Defining Composites (long)

Here's a short article published in CF magazine on the subject of defining
composites... it might help tie together some of the previous R.A.S. discussion
on the topic.

Bob
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Defining Composites

What are composites? As elementary as the question may seem to those in the
"composites" business, the exact definition of composites is somewhat elusive.

In the convolutions of the English language the term "composite" is used in a
number of contexts. For example, there is the field of composite photography,
where two or more photographs are joined to form a single image. The Air Force
operates composite wings, which are units made up of various types of aircraft.
Botanists speak of compositae, a plant family that includes dandelions.
Mathematicians define a composite number as an integer exactly divisible by at
least one number other than itself. And then of course there are composite
materials.

The term composite is derived from the Latin compositus, stemming from the root
word componere, to bring together: com - together + ponere - to put (to put
together).1 This accurately describes the application that puts together
materials to form a composite.

Defining composites as an engineering material requires a progressive
definition, which begins with the general and moves to the specific. The broad
general definition of a composite is: "Two or more dissimilar materials which
when combined are stronger than the individual materials"2

This definition draws attention to the synergistic effects of combining
materials that have different properties, to create a new material with
superior properties to the individual components. This definition can apply to
both natural and synthetic (manmade) composites: Wood is an example of a
natural composite that falls into the broadest definition of composites. Wood
is a combination of cellulose fiber and lignin. The cellulose fiber provides
strength and the lignin is the "glue" that bonds and stabilizes the fiber.

Manmade composites can be constructed using natural materials. Adobe bricks are
a perfect example of a composite material; the combination of mud and straw
forms a material that is stronger than either the mud or the straw.

There are many forms of synthetic composites. Steel and concrete combine to
create structures that are strong and rigid. In this case the synergy results
from the high stiffness and compression strength of the concrete, and the high
tensile strength of steel, creating a structure that is strong and stiff. A
very different composite is automobile tires. A steel belted radial tire uses
rubber as a strong but flexible matrix to encapsulate steel strands which have
high tensile strength.

This broad definition however, is too general to describe the specialized form
of materials from which the composites industry takes its name. A definition
is required that adequately segregates these structural materials from other
engineering materials. Brent Strong uses this definition in his book,
"Fundamentals of Composites Manufacturing" - The combination of a reinforcement
material (such as a particle or fiber) in a matrix or binder material.3 Dr.
Strong points out, "That the term composite also implies that the materials are
macroscopically identifiable, that is, the materials are not merely different
at the molecular level but have distinctive component properties and they are
generally mechanically separable. This definition excludes many materials,
which might have been included in the broader definition such as: metal alloys,
plastic copolymers, minerals, glasses, and wood."

To hone a finer edge, the definition must be developed to the next level. One
must examine the engineering properties of the component materials that form a
composite. Considering that a composite is a combination of reinforcement in a
matrix, it becomes necessary to define the terms reinforcement and matrix.

In engineering terms, one of the functions of a reinforcement in a composite is
to take up the load strain transferred through the matrix. The load must then
be distributed throughout the matrix and reinforcements.

Particles generally have a low aspect ratio (a comparison of length to width)
and are roughly spherical in shape. Generally referred to as fillers, particles
consist of both organic and inorganic materials. The most common particles
found in plastic materials are, calcium carbonate (limestone), calcium sulfate,
and alumina trihydrate. Additionally, hollow and solid spheres of glass or
other materials may be used as fillers.

Particles in a matrix (such as a resin) produce isotropic properties. That is,
the material will have the same tensile, compression and elongation properties
in the X, Y and Z-axis. A particle filled matrix will be homogenous (the same
throughout), as are metals. Particles however, are not effective as
reinforcements. By virtue of having low aspect geometry (rough spherical
shapes), they do not effectively transfer loads from particle to particle.
Therefore, particles are not considered, or referred to, as reinforcements in
composites materials.

Fibers are reinforcements, having one long axis compared and one short axis. -
a high aspect ratio. In the matrix, fibers overlap to a degree that strain
within the matrix is transferred to a series of fibers. Where fibers overlap,
the load is distributed to adjacent reinforcements, with the matrix holding the
fibers in place and transferring the strain from fiber to fiber. By nature of
having a high aspect ratio (long and narrow), fibers may be oriented in a
specific direction and have the capability to produce a material with
aniso-tropic properties; a material that is stronger in one direction than the
other. Fibers can be used to produce a non-homogeneous structure, which has
different properties in different directions. This is a distinct advantage in
an engineering material.

The function of the matrix in a composite is to provide a relatively rigid
media that is capable of transferring loads to the fiber components of the
material. The matrix encapsulates the reinforcement creating the physical
properties synergy between the two materials. In forming "composites" a
critical aspect in the amalgamation of the matrix and the reinforcement is that
a chemical bond is formed between matrix and the reinforcement.

To explore this concept further, consider the combination of a thermoplastic
resin (such as polypropylene) and glass fiber; in this case the reinforcement
fiber is merely encapsulated by the resin matrix but not molecularly bonded to
the resin. Whereas, the combination of glass fiber and thermoset polyester
resin produces a chemical bond at the interface of the fiber and the resin.
Therefore, we arrive at one of the distinctive characteristics of a composite
engineering material - the reinforcement is not merely encapsulated by the
matrix, but is molecularly bonded to the matrix. The bonding of reinforcement
and matrix produces the superior physical properties, chemical resistance and
fatigue endurance, which characterize composite materials.

Now, having the technical aspects of composites characterization in hand, the
definition of these materials can move to the final step of refinement.

"Composites are a combination of a reinforcement fiber in a polymer resin
matrix, where the reinforcement has an aspect ratio that enables the transfer
of loads between fibers, and the fibers are chemically bonded to the resin
matrix." 4
This precise definition accounts for the attributes of thermoset composites as
an engineering material, and differentiates them from a host of combined
materials having lesser degrees of synergy between the individual components.v


Author:
Robert R. Lacovara, CCT-I
Technical Director
Composites Fabricators Association


References:
1 "College Dictionary", Houghton Mifflin Company, 1986
2 "Certified Composites Technician Study Guide", Lacovara, CFA, 1999
3 "Fundamentals of Composites Manufacturing", Strong, SME, 1989
4 "Defining Composites", Lacovara, CFA 2000

Other References:
"Certified Composites Technician Instructors Guide", Lacovara, CFA, 2000
"Industrial Plastics", Richardson and Lokensgard, Delmar
Publishers, 1996
"Introduction to Composites", SPI, 1992
"Composite Basics", Marshall, Marshall Publishing, 1994
"Reinforced Plastics Handbook", Murphy, Elsevier Science
Publishers, 1994
"Composite Materials", Schwartz, Prentice Hall Inc., 1997

In article ,
says...
Here's a short article published in CF magazine on the subject of defining
composites... it might help tie together some of the previous R.A.S. discussion
on the topic.

Bob
------------------------------------------------------------
Defining Composites

What are composites? As elementary as the question may seem to those in the
"composites" business, the exact definition of composites is somewhat elusive.

snip

Thanks, Bob. I was hoping a practioner in the field could give us some
guidelines. The ras discussion was becoming somewhat unfocused, as it
spread from FRP materials to wood, plywood, and even corrugated
cardboard!
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)