials, aligned continuous fibers, discontinuous fibers, whiskers (elongated single crystals), particles, and numerous forms of fibrous architectures which are produced by textile technology, such as fabrics and braids.

Carbon nano tubes are similar to discontinuous fibers. Two-dimensional fabrics are shown. There are also a wide range of three-dimensional woven and braided reinforcements.

A common way to represent the fiber-reinforced composites which are able to show the fiber and matrix separated by using a slash. For example, carbon fiber-reinforced epoxy is typically written carbon/epoxy, or in abbreviated form, C/Ep.

We represent particle reinforcements by enclosing them in parentheses followed by “p.” Using this convention, silicon carbide (SiC) particle-reinforced aluminum appears as (SiC) p/Al.

Composites are strongly heterogeneous materials. That is, the properties of a composite material varying in a considerably manner from point to point in the material, depending on which material phase the point is located in.

Monolithic ceramics and metallic alloys are usually considered to be isotropic materials to a first approximation, although rolled aluminum alloys have anisotropic strength properties.

Many artificial composites, especially those which are reinforced with fibers, are anisotropic in nature, which means their properties vary with the direction while the properties of isotropic materials are the same in every direction. This is a characteristic that they shared with a widely used natural fibrous composite material, wood.

As for wood, when structures made from artificial fibrous composites are required to carry load in more than one direction, they are used in laminated form (plywood). Particulate composites can be effectively isotropic if the reinforcements are equiaxed, that is, have roughly the same dimensions in three orthogonal directions (think of sand particles).

Many fiber-reinforced composites, especially PMCs, MMCs, and CAMCs, do not display plastic behavior as metals do, which makes them more sensitive to stress concentrations.

However, the absence of plastic deformation does not mean that composites are brittle materials like monolithic ceramics. The heterogeneous nature of composites results in complex failure mechanisms which impart toughness.

Fiber-reinforced materials have been found that to produce a durable and reliable structural component in countless applications. The special characteristics of composite materials, especially anisotropy, required the use of special designing methods.

Types of Matrices

Generally, composite materials are typically classified on basis of matrix which is main constituent. The major composite classes are including that Organic Matrix Composites (OMCs), Metal Matrix Composites (MMCs) and Ceramic Matrix Composites (CMCs).

The term organic matrix composite is generally assumed to include that two classes of composites, namely Polymer Matrix Composites (PMCs) and carbon matrix composites commonly referred to as carbon-carbon composites.

There are three types of matrices produced three common types of composites as follows:

1. Polymer matrix composites (PMCs), of which GRP is the best-known example, use ceramic fibers in a plastic matrix.

2. Metal-matrix composites (MMCs) mostly used silicon carbide fibers embedded in a matrix made from an alloy of aluminum and magnesium, but other matrix materials such as titanium, copper, and iron are increasingly being used.
Specially, the applications of MMCs include bicycles, golf clubs, and missile guidance systems; an MMC made from silicon carbide fibers in a titanium matrix is currently being developed for use as the skin means its fuselage material for the US National Aerospace Plane.

3. Ceramic-matrix composites (CMCs) are the third major type of composites and examples include silicon carbide fibers fixed in a matrix made from a borosilicate glass.

The ceramic matrix makes them particularly suitable for the purpose of use in lightweight, high-temperature components, such as parts for airplane jet engines.

Polymer Matrix Composites (PMC)/Carbon Matrix Composites/Carbon-Carbon Composites (CCC)

Polymer makes an ideal materials as they can be processed easily which possess light in weight, and desirable mechanical properties. It follows that high temperature resins are extensively used in aeronautical applications.

There are two main kinds of polymers are thermosets and thermoplastics. Thermosets have some different qualities such as a well-bonded three-dimensional molecular structure after curing up.

They decompose instead of melting on hardening. Simply changing the basic composition of the resin is good enough to alter the conditions suitably for curing and determine its other characteristics.

They can be retained in a partially cured condition too over a prolonged period of time, rendering Thermosets very flexible.

