Composite Structures
Components of Composite Materials
• Matrix phase: bulk materials such as:
Metals
Ceramics
Polymers
• Reinforcement: fibers and particulates such
as:
Glass
Carbon
Kevlar
Silicon Carbide Boron
Ceramic
Ceramic
Metallic
Aggregate
• Interface: area of mechanical
A composite material is basically
a combination of two or more
materials, each of which retains
it own distinctive properties.
Multiphase metals are composite
materials on a micro scale, but
generally the term composite is
applied to materials that are
created by mechanically bonding
two or more different materials together. The resulting material has
characteristics that are not characteristic of the components in isolation. The
concept of composite materials is ancient. An example is adding straw to mud for
building stronger mud walls. Most commonly, composite materials have a bulk
phase, which is continuous, called the matrix; and a dispersed, non-continuous,
phase called the reinforcement. Some other examples of basic composites
include concrete (cement mixed with sand and aggregate), reinforced concrete
(steel rebar in concrete), and fiberglass (glass strands in a resin matrix).
In about the mid 1960’s, a new group of composite materials, called advanced
engineered composite materials (aka advanced composites), began to emerge.
Advanced composites utilize a combination of resins and fibers, customarily
carbon/graphite, kevlar, or fiberglass with an epoxy resin. The fibers provide the
high stiffness, while the surrounding polymer resin matrix holds the structure
together. The fundamental design concept of composites is that the bulk phase
accepts the load over a large surface area, and transfers it to the reinforcement
material, which can carry a greater load. The significance here lies in that there
are numerous matrix materials and as many fiber types, which can be combined
in countless ways to produce just the desired properties. These materials were
first developed for use in the aerospace industry because for certain application
they have a higher stiffness to weight or strength-to-weight ratio than metals.
This means metal parts can be replaced with lighter weight parts manufactured
from advanced composites. Generally, carbon-epoxy composites are two thirds
the weight of aluminum, and two and a half times as stiff. Composites are
resistant to fatigue damage and harsh environments, and are repairable.
Composites meeting the criteria of having mechanical bonding can also be
produced on a micro scale. For example, when tungsten carbide powder is mixed
with cobalt powder, and then pressed and sintered together, the tungsten carbide
retains its identity. The resulting material has a soft cobalt matrix with tough
tungsten carbide particles inside. This material is used to produce carbide drill
bits and is called a metal-matrix composite. A metal matrix composite is a type of
metal that is reinforced with another
Composites
A composite is commonly defined as a combination of two or more distinct
materials, each of which retains its own distinctive properties, to create a new
material with properties that cannot be achieved by any of the components acting
alone. Using this definition, it can be determined that a wide range of engineering
materials fall into this category. For example, concrete is a composite because it
is a mixture of Portland cement and aggregate. Fiberglass sheet is a composite
since it is made of glass fibers imbedded in a polymer.
Composite materials are said to have two phases. The reinforcing phase is the
fibers, sheets, or particles that are embedded in the matrix phase. The
reinforcing material and the matrix material can be metal, ceramic, or polymer.
Typically, reinforcing materials are strong with low densities while the matrix is
usually a ductile, or tough, material.
Some of the common classifications of composites are:

Reinforced plastics

Metal-matrix composites

Ceramic-matrix composites

Sandwich structures

Concrete
Composite materials can take many forms but they can be separated into three
categories based on the strengthening mechanism. These categories are
dispersion strengthened, particle reinforced and fiber reinforced. Dispersion
strengthened composites have a fine distribution of secondary particles in the
matrix of the material. These particles impede the mechanisms that allow a
material to deform. (These mechanisms include dislocation movement and slip,
which will be discussed later). Many metal-matrix composites would fall into the
dispersion strengthened composite category. Particle reinforced composites
have a large volume fraction of particle dispersed in the matrix and the load is
shared by the particles and the matrix. Most commercial ceramics and many
filled polymers are particle-reinforced composites. In fiber-reinforced composites,
the fiber is the primary load-bearing component. Fiberglass and carbon fiber
composites are examples of fiber-reinforced composites.
If the composite is designed and fabricated correctly, it combines the strength of
the reinforcement with the toughness of the matrix to achieve a combination of
desirable properties not available in any single conventional material.