Composite Materials

                  The word ‘composite’ means ‘consisting of two or more distinct parts’. Thus a material having two or more distinct constituent materials or phases may be considered as composite materials.
       When the constituent phases have significantly different physical properties and   the composite properties are noticeably different physical properties then they are said to be composites. Generally metals consists of  unwanted impurities or alloying elements, but they must not be called as composites  because the constituent phases will have nearly identical properties ,the phases are not generally fibrous in character, and one of the phases is usually present in small fractions.
 Thus classification of certain materials as composites is often based on cases where significant property changes occur as a result of combination of constituents, and these property changes will be generally be most obvious when one of the phase is in platelet or fibrous form, when the volume fraction is greater than 10%, and when the property of one constituent is much greater (>5 times) than the other.
 So composites can be considered as materials consisting of two or more chemically distinct constituents, on a macro scale, having a distinct interface separating them whose mechanical properties are superior to those of individual components acting independently.

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 4.2  Constituents. :


                One phase is discontinuous, stiffer and stronger is called reinforcement. Other phase is continuous, less stiffer and weaker is called matrix.  Sometimes, a third phase exists between reinforcement and the matrix because of chemical interactions or other processing effects is called interphase which plays an important role in controlling failure mechanisms, fracture toughness. The reinforcing phase, is in the form of fibers, flakes, or particles, and is embedded in the other phase called the matrix. For example: polymer/ceramic composites have a greater modulus than the polymer component, but aren't as brittle as ceramics.                      The phases retain their physical identities on a macroscopic scale, which do not  dissolve in to one another completely, but they are perfectly bonded at the interfaces so that they complement each other in the action. These facts indicate that the composite material is physically non-homogenous at macroscopic level. On the other hand the phase of single metallic alloys combine at microscopic level, looses their physical identities and they have mechanical properties, more or less of same order.

ADVANTAGES OF COMPOSITES: -        
     The applications of composite materials in the field of cryogenic technology is consistently gaining reputation due to following chief advantages:
·         GOOD SPECIFIC STRENGTH
·         GOOD INSULATING PROPERTIES
·         HIGH CORROSION RESISTANCE
·         LONG DURABILITY
·         COST ADVANTAGES.

                    GOOD SPECIFIC STRENGTH: -  We know that specific strength is the ratio of strength to unit weight.We know that aluminum is the lightest material, which has high specific strength, so we use it, has aeroplane body. But the composites now developed have specific strength even less than that it is about 40 to 60% less than aluminum.
 GOOD INSULATING PROPERTIES: -
When we are dealing with low temperatures the main preventive measure we can follow is controlling flow of heat because refrigeration cost is proportional to the effectiveness of insulation.
   By using composite materials as supporting members in refrigerated units. We can minimize heat transfer such as using as supports for the super conductivity magnets or panels of refrigerants or walls of tanks etc.
HIGH CORROSION RESISTANCE: -
   Comparing to metals these have good corrosion resistance with the   floods and with ambient moisture. The first important thing is selecting the correct combination of composites and materials to be stored. If they are chemically stable then the corrosion resistance is very good.
LONG DURABILITY: -    
            The life of the composite material is highly sufficient for its performance in its applications. The trend is now changing; the conventional materials are replaced by composites, as they are reusable, which is particularly in applications of space shuttles.

COST ADVANTAGES: -
   Even it seems costly to manufacture composites material at now. But mass production may leads to provide some cost advantages such that they are economical than conventional metals.
HIGH SPECIFIC MODULUS :-
     Specific modulus may be defined as ratio of Youngs  Modulus (E) and density (p).
   The specific modulus is high for composite material which means the rod cross section of graphite/epoxy would only be one third of steel of same strength. This reduction in cross cross sectional area and mass translates to reduced space requirements an lower energy and material costs.
LIMITATIONS :-
 HIGH COST:-
                       High cost of fabrication of composites is a critical issue. For example, a part made of graphite/epoxy composite may cost up to 10 to 15 times the material costs. Improvement in processing and manufacturing techniques will lower these costs in future. Already manufacturing techniques such as SMC(sheet molding compound) and SRIM(structural reinforcement injection molding) are lowering the cost and production time in manufacturing automobile parts.

MECHANICAL CHARACTERISATIONS :
                      Mechanical characterization of a composite structure is more complex than of a metal structure. Unlike metals, composite materials are not isotropic, that is, their properties are not the same in all directions. So require more material parameters. In the case of a monolithic material such as steel, one requires only four stiffness and strength constants.


REPAIRING CAPABILITY:
                                       Repair of composites is not a simple process as compared to metals. Sometimes critical flaws and cracks in composite structures may go undetected.

MECHANICAL PERFORMANCE:
                       Composites do not have a high combination of strength and fracture toughness as compared to metals. Metals show an excellent combination of strength and fracture toughness as compared to composites.

ALROUND PERFORMANCE:-      
                        Composites do not necessarily give higher performance in all the properties used for material selection. Six primary material selection parameters are strength, toughness, and formability; join ability, corrosion resistance, and affordability. If the values at the circumference are considered as the normalized required property level for a particular application, certain areas will show values provided by ceramics, metals, and metal-ceramic composites. Clearly, composites show better strength than metals but lower values for other material selection parameters.