Thus, they are most suitable as matrix bases for the advanced conditions fiber reinforced composites.

Thermosets are found that a wide ranging applications in a chopped fiber composites form particularly when a premixed or moulding compound with fibers of specific quality and the aspect ratio happens to be in starting material as in epoxy, polymer and phenolic polyamide resins etc.

Thermoplastics have one or two-dimensional molecular structure and they tends to at an elevated temperature and show exaggerated melting point.

Another advantage is that the process of softening at an elevated temperatures can reversed to regain its properties during cooling phase, facilitating applications of conventional compressing techniques to mould the compounds.

Resins reinforced with thermoplastics and get comprised an emerging group of composites. The structure of most of the experiments in this area to improve the base properties of the resins and extract the greatest functional advantages from them in new avenues, including attempts to replace metals in die-casting processes.

In a crystalline thermoplastic, the reinforcement affects the morphology to a considerable extent, prompting the reinforcement to empower the nucleation.

Whenever, a crystalline or amorphous, these resins possess the facility to alter their creep over a considerable range of temperature.

But this range includes that the point at which the usage of resins is constrained, and the reinforcement in such systems can increase the possibility of the failure load as well as creep resistance. Figure 2 shows kinds of thermoplastics.

Figure 2. Types of Thermoplastics

A small quantum of shrinkage and the tendency of the shape to retain into its original form are also to be accounted but reinforcements can change this condition too.

The advantages of thermoplastics systems over the thermosets are that there are no chemical reactions get involved, which often result in the release of gases or heat.

Manufacturing is limited by the time required for heating, shaping and cooling the structures.

Thermoplastics resins are sold as moulding compounds. Fiber reinforcement is apt for these resins. Since the fibers are randomly dispersed, the reinforcement will be almost isotropic. However, when subjected to moulding processes, they can be aligned directionally.

There are a few options to increase the heat resistance in thermoplastics. Addition of fillers in thermoplastics raises the heat resistance.

But all thermoplastic composites tends lose their strength at an elevated temperatures. However, their compensating qualities like rigidity, toughness and ability to repudiate creep, place thermoplastics in the important composite materials bracket. They are used in automotive control panels/ dashboards, electronic products encasement etc.

Newer developments augur the broadening of the scope of applications of thermoplastics. In market, huge sheets of reinforced thermoplastics are now available and they only required sampling and heating to be moulded into the required shapes. This has facilitated easy fabrication of bulky components, doing away with the more cumbersome moulding compounds.

Thermosets are the most popular material of the fiber composite matrices without which, research and development in structural engineering field could get dependant.

Mostly in aerospace components, automobile parts, defense systems etc., use a great deal of this type of fiber composites. Epoxy matrix materials are used in printed circuit boards and other similar electric and electronic field. Figure 3 shows some kinds of thermosets.

Figure 3. Types of Thermosets

Direct condensation polymerization followed by rearrangement reactions to form heterocyclic entities is the method generally used to produce thermoset resins. Water, a product for the reaction, in both the methods, hinders production of void-free composites.

These voids have a negative effect on properties of the composites in terms of strength and dielectric properties. Polyesters phenolic and Epoxies are the two important classes of thermoset resins.

Epoxy resins are widely used in filament-wound composites and are suitable for moulding prepress. They are reasonably stable to chemical attacks and are excellent adherents having slow shrinkage during curing and no emission of volatile gases.

These advantages, however, make the use of epoxies rather expensive. Also, they cannot be expected beyond the temperature limit of 140ºC. Their use in high technological areas where service temperatures are gets higher, as a result, is ruled out.

Polyester resins on the other hand which are quite easily accessible, cheap and find in use of a wide range of fields.

Liquid polyesters are stored at room temperature for couple of months; sometimes for years and the addition of a catalyst can be cure the matrix material within a short period of time. They are widely used in automobiles components and structural applications.

