199
7
providing support to 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, that occur in composites.
7.2.4 Improper Volume Fractions
Improper volume fractions lead to either resin-rich areas or resinstarved areas. Both of these situations are detrimental to the performance of composites. For the best performance of polymeric
composites, equal fractions of volumes of reinforcement and matrix
must be kept.
7.2.5 Uncontrolled Temperature, Pressure,
and Time
One of the major challenges in manufacturing of composites is the
efficient handling of the fibres and the application of uniform pressure during the curing of the resin. The 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 that is placed in a mould where the 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. The 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 determining the quality and reliability of the
manufactured component.
A temperature gradient through the thickness of the composite
does occur during the curing process. The insulating character of
composites restricts the heat transfer and increases the level of
residual stresses that are formed in the composite. 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
exotherm from curing, leading to high stresses in off-axis plies and
increased risk of waviness and wrinkles in the plies (specially with
complex shapes).
7.2.6 Improper Closing of the Mould
Improper closing of the mould leads to dimensional instability of
the cured product. It also may lead to the resin oozing out upon
curing. The oozing resin can lead to an imbalance in volume fractions of both the reinforcement and the matrix, causing the
composite to have low mechanical properties.
7.2 · Processing-Induced Defects in Polymeric Composites
7
providing support to 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, that occur in composites.
7.2.4 Improper Volume Fractions
Improper volume fractions lead to either resin-rich areas or resinstarved areas. Both of these situations are detrimental to the performance of composites. For the best performance of polymeric
composites, equal fractions of volumes of reinforcement and matrix
must be kept.
7.2.5 Uncontrolled Temperature, Pressure,
and Time
One of the major challenges in manufacturing of composites is the
efficient handling of the fibres and the application of uniform pressure during the curing of the resin. The 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 that is placed in a mould where the 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. The 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 determining the quality and reliability of the
manufactured component.
A temperature gradient through the thickness of the composite
does occur during the curing process. The insulating character of
composites restricts the heat transfer and increases the level of
residual stresses that are formed in the composite. 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
exotherm from curing, leading to high stresses in off-axis plies and
increased risk of waviness and wrinkles in the plies (specially with
complex shapes).
7.2.6 Improper Closing of the Mould
Improper closing of the mould leads to dimensional instability of
the cured product. It also may lead to the resin oozing out upon
curing. The oozing resin can lead to an imbalance in volume fractions of both the reinforcement and the matrix, causing the
composite to have low mechanical properties.
7.2 · Processing-Induced Defects in Polymeric Composites