Composite Materials Research Progress

168
Giangiacomo Minak and Andrea Zucchelli
Numerous methods have been attempted to identify damage mechanisms from AE data
[40,41] and in general many carefully controlled laboratory experiments are necessary to
develop relationships between measured AE signals and a specific damage mechanism.
The results from AE monitoring have been used in attempts to estimate the residual
strength or life of a structure [34]. Most strength assessments from AE are based on empirical
correlations developed from failure tests on a large number of nominally identical structures
[42].
Even if recently the research on AE, especially as regards composite laminates focused
on modal analysis [43-45] in this chapter we consider classical feature-based (also known as
parametric) AE analysis [38], in which for each acoustic emission a set of meaningful
parameters (shown in figure 1) are detected such as:
- progressive event number
- counts per event
- maximum amplitude within the event
- event duration
- event energy
Figure 1. Acoustic emission parameters.
This approach has been used in composite laminates with different AE interpretations.
Many authors (e.g. Siron and tsuda [40] ) report that fibre breakage produces large amplitude
signals while matrix cracking results in much smaller amplitudes and delamination is thought
to produce medium amplitude signals. Other studies conclude that matrix cracking causes
large amplitude signals while fibre breakage produced low amplitudes [47].
In reality the amplitude depends on a number of factors including the local stress
conditions and the energy released. In fact, for example, in [43] a very small increment of
matrix crack growth produces a much smaller amplitude signal than a large matrix crack.
Moreover, long duration events are attributed to delamination [44].