Acoustic Emission Monitoring of CFRP Laminated Composites ...

12 Chapter 2. Carbon Fibre-Reinforced Polymer Composites
these standards are based on quasi-static loading of coupons. Quasi-static
loading does not provide adequate characterization of the composite material
under dynamic loading because the polymer responds dierently to low
and high strain rates. 4244
Random defects can be introduced during the
production and handling of bres, so the material property parameters, like
all other experimentally obtained data, follow probability distributions.
2.2.2 Defects in Laminated Composites
Careful quality control is important throughout the manufacturing process
of a CFRP composite, from lay-up to nal nishing. This is because
of the large number of defects which can be generated and other factors
which can aect the mechanical properties of the composite. During the
lay-up process, it is important to remove wrinkles when new layers are
added. Wrinkles (Fig. 2.3) can cause air entrapment and resin build-up. 45
They can therefore weaken the composite and render the design useless.
Air entrapment, sometimes called voids, can occur between the bre lay-
Figure 2.2
Shows what happens when air gets trapped between layers.
ers (Fig. 2.2) because air gets trapped between layers during the lay-up process.
According to Schwartz 26
voids and porosity can be caused by volatile
entrapment during resin curing. Schwartz also remarks that if they are only
partially entrapped then blisters are generated. Blisters are generated in
the outermost layers. Voids and blisters weaken the composite and can
also induce the formation of other types of damage. Foreign objects that
get trapped between layers weaken the composite (Fig. 2.3). Examples of
foreign inclusion are: oily residues left on the prepregs due to hand lotion