Composites
Issues to address:
What are the classes and types of composites?
Why are composites used instead of metals, ceramics, or polymers?
How do we estimate composite stiffness and strength?
What are some typical applications?
Definition of composite materials:
Is Considered to be any multiphase materials that exhibit a significant proportion of the properties of both constituent phases such that a better combination of properties is realized.
From definition
It contains two or more physically distinct and mechanically separable materials
It is made by dispersing one material in the other in a controlled way to achieve optimum properties
The properties of the composite are superior and possibly unique in some specific respects to the properties of individual components
<< Many composite materials are composed of just two phases one phase is matrix and the other is dispersed phase>>
Matrix :
The phase in a composite or two-phase alloy microstructure that is continuous or completely surrounds the other (or dispersed)phase.
Purpose is to:
Transfer stress to other phases, and protect phases from environment
Classification :MMC, CMC, PMC
Dispersed phase:
For composites and some two-phase alloys, the discontinuous phase that is surrounded by the matrix phase.
Purpose: enhance matrix properties.
MMC: increase sy, Ts, creep resistant.
CMC: increase Kc
PMC: increase E,sy, Ts, creep resistant
Classification: particle, fiber, structural
Note :
The properties of composites are a function of the properties of the constituent phases, their relative amounts, and the geometry of the dispersed phase.
Dispersed phase geometry means:
The shape of the particle
The particle size
Distribution and orientation
See fig. (16.1) in your text book.
Classification of the composite materials:
Particle- reinforced composites:
Large particle and dispersion-strengthened composites are the two subclassifications of particle-reinforced composites
Large particle composite: a type of particle-reinforced composite wherein particle-matrix interactions cannot be treated on an atomic or molecular level; the particle reinforced the matrix phase.
The degree reinforcement or improvement of mechanical behavior depends on strong bonding at the matrix- particle interface.
For dispersion-strengthened composites, particles are normally much smaller with diameters between 0.01 & 0.1 µm (10 & 100nm).
large-particle composite:
Some polymeric materials to which fillers have been added are really large- particle composites.
The fillers modify or improve the properties of the material.
Example of large-particle composite is concrete, which is composed of cement (the matrix), and sand and gravel (the particulates).
Particles can have quite a variety of geometries, but they should be of approximately the same dimension in all direction (equiaxed).
For effective reinforcement, the particles should be small and evenly distributed throughout the matrix.
The volume fraction of the two phases influences the behavior; mechanical properties are enhanced with increasing particulate content.
Rule of mixture: equation predict that the elastic modulus should fall between an upper and lower bound as shown:
Where:
Ec: elastic modulus of composite
Ep: elastic modulus of particle
Em: elastic modulus of matrix
Vm: volume fraction of matrix
Vp: volume fraction of particle
Example : fig.16.3 plots upper and lower bound Ec – versus Vp curves for a copper – tungsten composite; in which tungsten is the particulate phase.
Application to other properties:
Electrical conductivity,se:replace E by se .
Thermal conductivity: K: replace E by K.
Dispersion strengthened composite :
Dispersion-strengthened means of strengthening materials where in very small particles (usually less than 0.1 µm) of a hard yet inert phase are uniformly dispersed within a load – bearing matrix phase.
The dispersed phase may be metallic or nonmetallic, oxide materials are often used.
Fiber- reinforcement composite:
A composite in which the dispersed phase is in the form of a fiber (i.e., a filament that has a large length to diameter ratio).
Basic principles of reinforcement
Types of reinforcing elements
Forms of reinforcing elements
Direction of reinforcing
Load transferring ability
Volume fraction of reinforcements
Continuous vs. discontinuous reinforcement
Characteristics of fiber-reinforced Composites
Amount of fibers
Orientation of fibers
Types of fibers
Fiber aspect ratio
Fiber orientation effects
Strain rate effects
Types of matrix
Interfacial bonding conditions.
Types of fibers
Inorganic fibers
Glass
Graphite (carbon)
Silicon carbide
Organic fibers
Aramid
Thermoplastic
Metallic fibers
Stainless steel
Nickel alloy.