STUDY OF COMPOSITE MATERIAL AND EFFECT OF HEAT TREATMENT ON MECHANICAL PROPERTIES OF HYBRID COMPOSITE

Introduction

Many of our modern technologies require materials with unusual combinations of properties that can not be by the conventional metal alloys, ceramics and polymeric materials. This is especially true for materials that are needed for aerospace, underwater and transportation applications. For example aircraft engineers are increasingly searching for structural materials that have low density are strong, stiff and abrasion and impact resistance, and are not usually corroded. This is a rather formidable combination of characteristics. Frequently, strong materials are relatively dense; also, increasing the strength or stiffness generally results in a decrease in impact strength.

Material property combinations and ranges have been, and are yet being experimented by the development of composite materials. Generally speaking a composite is considered to be a multiphase material that exhibits a significant proportion of the property of both the constituents phases such that a better combination of properties is realized. According to this principle of combined action, better property combinations are fashioned by the judicious combination of two or more distinct materials. Property trade-offs are also made for many composites.

COMPOSITE MATERIAL

Composite materials are engineered materials made from two or more constituents’ materials with significantly different mechanical properties and which remain separate and distinct within the finished structure. Although many man maid materials have two are more constituents, they are generally not referred to as composites if the structural unit is formed at the microscopic level rather than at the macroscopic level.

TYPES

Different types of composites are as follows:

1. Particle-reinforced
2. Fiber-reinforced
3. Structural

This paper mainly deals with the fiber-reinforced composites. Every fiber-reinforced composite has two categories of constituent materials: matrix and reinforcement. At least one portion (fraction) of each type is required. . The reinforcements impart special physical (mechanical and electrical) properties to enhance the matrix properties. A synergism produces material properties unavailable from naturally occurring materials.

Fibers are used as reinforcement material in such type of composites. Fibrous reinforcement is so effective because many materials are much stronger and stiffer in fiber form than they are in bulk form, because the probability of the presence of the critical surface flaw that can lead to fracture diminishes with decreasing specimen volume and this feature is used to advantage in the fiber reinforced composites. There can be no doubt that fibers allow us to obtain the maximum tensile strength and stiffness of a material, but there are obvious disadvantages of using a material in the fiber form. Fibers alone can not support longitudinal compressive loads and their transverse mechanical properties are generally not so good as the corresponding longitudinal properties. Thus, fibers are generally useless as structural materials unless they are held together in a structural unit with a binder or matrix material and unless some transverse reinforcement is provided.

The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The matrix holds the fibers together in a structural unit and protects them from external damage, transfer and distributes to the fibers and in many cases contribute some needed property such as ductility, toughness, or electrical insulation. A strong interface bond between the fiber and matrix is obviously desirable, so the matrix must be capable of developing a mechanical or chemical bond with the fiber. The fiber and matrix materials should also be chemically compatible, so that undesirable reactions do not take place at the interface. Such reaction tends to be more of a problem in high-temperature composites. Service temperature is often the main consideration in the selection of a matrix material

The need for fiber placement in different directions according to the particular applications has led to various types of composites as given bellow:

1. Continuous fiber composite
2. Woven fiber composite
3. Chopped fiber composite
4. Hybrid composite

In the continuous fiber composite laminate individual continuous fiber/matrix laminae are oriented in the required directions and bonded together to form a laminate. Although the continuous fiber laminate is used extensively, the potential for delamination

or separation of the laminae, is still a major problem because the interlaminar strength is Matrix-dominated.

Woven fiber composites not have distinct laminae and or not susceptible to delamination, but strength and stiffness are sacrificed due to the fact that the fibers are not so straight as in the continuous fiber laminate.

Chopped fiber composites may have short fibers randomly dispersed in the matrix. Chopped fiber composites are used extensively in high-volume applications due to low manufacturing cost, but their mechanical properties are considerably poorer than those of continuous fiber composites.

Hybrid composites may consists of chopped and continuous fibers or mixed fiber types such as glass/graphite.

HYBRID COMPOSITES

A relatively new fiber-reinforced composite is hybrid, which is obtained by using two or more different kinds of fibers in a single matrix; hybrids have a better all-around combination of properties than composites containing only a single fiber type. A variety of fiber combinations and matrix material are used, but in the most common system, both carbon and glass fiber are incorporated into a polymeric resin. The carbon fibers are strong and relatively stiff and provide low density reinforcement; however, they are expensive. Glass fibers are inexpensive and lack the stiffness of carbon. The glass-carbon hybrid is stronger and tougher, has a higher impact resistance, and may be produced at a lower cost than either of comparable all carbon or all glass reinforced plastics.