The cured polyester is usually in a form of rigid or flexible as the case may be a transparent. Polyesters withstand against the variations of the environment and stable against chemical reactions.

Depending upon the formulation of the resin or service requirement of the application, they can be used up to about 75ºC or higher than 75°C. Other advantages of the polyesters include that easy compatibility with few glass fibers and can be used with verified version of reinforced plastic accoutrey.

Aromatic Polyamides are the most seeking after candidates as the matrices of advanced fiber composites for the structural applications which demanding a long duration exposure for continuous service at around 200 to 250ºC.

Metal Matrix Composites (MMC)

Metal matrix composites, in today’s condition its generating a wide interest in research fraternity, are not as widely in use as their plastic counterparts.

High strength, fracture toughness and stiffness are getting offered by the metal matrices than those which are offered by their polymer counterparts.

They can withstand at an elevated temperature in the corrosive environment than the polymer composites.

Most of the metals and alloys could be used as matrices and they required reinforcement materials which need to be stable over a range of temperature and non-reactive too.

However the guiding aspect for the choice actually depends essentially on the matrix material. Light metal forms the matrix for the temperature application and the reinforcements in addition to the some reasons are characterized by high moduli.

Most of the metals and alloys make good matrices. However, practically, the choices for low temperature applications are not so many. Only light metals are responsive in nature, with their low density proving an advantage.

Such as Titanium, Aluminium and magnesium are the popular matrix metals currently in vogue, which are particularly useful for the aircraft applications.

If the metallic matrix materials have to offer high strength, they required high modulus reinforcements. The strength-to-weight ratios of the resulting composites can be higher than most of the alloys.

The melting point, physical and mechanical properties of the composite material at various temperatures to determine the service temperature of composites.

Most of the metals, ceramics and compounds can be used with the matrices of low melting point alloys. The choice of reinforcements becomes more stunted with increase in the melting temperature of the matrix materials.

Ceramic Matrix Materials (CMM)

Ceramics can be described as the solid materials which exhibits very strong ionic bonding in general and in few cases covalent bonding shown.

High melting points, good corrosion resistance and stability at an elevated temperatures and high compressive strength, rendered ceramic-based matrix materials avails for the applications which requiring a structural material that doesn’t give any other option at the temperature above 1500ºC.

Naturally, ceramic matrices are the obvious best choice for the high temperature applications.

High modulus of elasticity and low tensile strain, which most of the ceramics posses, have combined to cause the failure of attempts to add reinforcements to obtain improvement in the strength. This is because of the stress levels at which ceramics ruptured, there is insufficient elongation of the matrix produced which keeps the composite from transferring to an effective quantum of load to the reinforcement and the composite may fail unless the percentage of the fiber volume is getting high enough.

A material is reinforcement to utilize the higher tensile strength of the fiber, to produce an increase in the load bearing capacity of the matrix. In addition to this high-strength fiber to a weaker ceramic has not always been successful and often the resultant composite has proved to be a weaker.

Use of reinforcement with high modulus of elasticity may take care of the problem to some amount and presents pre-stressing of the fiber in the ceramic matrix is being increasingly resorted to as an option.

When ceramics having a higher thermal expansion coefficient than the reinforcement materials, the resultant composite is getting unlikely to have a superior level of strength.

In that case, the composite material will develop the strength within the ceramic at the time of cooling resulting in microcracks extending from fiber to fiber within the matrix. Microcracking can result in a composite with lower tensile strength than that of the matrix.

Role of matrix materials

The choice of a matrix alloy for MMC is dictated by the several considerations of particular importance is whether the composite is to be continuously or discontinuously reinforced.

The use of continuous fibers as a reinforcement may results in the transfer of most of the load to the reinforcing filaments and hence composite strength will becomes primarily by the fiber strength.

The primary roles of the matrix alloys are to provide efficient transfer of load to the fibers and to the blunt cracks in the event that fiber failure occurs and so the matrix alloy for continuously reinforced composites may be chosen more for toughness than the strength.

