Laminar Composites - Prof. Supriya PDF

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Prof. Supriya...

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LAMINAR COMPOSITES • These can be described as materials comprising of layers of materials and bonded together. • These may be of several layers of two or more materials occurring alternately or in a sequential order more than once, and in as many numbers as required for a specific purpose • Laminar composites include thin coatings, cladded metal, Bimetallic, laminates etc. • Many laminar composites are designed to improve corrosion resistance retaining low cost, light weight and high strength. Other important characteristics include superior wear resistance, improved appearance and unusual thermal expansion characteristics. •

Laminar composites are extremely anisotropic and the properties vary significantly with orientation Two types of multi-layered constructions

Particulate composites The characteristics of particulates is dependent on the size and distribution of particles, type of matrix and particles and the interface bond strength. Smaller size and increased volume fraction lead to improved properties. Debonding and pull out of particles are invariably the reasons for poor bond strength. Particulate composites are popular in Metal Matrix and Ceramic Matrix composites. The particulates are classified as •

dispersoids (less than 10 mm)

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large particulates (greater than 10 mm). Dispersoid reinforced composites are mainly used for high temperature applications to provide creep strength and high temperature tensile strength. The dispersoids that act as obstacles to moving dislocations are the ones which are ideal.

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Large particle reinforced composites • Particle size is 1-50µm • Concentration usually ranges from 15-40% by volume • •

The particulate phase is harder and stiffer than the matrix. Particles provide strength to the composite.

Dispersion strengthened composites • • •

Uniformly dispersed fine, hard and inert particles of size less than 0.1µm are used as reinforcement These particles are stronger than the pure metal matrix and can be metallic, inter-metallic or nonmetallic. Matrix is the load bearing phase

Advantages of Particulate composites • Provide reinforcement to the matrix material • Improved material properties • Tailored material properties • •

Manufacturing flexibility High creep resistance

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High tensile strength at elevated temperatures High toughness

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High strength to weight ratio

Pre-pregs • • • • •

Pre-pregs are composite materials in which a reinforcement fiber is pre-impregnated with a thermoplastic or thermoset resin matrix in a certain ratio. Prepregs have unique properties as they are cured under high temperatures and pressures. Generally, the resin matrix in pre-pregs is partially cured for ease of handling and is stored in a cool place to prevent complete polymerization. In the Next stage pre-preg need to be heated in an autoclave or oven during manufacture of composite materials to achieve full polymerization. The reinforcement in a pre-preg can be unidirectional fibers Sandwich Construction

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Sandwich panels are used in applications where high stiffness combined with low structural weight is required

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Sandwich panels are based on a simple structure of two high strength skins bonded to either side of a light weight core

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This provides a very light and very stiff structure. The design of the sandwich structure ensures that the skins carry the large majority of the tensile and compressive stresses that are produced under load.

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