komposit
Post on 24-May-2015
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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 sometimesreferred 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 Materials1. 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
FibersFilaments 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 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 compositematerial 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
Computing composite propertiesDetermine the mechanical property of Continuous
and Aligned Fiber compositesLongitudinal 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 LoadingThe assumption of an isostress state :
Determine the mechanical property of Discontinuous and Aligned Fiber CompositesIf 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 = 0.1 -0.6.
EXAMPLE PROBLEMA 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 flakesCommercially, PMCs are more important
than MMCs or CMCsExamples: 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 fibersPolymer matrix materials:Usually a thermosetting (TS) plastic such as
unsaturated polyester or epoxyCan 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 PMCsVarious forms: discontinuous (chopped),
continuous, or woven as a fabricPrincipal fiber materials in FRPs are glass,
carbon, and Kevlar 49Less common fibers include boron, SiC, and
Al2O3, and steelGlass (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 phaseReinforcing phases:1. Particles of ceramic (these MMCs are
commonly called cermets)2. Fibers of various materials: other metals,
ceramics, carbon, and boron
Cemented CarbidesOne or more carbide compounds bonded in a
metallic matrixCommon 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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