258
9
uniform stressing. These modifications are due to strain in the presence of local flaws (defects) under the surface of the composite. The
visible deformation due to the generation of stresses occurs by illuminating the composites with laser light. All visual information on
the illuminated composite surface is stored as a hologram on a photographic plate. The visibility of deformation on the composites
provides information on flaws and defects.
One of the difficulties of holographic nondestructive testing is
finding the method for stressing that is able to produce singularities
in the strain state at the surface of composite being tested. This is a
useful method for detecting local defects (such as flow marks,
delaminations, voids, and broken reinforcement) in composites.
9.5 Structure–Property Relationship
Mechanical properties of polymer composites can be optimized by
studying their structure–property relationships. Composites are
characterized by the proportions of each phase, the shape of the reinforcement, and their arrangement in space. The phases of a composite often have different transport or related properties (i.e. thermal or
electrical conductivity, gas permeability, modulus of elasticity). A
given property of a composite is a function of the structure of the
composite and the corresponding property of each component.
Structure–property analytical relationships are interlinked [28].
9.5.1 Influence on Thermal Properties
The structure of a material influences its thermal properties greatly.
The glass transition tempeature (T g ), melting point curing temperature, degradation temperature, etc. are influenced by the structure
of the matrix, fibres, and their interphase. For example, an aromatic
polyamide, such as Kevlar, has a very high melting point and high
strength compared to an aliphatic polyimide, such as nylon 6 or
nylon 6,6. In Kevlar, much of its thermal stability is provided by a
rigid benzene ring. The melting point of Kevlar is 650 °C, while the
melting point of nylon 6 is 240 °C. The chemical molecules in any
material used for the reinforcement and the matrix are arranged in
a special fashion, which is called their stereochemistry. The stereochemistry is responsible at a microlevel for any localized motion of
molecules. The glass transition temperature (T g ) is highly dependent on the stereo arrangement of groups in the molecules. The
temperature required for any translational movement of molecular
segments is dependent upon how strongly the molecules in the
polymers are arranged. For regular arrangements of the molecules,
the heat energy required to initiate translational movement will be
higher than the polymeric molecules that are not arranged in a
regular fashion. For example, the well arranged PP molecules,
which are crystalline and called isotactic PP, have a T g of 0 °C,
whereas atactic PP molecules, which have no regular arrangement,
are completely amorphous and have a T g of −20 °C. This behaviour
Chapter 9 · Characterization and Testing of Polymeric Composites
9
uniform stressing. These modifications are due to strain in the presence of local flaws (defects) under the surface of the composite. The
visible deformation due to the generation of stresses occurs by illuminating the composites with laser light. All visual information on
the illuminated composite surface is stored as a hologram on a photographic plate. The visibility of deformation on the composites
provides information on flaws and defects.
One of the difficulties of holographic nondestructive testing is
finding the method for stressing that is able to produce singularities
in the strain state at the surface of composite being tested. This is a
useful method for detecting local defects (such as flow marks,
delaminations, voids, and broken reinforcement) in composites.
9.5 Structure–Property Relationship
Mechanical properties of polymer composites can be optimized by
studying their structure–property relationships. Composites are
characterized by the proportions of each phase, the shape of the reinforcement, and their arrangement in space. The phases of a composite often have different transport or related properties (i.e. thermal or
electrical conductivity, gas permeability, modulus of elasticity). A
given property of a composite is a function of the structure of the
composite and the corresponding property of each component.
Structure–property analytical relationships are interlinked [28].
9.5.1 Influence on Thermal Properties
The structure of a material influences its thermal properties greatly.
The glass transition tempeature (T g ), melting point curing temperature, degradation temperature, etc. are influenced by the structure
of the matrix, fibres, and their interphase. For example, an aromatic
polyamide, such as Kevlar, has a very high melting point and high
strength compared to an aliphatic polyimide, such as nylon 6 or
nylon 6,6. In Kevlar, much of its thermal stability is provided by a
rigid benzene ring. The melting point of Kevlar is 650 °C, while the
melting point of nylon 6 is 240 °C. The chemical molecules in any
material used for the reinforcement and the matrix are arranged in
a special fashion, which is called their stereochemistry. The stereochemistry is responsible at a microlevel for any localized motion of
molecules. The glass transition temperature (T g ) is highly dependent on the stereo arrangement of groups in the molecules. The
temperature required for any translational movement of molecular
segments is dependent upon how strongly the molecules in the
polymers are arranged. For regular arrangements of the molecules,
the heat energy required to initiate translational movement will be
higher than the polymeric molecules that are not arranged in a
regular fashion. For example, the well arranged PP molecules,
which are crystalline and called isotactic PP, have a T g of 0 °C,
whereas atactic PP molecules, which have no regular arrangement,
are completely amorphous and have a T g of −20 °C. This behaviour
Chapter 9 · Characterization and Testing of Polymeric Composites