On that basis, lower strength, more ductile, and tougher matrix alloys can be utilized in continuously reinforced composites. For discontinuously reinforced composites, the matrix may covers the composite strength.

Then, the choice of matrix will be impact by the consideration of the required composite strength and higher strength matrix alloys will be required.

Some additional considerations in the choice of the matrix includes that potential reinforcement, matrix reactions, either during processing or in service stage, which might be result in degraded composite performance; thermal stresses due to thermal expansion mismatching between the reinforcements and the matrix; and the influence of matrix fatigue behavior on the cyclic response of the composite material.

Actually, the behavior of composites under cyclic loading conditions is an area requiring the special consideration.

In composites, intended use at an elevated temperature, an additional consideration is the difference in melting temperatures between the matrix and the reinforcements.

Due to large melting temperature difference may results in matrix creep while the reinforcements remain elastic and even at temperatures approaching the matrix melting point.

Wherever, creep in both the matrix and reinforcement must be taken into consideration when there is a small melting point difference in the composite.

Functions of a Matrix

In composite material, the matrix material which plays the following functions:

1. Matrix holds the fibres together with strong bonding.

2. Matrix protects the fibres from environmental impact.

3. Matrix helping to distributes the load evenly between the fibres so that all fibres are subjected to the same amount of strain.

4. Matrix enhances the transverse properties of a laminate.

5. It improves the impact and fracture resistance of a component.

6. It helps to avoid propagation of crack growth through the fibres by providing alternate failure path along the interface between the fibres and the matrix.

7. It carries inter laminar shear.

8. Matrix plays a minor role in the tensile load-bearing capacity of a composite structure.

However, the selection of a matrix has a major influence on the inter-laminar shear as well as in-plane shear properties of the composite material. Inter-laminar shear strength is an important for the design consideration point of view specially for the structures under the bending loads, whereas in-plane shear strength is important under torsion loads.

The matrix provides lateral support against the possibility of fibre buckling under the compression loading, thus influencing to some extent the compressive strength of the composite material.

The interaction between fibres and matrix is also important in designing the damage tolerant structures. Finally, the processability and defects in a composite material depends strongly on the physical and thermal characteristics, such as viscosity, melting point, and curing temperature of the matrix.

Advantages and Limitations of Composites Materials

Advantages of Composites

List of the advantages exhibited by the composite materials, which are of significant use in aerospace industry are as follows:

1. Composite materials have high resistance to fatigue and corrosion degradation.

2. Composite materials having 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 which are to be made.

3. Due to greater reliability of composites, there are fewer inspections and structural repairs occurred.

4. Composite’s directional tailoring capabilities to meet the design requirements. The fibre pattern can be formed in such a manner that will customize the structure to efficiently bear the applied loads.

5. Composite material has fibre to fibre redundant load path.

6. It improved dent resistance is normally achieved. Composite panels do not sustain the damage as easily as thin gauge sheet metals.

7. For composite material, it is easier to achieve smooth aerodynamic profiles for drag reduction. Complex double-curvature parts with a smooth surface finish can be made only in one manufacturing operation.

8. Composite materials offer an improved torsional stiffness. This implies that high whirling speeds, reduced number of intermediate bearings and supporting to the structural elements. The overall part count and manufacturing & assembly costs are thus gets reduced.

9. Composites have high resistance to impact damage.

10. Polymer composite materials such as thermoplastics having a rapid process cycle, which making them attractive for high volume commercial applications that traditionally have been the domain of sheet metals. Moreover, thermoplastics can also be reformed.

11. As comparing with metals, thermoplastics have indefinite shelf life.

12. Composite materials are dimensionally stable that is they have low thermal conductivity and low coefficient of thermal expansion. Composite materials can be customizing to comply with the broad range of thermal expansion design requirements and to minimize the thermal stresses.

13. For composite material, manufacture and assembly are simplified because of its part integration such as joint, fastener reduction because of that reducing the cost.

