Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 37

• Warping
• Waterabsorption
• Frictionandwear
• Fatigue
• Aestheticproperties
3.3.1 Density
Density is the mass (weight) per unit volume of a material or product. It is commonly
expressed as g/cm 3 in the metric system but the appropriate SI-derived unit of density
is kg/m 3 . Density is an important property for composites. For example, lightweight
composites play a key part in applications in aircraft, aerospace and automobile
applications, whereas denser composites are required for outdoor applications such
as decking. The relative density of a plastic is the density of a given substance divided
by the density of water. In the production of polymeric composites, addition of fillers
will increase density but composite designers may also use foaming agents and other
additives to decrease density.
3.3.2 Compressive Strength
Compressive strength is the amount of force required to crush or rupture a material.
Values are expressed as multiples of the unit Pascal, such as kilopascal (kPa) or larger
units. Compressive strength is calculated by dividing the maximum load (force) in
the unit Newton by the area of the specimen being tested (in square metres). Values
of compressive strength may be useful for distinguishing between different plastics
to be used as matrices. Such values may be especially useful for the testing of filled
or foamed composite products.
3.3.3 Impact Strength
In practice, nearly all polymer composites are subject to impact loads. Impact strength
is the resistance of any material to withstand constant pressure under strain is and the
point before fracture. Many polymers are tough and ductile, so they are very often
well suited for this type of loading. The impact strengths of composites made from
copolymers and polymer blends can be quite different. For example, composites using