material komposit

Composite Material Defined

• A materials system composed of two or more physically distinct phases whose combination produces aggregate properties that are different from those of its constituents

Advantages

• Composites can be very strong and stiff, yet very light in weight, so ratios of strength-to-weight and stiffness-to-weight are several times greater than steel or aluminum

• Fatigue properties are generally better than for common engineering metals

• Toughness is often greater too

• Composites can be designed that do not corrode like steel

• Possible to achieve combinations of properties not attainable with metals, ceramics, or polymers alone

Components in a Composite Material Nearly all composite materials consist of two phases:

• 1. Primary phase - forms the matrix within which the secondary phase is imbedded

• 2. Secondary phase - imbedded phase sometimes

referred to as a reinforcing agent, because it usually serves to strengthen the composite

The reinforcing phase may be in the form of fibers,particles, or various other geometries

Classification Scheme for Composite Materials

1. Metal Matrix Composites (MMC) - mixtures of ceramics and metals, such as cemented carbides and other cermets

2. Ceramic Matrix Composites (CMC) – Al2O3 and SiC imbedded with fibers to improve properties, especially in high temperature applications

The least common composite matrix

3. Polymer Matrix Composites (PMC) - thermosetting resins are widely used in PMC Examples: epoxy and polyester with fiber reinforcement,and phenolic with powders

Functions of the Matrix Material(Primary Phase)

Provides the bulk form of the part or product made of the composite material

Holds the imbedded phase in place, usually enclosing and often concealing it

When a load is applied, the matrix shares the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent

The Reinforcing Phase (Secondary Phase)

• Function is to reinforce the primary phase

• Imbedded phase is most commonly one of the following shapes:

Fibers

Particles

Flakes

• In addition, the secondary phase can take the form of an infiltrated phase in a skeletal or porous matrix

Example: a powder metallurgy part infiltrated with polymer

Fibers

Filaments of reinforcing material, usually circular incross-section

Diameters range from less than 0.0025 mm to about 0.13 mm, depending on material

Filaments provide greatest opportunity for strength enhancement of composites

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The filament form of most materials is significantly stronger than the bulk form

As diameter is reduced, the material becomes oriented in the fiber axis direction and probability of defects in the structure decreases significantly

Continuous vs. Discontinuous Fibers

Continuous fibers - very long; in theory, they offer a continuous path by which a load can be carried by the composite part

Discontinuous fibers (chopped sections of continuous fibers) short lengths (L/D = roughly 100)

Important type of discontinuous fiber are whiskers –hair like single crystals with diameters down to about 0.001 mm (0.00004 in.) with very high strength

Fiber Orientation – Three Cases

One-dimensional reinforcement, in which maximum strength and stiffness are obtained in the direction of the fiber

Planar reinforcement, in some cases in the form of a two-dimensional woven fabric

Random or three-dimensional in which the composite

material tends to possess isotropic properties

Particles and Flakes

A second common shape of imbedded phase is particulate, ranging in size from microscopic to macroscopic

Flakes are basically two-dimensional particles - small flat platelets

The distribution of particles in the composite matrix is random, and therefore strength and

other properties of the composite material are usually isotropic

Strengthening mechanism depends on particle size

The Interface

There is always an interface between constituent phases in a composite material

For the composite to operate effectively, the phases must bond where they join at the interface

Interphase

In some cases, a third ingredient must be added to achieve bonding of primary and secondary phases

Called an interphase, this third ingredient can be thought of as an adhesive

Properties are Determined by Three Factors:

1. The materials used as component phases in the composite

2. The geometric shapes of the constituents and resulting structure of the composite system

3. The manner in which the phases interact with one another

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Computing composite properties

Determine the mechanical property of Continuous and Aligned Fiber composites

Longitudinal direction

Fc =Fm + Ff

The assumption of an isostrain state :

The ratio of the load carried by the fibers to that carried by the matrix is

Transverse Loading

The assumption of an isostress state :

Determine the mechanical property of Discontinuous and Aligned Fiber Composites

If the fiber length (l) is less than critical(lc), the longitudinal strength

If (l >lc), the longitudinal strength

Where

Determine the mechanical property of Discontinuous and Randomly Oriented Fiber Composites

Where K = fiber efficiency parameter

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= 0.1 -0.6.

EXAMPLE PROBLEM

A continuous and aligned glass fiber-reinforced composite consists of 40 vol% of glass fibers having a modulus of elasticity of 69 GPa and 60 vol% of a polyester resin that, when hardened, displays a modulus of 3.4 GPa.

(a) Compute the modulus of elasticity of this composite in the longitudinaldirection.

(b) If the cross-sectional area is 250 mm2 and a stress of 50 MPa is applied in this longitudinal direction, compute the magnitude of the load carried by each of the fiber and matrix phases.

(c) Determine the strain that is sustained by each phase when the stress inpart (b) is applied.

Polymer Matrix Composites (PMCs)

A polymer primary phase in which a secondary phase is

imbedded as fibers, particles, or flakes

Commercially, PMCs are more important than MMCs or CMCs

Examples: most plastic molding compounds, rubber reinforced with carbon black, and fiber-reinforced polymers (FRPs)

Fiber-Reinforced Polymers (FRPs)

A PMC consisting of a polymer matrix imbedded with high-strength fibers

Polymer matrix materials:

Usually a thermosetting (TS) plastic such as unsaturated polyester or epoxy

Can also be thermoplastic (TP), such as nylons (polyamides), polycarbonate, polystyrene, and polyvinylchloride

Fiber reinforcement is widely used in rubber products such

as tires and conveyor belts

Fibers in PMCs

Various forms: discontinuous (chopped), continuous, or woven as a fabric

Principal fiber materials in FRPs are glass, carbon, and Kevlar 49

Less common fibers include boron, SiC, and Al2O3, and steel

Glass (in particular E-glass) is the most common fiber material in today's FRPs;

Metal Matrix Composites (MMCs)

A metal matrix reinforced by a second phase

Reinforcing phases:

1. Particles of ceramic (these MMCs are commonly called

cermets)

2. Fibers of various materials: other metals, ceramics, carbon, and boron

Cemented Carbides

One or more carbide compounds bonded in a metallic matrix

Common cemented carbides are based on tungsten carbide(WC), titanium carbide (TiC), and chromium carbide(Cr3C2)

Tantalum carbide (TaC) and others are less common

Metallic binders: usually cobalt (Co) or nickel (Ni)

Ceramic Matrix Composites (CMCs)

A ceramic primary phase imbedded with a secondary phase, which usually consists of fibers

Attractive properties of ceramics: high stiffness, hardness, hot hardness, and compressive strength; and relatively low density

Weaknesses of ceramics: low toughness and bulk tensilestrength, susceptibility to thermal cracking

CMCs represent an attempt to retain the desirable properties of ceramics while compensating for their weaknesses

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