14. Improved weatherability of composites in a marine application environment as well as their corrosion resistance property and durability reduced at the down time for maintenance.

15. In composites, close tolerances can be achieved without machining.

16. Wastage of material is reduced while production because composite parts and structures are frequently built to shape rather than machined to the required configuration, as is common with metals.

17. Due to excellent heat sink properties of composites, especially Carbon-Carbon, combined with their lightweight has extended their use for aircraft brakes.

18. Composites have improved friction and wear properties.

19. Composites have ability to tailor the basic material properties of a Laminate has allowed new approaches to the design of aeroelastic flight structures.

Advantages which are given above translate not only use into airplane, but also into common implements and equipment such as a graphite racquet that has inherent damping, and causes less fatigue and pain to the user of it.

Limitations of Composites

Some of the associated disadvantages of advanced composites are as follows:

1. For composites, it has high cost of raw materials and fabrication.

2. Composite materials are more brittle than the wrought metals and thus they are more easily damaged.

3. Transverse properties of composite materials may be weak.

4. Composite Matrix is weak in strength, therefore, low toughness.

5. Composite materials reuse and disposal may be difficult and difficult to attach.

Repair introduces new problems, for the following reasons:

1. Composite Materials required a refrigerated transport and storage because it has limited shelf life.

2. In some composite materials hot curing is necessary in many cases which required special tooling. And hot or cold curing takes time.

3. Analysis of composite material is difficult.

4. Composite materials matrix is subjected to environmental degradation.

However, proper design and material selection can affect many of the above disadvantages.

New technology is to be provided a variety of reinforcing fibres and matrices those can be combined with to form the composites which having a wide range of exceptional properties.

Since, the advanced composite materials are capable to provide the structural efficiency at lower weights as compared to the equivalent metallic structures; they have been emerged as the primary materials for the future use.

In aircraft application, advanced fibre reinforced composites are now being used in many structural applications, for example, floor beams, engine cowlings, flight control surfaces, landing gear doors, wing-to-body fairings, etc., and also major load carrying structures including the vertical and horizontal stabilizer main torque boxes.

Composite materials are also taken into consideration for use in improvements into civil infrastructures, for examples, earthquake proof highway supports; power generating wind mills, long span bridges, etc.

Comparison with Metals

Requirements commanding the choice of materials which is apply to both metals and reinforced plastics.

Composites offer significant weight saving over existing metals. Composites can provide structures which are 25% to 45% lighter than the conventional aluminium structures which designed to meet the same functional requirements. This is due to the lower density of the composites.

Depending on material form, composite densities range from 1260 to 1820 kg/in³(0.045 to 0.065 lb/in³) as compared to 2800 kg/in³(0.10 lb/in³) for aluminum. Some other applications to be required thicker composite sections to meet the strength/stiffness requirements, however, weight savings will still result.

Unidirectional fibre composites have specific tensile strength (ratio of material strength to density) about 4 to 6 times greater than that of steel and aluminium.

Unidirectional composite materials have specific –modulus that is the ratio of the material stiffness to density of about 3 to 5 times greater than that of steel and aluminium.

Fatigue endurance limit of composite materials can approach about 60% of their ultimate tensile strength. For steel and aluminium, this value is lower while taken into consideration.

Fibre composites are more versatile in nature than the metal, and it can be customized so as to meet the performance needs and complex design requirements such as aero-elastic loading on the wings and the vertical & horizontal stabilizers of aircraft.

Fibre reinforced composites can be designed with excellent structural damping features. While comparing with metals, they are less noisy and provide lower vibration transmission than metals.

High corrosion resistance of fibre composite materials mainly contributes to reduce the life- cycle cost.

Composite materials offer reduced manufacturing cost principally by significantly reducing the number of detailed parts and expensive technical joints which required to form the large metal structural components.

Alternatively we can say that, composite parts can eliminate joints or fasteners thereby providing parts simplification and integrated design.

Long term service experience environment and durability behaviour of composite material is limited in comparison with the metals.

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