Acoustic Emission Monitoring of CFRP Laminated Composites ...

14 Chapter 2. Carbon Fibre-Reinforced Polymer Composites
Lay-ups are commonly consolidated in autoclaves using vacuum bagging,
which is used to compress the part and squeeze out excess resin. This
reduces the resin content, removes entrapped air, and as a result makes the
part both lighter and stronger. During vacuum bagging, layers can slide
and hence alter the design. This process depends on a large number of variables,
including the mould geometry, locations of the resin outlets, resin
temperature, vacuum pressure, and curing. The last three are functions of
both time and location and must be repeated exactly the same way every
time. Also, the moulds must be designed in order to ensure correct vacuum
pressure and resin ow throughout the part. During the curing of the
resin it warms up and its volume reduces due to polymerization shrinkage
but the bres are unaected. This generates internal stresses between
the matrix and the bres. 54
When the composite cools down, additional
stresses are generated because the matrix and the bres have a dierent coecient
of thermal expansion. 54
The stresses generated are called residual
stresses. The residual stresses aect several properties of the composite,
e.g. strength, fatigue and chemical resistance. The stresses can be reduced
by extending the curing time.
The nal nishing touches include cutting/drilling, grinding and assembling.
These operations can easily cause damage to the composite.
The most common reason for damage is forces that are applied perpendicularly
to the direction of the bres. Composites, however, have very
little strength in that direction. 55
When the cutting tool used for the
cutting/drilling operations enters and exits the material it can induce delamination.
56
Furthermore, localized defects such as bre pullouts, and
55, 57, 58
burning of the resin can also be initiated. The grinding operations
can result in a higher temperature than the curing temperature; hence,
the material property may be degraded. 59
Low-velocity impacts such as
dropping the composite part, dropping tools on the part, and mechanically
hitting the composite during assembly can initiate delamination, break -
46, 58, 6063
bres, and generate cracks. Furthermore, mechanical joining of
composite parts, such as bolting, will produce stress concentrations which
can initiate damage growth. 64 Hence, improper selection of washer sizes 65
and over-torquing 58
can cause detrimental local damage around the bolt
holes. Cut or broken bres weaken the composite. Fibre failure can be
attributed to improper handling, imperfections, and both tensile and compressive
stresses. Stresses can develop around the area where the bre