261
9
The smaller the size of the reinforcement, the more the physical
barrier is present, affecting the translational movement of polymeric chains. Also, the reinforcement acts as a heat-soaking
medium; therefore, it increases the performance of the composite at
high temperatures. For example, in the ablation process, discussed
earlier, the fibres provide a medium of heat soaking, and the matrix
gets charred into a perforated mass like open cell foam. This cellular
structure of charred matrix along with fibre reinforcement protects
the composite structure from extreme heat in ablative composites.
9.5.3 Influence on Mechanical Properties
The mechanical properties are also greatly affected by the structure
of composites. The structure of composites is guided by properties
of reinforcement, matrices, and their interphases. For example, the
rigidity and strength of Kevlar fibres are provided by the presence of
a rigid benzene ring that is also responsible for the fibre’s high melting temperature. Whereas nylon, which is aliphatic polyamide, has
a lower strength and melting point compared to Kevlar. The factors
of composite structure that influence mechanical properties are
given here.
5 Interphase
5 Volume fractions
5 Dimensions of reinforcement
5 Uniformity of reinforcement
5 Voids and air gaps
5 Morphology
5 Crystallization (for thermoplastic matrices )
5 Cross-link density and its uniformity (for thermosetting
matrices)
The prediction of performance depends upon the service environment. It has been always a challenge to predict the performance of
polymeric composites. Generally, polymeric composites undergo
catastrophic failures due to the brittleness of the material. Creep
and stress relaxation are the main deformation mechanisms in
polymeric composites.
Creep is a common phenomenon in polymers and also in polymeric composites. Creep is a permanent deformation under stress
over a period of timethat is caused by the delayed response of the
polymeric chains in polymers. The deformation stops once the
polymeric chains reach a new equilibrium. Creep is also dependent
on temperature. Polymeric composites creep faster at high temperatures.
Stress relaxation is another characteristic of polymeric materials
and is a consequence of delayed molecular motions (as in creep) .
Contrary to creep, stress relaxation is a reduction in the force
required to maintain a constant deformation under constant strain.
This is also a property of composite structures in which both reinforcement and matrix participate.
9.5 · Structure–Property Relationship
9
The smaller the size of the reinforcement, the more the physical
barrier is present, affecting the translational movement of polymeric chains. Also, the reinforcement acts as a heat-soaking
medium; therefore, it increases the performance of the composite at
high temperatures. For example, in the ablation process, discussed
earlier, the fibres provide a medium of heat soaking, and the matrix
gets charred into a perforated mass like open cell foam. This cellular
structure of charred matrix along with fibre reinforcement protects
the composite structure from extreme heat in ablative composites.
9.5.3 Influence on Mechanical Properties
The mechanical properties are also greatly affected by the structure
of composites. The structure of composites is guided by properties
of reinforcement, matrices, and their interphases. For example, the
rigidity and strength of Kevlar fibres are provided by the presence of
a rigid benzene ring that is also responsible for the fibre’s high melting temperature. Whereas nylon, which is aliphatic polyamide, has
a lower strength and melting point compared to Kevlar. The factors
of composite structure that influence mechanical properties are
given here.
5 Interphase
5 Volume fractions
5 Dimensions of reinforcement
5 Uniformity of reinforcement
5 Voids and air gaps
5 Morphology
5 Crystallization (for thermoplastic matrices )
5 Cross-link density and its uniformity (for thermosetting
matrices)
The prediction of performance depends upon the service environment. It has been always a challenge to predict the performance of
polymeric composites. Generally, polymeric composites undergo
catastrophic failures due to the brittleness of the material. Creep
and stress relaxation are the main deformation mechanisms in
polymeric composites.
Creep is a common phenomenon in polymers and also in polymeric composites. Creep is a permanent deformation under stress
over a period of timethat is caused by the delayed response of the
polymeric chains in polymers. The deformation stops once the
polymeric chains reach a new equilibrium. Creep is also dependent
on temperature. Polymeric composites creep faster at high temperatures.
Stress relaxation is another characteristic of polymeric materials
and is a consequence of delayed molecular motions (as in creep) .
Contrary to creep, stress relaxation is a reduction in the force
required to maintain a constant deformation under constant strain.
This is also a property of composite structures in which both reinforcement and matrix participate.
9.5 · Structure–Property Relationship