Acoustic Emission Monitoring of CFRP Laminated Composites ...

32 Chapter 3. Acoustic Emission
Figure 3.3
Illustration of conventionally used AE hit-based features.
studying its statistical parameters and its correlation with other features.
The statistical distribution of a feature can be estimated from a histogram
of its values. The shape of the histogram can provide useful information,
e.g. the shape of the amplitude histogram can help to identify
the damage mechanism which generates the AE. 135
Correlation analysis
is often performed using cross-plots by plotting one feature as a function
of another. For this task, the following feature pairs are often used: AE
counts vs. amplitude, AE counts vs. duration, and duration vs. amplitude.
These plots can further help with the identication of the damage mechanism.
Correlation analysis can also be performed in higher dimensions,
but the correlation of four and more features may be dicult to visualise.
One of the advantages of using AE features is that they provide a huge
data reduction compared with a full waveform acquisition, but there are
several practical disadvantages. First, the fact that the same type of damage
can emit AE signals with dierent amplitudes makes it dicult to
set the threshold. Second, AE signals in composites suer from high attenuation,
which means that signals close to the transducer are stronger
and more likely to be detected than those generated further away. Third,
the rugged frequency response of resonance AE transducers means that
frequencies which are spaced only a few tens of kilohertz apart, will be