There are a number of ways in which the two different fibers may be combined, which will ultimately affect the overall properties. For example, the fibers may all be aligned and intimately mixed with one another; or laminations may be constructed consisting of layers, each of which consists of a single fiber type, alternating one with another. In virtually all hybrids the properties are anisotropic.

When hybrid composites are stressed in tension, failure is usually noncatastrophic (i.e., does not occur suddenly). The carbon fibers are the first to fail, at which time the load is transferred to the glass fibers. Upon failure of the glass fibers the matrix placed must sustain the applied load. Eventual composite failure concurs with that of the matrix phase.

APPLICATION OF COMPOSITES

Principle applications for hybrid composites are light weight land, water and air transport structural components, sporting goods and light weight orthopedic components.

Composite structural elements are now used in a variety of components for automotive, aerospace, marine, and architectural structures in addition to consumer products such as skis, golf clubs, and tennis rackets.

CONSTITUENT MATERIALS FOR COMPOSITES

The constituent materials for composites are as follows:

1.Matrix Material

2. Fiber Materials

3. Filler Material

MATRIX MATERIALS

1. Polymer
2. Metal
3. Ceramics

Aluminium based composites have received considerable attention for aerospace applications because of their low density and high stiffness. The introduction of a ceramic material into a metal matrix produces a composite material that results in an attractive combination of physical and mechanical properties, which cannot be obtained with monolithic alloys. Interest in particulate reinforced MMCs is mainly due to easy availability of particles and economic processing technique, adopted for producing the particulate-reinforced MMCs.

A literature survey reveals that only a few materials namely SiC, Graphite and Alumina are being used widely to reinforce metal alloys, while materials such as zircon, mica, and E-Glass are being sparingly. Interest in reinforcing Al alloy matrices with ceramic particles is mainly due to low density, low coefficient of thermal expansion and high strength of the resulting ceramics and also due to their wide availability.

FILLER MATERIAL

The third constituent material of a composite, the filler material is mixed with the matrix material during fabrication. Fillers are not generally used to include mechanical properties but, rather are used to enhance some other aspects of composite behavior.

FILLER MATERIAL	EFFECT ON COMPOSITE BEHAVIOR
Hollow glass micro spheres	Reduces weight
Clay or mica particles	Reduces cost
Carbon black particles	Protection against ultra violet radiation
Alumina tri hydrate	Flame and smoke suppression

Filler truly add another dimension to the design flexibility in composites.

EFFECT OF HEAT TREATMENT ON MECHANICAL PROPERTIES OF HYBRID COMPOSITES

The effect of heat treatment (T6 heat treatment) on mechanical properties of hybrid composite, Al (6061) alloy reinforce with SiC particulate and E glass fiber subjected wit different ageing duration at 175^o C.

EFFECT ON ULTIMATE TENSILE STRENGTH

Addition of reinforcement to Al alloys improves the yield strength and the ultimate tensile strength of the composites whereas the strain to failure decreases as the weight percentage of reinforcements increases. It is possible because ageing allows diffusion of segregation components, which has enhanced those UTS of hybrid composites. Young’s modulus also increases with the increasing ageing durations.

EFFECT ON COMPRESION STRENGTH

Upon T6 heat treatment the hybrid composite behave as a ductile material, which may be due to the influence of dislocation of sub structure on the precipitation process. The increase in dislocation density and dislocation acts as preferential nucleation sites for precipitates obtained during ageing. Due to dislocation, finer precipitate distribution appears in the composite. With the increase in ageing duration fracture strength (compressive) also increases.

EFECT ON HARDNESS

Addition of ceramic reinforcements brings about a considerable increase in hardness of composites. The increase in hardness is due to the fact that ceramic reinforcements are very hard and act as a barrier to the movement of dislocation within the matrix. Increasing ageing duration brings marginal increase in hardness of the composite. Some of the researchers have reported accelerated ageing responses of the composites in comparison with the base alloy, decreases beyond the peak hardness ageing duration.

EFFECT ON DUCTILITY

With the increase in aging duration, the ductility increases by slender margin. But with increase in the reinforcement content ductility deceases substantially. Increased aging increases ductility due to the fine dispersion of precipitate clusters form during heat treatment process.

CONCLUSIONS

the final conclusion s about the effect of heat treatment on hybrid composite (Al 6061) alloy +SiC particulate + E-glass fiber are as follows :-

1. The ultimate tensile strength, compression strength, young’s modulus and hardness increases with the increases in reinforcement content, but the ductility decreases substantially.

2. The ultimate tensile strength, compression strength, young’s modulus and hardness increases with the increases in the ageing duration with the marginal improvement in the ductility, which is due to the formation of a precipitate in the matrix alloy.

3. The enhancement in the mechanical properties can be well attributed to the high dislocation density.