223
8
during high-temperature curing due to the formation of water
vapours. The accumulation of dust and foreign particles on the surface of the reinforcement or inside the matrix reduces all of the
properties of composites.
8.2.3 Wrong Placement of Fibres
Tailoring of the in-plane stiffness of composite laminates can
improve the buckling and post-buckling characteristics by redistributing the applied loads. The fibre placement technique used is
the core technology underpinning variable stiffness composites.
The mismatch between designed and real fibre paths increases as
the width of a single tow and the number of tows laid at the same
time increase. This causes nonuniformity of the fibre orientation,
and some defects such as tow gap and overlap and irregularity of the
thickness distribution will occur in composites.
8.2.4 Improper Volume Fractions
Improper volume fractions lead to either resin-rich areas or resinstarved areas, and both of these situations are detrimental to the
performance of composites. For the best performance of the polymeric composites, equal fractions of volumes of reinforcement and
matrix must be kept.
8.2.5 Uncontrolled Temperature, Pressure,
and Time
One of the major challenges in the manufacturing of composites is the
efficient handling of the fibres and the application of uniform pressure
during the curing of the resin. Fibres are stiff in nature and do not
flow. For this reason, continuous fibres are either stacked in layers that
are successively impregnated with the resin or formed into a preform,
which is placed in a mould where resin is supplied under pressure to
wet the fibres. In addition, getting a high-volume fraction of fibres in
the composites is a major concern. Properties of composites depend
upon the appropriate curing of the matrix. The pressure, temperature,
component geometry, and distribution of pressure over the surface of
the component are some of the deciding factors for maintaining the
quality and reliability of the manufactured component.
The temperature gradient (T g ) through the thickness of composites occurs during their curing process. The insulating characteristic of composites restricts the heat transfer and increased level
of residual stresses from forming. However, this increases the risk
of fibre waviness and wrinkles, leading to a risk of defects in the
composite. Thick laminates are subjected to combined effects of
cure exotherm, leading to high stresses in off-axis plies, which
increases the risk of waviness and wrinkles in the plies (especially
with complex shapes).
8.2 · Processing-Induced Defects in Polymeric Composites
8
during high-temperature curing due to the formation of water
vapours. The accumulation of dust and foreign particles on the surface of the reinforcement or inside the matrix reduces all of the
properties of composites.
8.2.3 Wrong Placement of Fibres
Tailoring of the in-plane stiffness of composite laminates can
improve the buckling and post-buckling characteristics by redistributing the applied loads. The fibre placement technique used is
the core technology underpinning variable stiffness composites.
The mismatch between designed and real fibre paths increases as
the width of a single tow and the number of tows laid at the same
time increase. This causes nonuniformity of the fibre orientation,
and some defects such as tow gap and overlap and irregularity of the
thickness distribution will occur in composites.
8.2.4 Improper Volume Fractions
Improper volume fractions lead to either resin-rich areas or resinstarved areas, and both of these situations are detrimental to the
performance of composites. For the best performance of the polymeric composites, equal fractions of volumes of reinforcement and
matrix must be kept.
8.2.5 Uncontrolled Temperature, Pressure,
and Time
One of the major challenges in the manufacturing of composites is the
efficient handling of the fibres and the application of uniform pressure
during the curing of the resin. Fibres are stiff in nature and do not
flow. For this reason, continuous fibres are either stacked in layers that
are successively impregnated with the resin or formed into a preform,
which is placed in a mould where resin is supplied under pressure to
wet the fibres. In addition, getting a high-volume fraction of fibres in
the composites is a major concern. Properties of composites depend
upon the appropriate curing of the matrix. The pressure, temperature,
component geometry, and distribution of pressure over the surface of
the component are some of the deciding factors for maintaining the
quality and reliability of the manufactured component.
The temperature gradient (T g ) through the thickness of composites occurs during their curing process. The insulating characteristic of composites restricts the heat transfer and increased level
of residual stresses from forming. However, this increases the risk
of fibre waviness and wrinkles, leading to a risk of defects in the
composite. Thick laminates are subjected to combined effects of
cure exotherm, leading to high stresses in off-axis plies, which
increases the risk of waviness and wrinkles in the plies (especially
with complex shapes).
8.2 · Processing-Induced Defects in Polymeric Composites