Acoustic Emission Monitoring of CFRP Laminated Composites ...

2 Chapter 1. Introduction
represented by a mathematical fatigue model. In other words, all fatigue
models will have a certain level of model uncertainty. In addition, fatigue
is also inuenced by the orientations, locations and the types of damage
mechanisms introduced in the composite. As a result, the fatigue model
needs to be continually updated and revised while the composite is inservice.
The information required for updating the model can be obtained
using non-destructive condition monitoring techniques.
By combining condition monitoring techniques with fatigue modelling,
critical material degradation processes can be identied, failure predicted,
and preventive actions planned. This approach is known as condition-based
maintenance. By using eective condition based maintenance, instead of
corrective (after failure) or preventive (calendar-based) maintenance, companies
can make substantial savings. The savings will be in the form of
extended part life, reduced number of unexpected breakdowns, lower risk
of secondary failure, and increased safety. Consequently, there is a denite
advantage in being able to detect, monitor and evaluate individual damage
mechanisms before the failure of a composite.
Acoustic Emission testing is a non-destructive condition monitoring
technique which can be used for in situ monitoring of composite fatigue.
Acoustic emissions (AE) are transient stress (pressure) waves which are
generated by the energy released when microstructural changes occur in
materials. 5, 6
The stress waves travel through the composite and when they
reach the surface, they cause it to vibrate. AE waves can be measured
using very sensitive transducers which respond to surface displacements
of several picometers. The AE technique can detect delamination, matrix
5, 710
cracking, debonding, bre cracking and bre pull-out. Hence, the high
sensitivity of the AE technique may potentially enable early detection of
damage.
However, there's no such thing as a free lunch; the high sensitivity of
the AE technique means that the measured AE signal may contain a high
number of AE transients from sources in both the composite and the environment.
The sources in the composite include damage growth, rubbing of
crack surfaces and friction between the bres and the matrix due to their
dierent material properties. The varying material properties will result in
an anisotropic speed of propagation. 11
In addition, reection and attenuation
of the AE waves add to the complexity. Attenuation can be caused
by geometric spreading, dispersion, internal friction and scattering. 12
Fur-