Acoustic Emission Monitoring of CFRP Laminated Composites ...

36 Chapter 3. Acoustic Emission
growth, i.e. a material change, produces AE only once, but the resulting
rubbing of damage surfaces generates AE many times.
Dierent and in some cases contradictory results have been published
using amplitude features. According to Duesing, 11 Barré and Benzeggagh
142 and Ativitavas et al. 143 ber breakage emits high AE amplitudes
whereas Ono explains that high amplitudes are caused by rapid advances
of delamination and carbon ber fracture is characterized by low amplitude
signals. 144
Godin et al. claimed that conventional AE feature analysis
cannot be used to distinguish between dierent AE sources and suggested
that more advanced methods, such as multivariate analysis and classiers,
should be used. 145 Similar comments were made by Tsamtsakis et al.; 8
they suggested that new parameters, such as force or displacement, should
be added.
Ativitavas et al. developed an amplitude ltering technique using a
normalized load to lter out low amplitudes, and claimed that a reliable
isolation of AE signals due to ber breakage was obtained. 143
The intuitive
reason behind this is that it requires more energy to break the bers than it
does to damage the matrix, because the bers have higher Young's modulus
and higher strength. Hence, high amplitude AE is generated when the
bers break. This idea was veried by comparing the cumulative signal
hits (with respect to the loads of the ltered signal) with cumulative signal
strength and the cumulative number of ber breaks. The results showed
strong correlation.
Acoustic Emission frequency analysis techniques have mostly been limited
to classical approaches like the FFT. When used, the aim has been to
determine frequency bands which can be related to certain damage mechanisms.
The results presented by Bochse determined the frequency bands
for three types of damage. 146
Frequencies due to matrix cracks were found
to reside in the 100 to 350 kHz band, and ber breakage was given the interval
between 350 to 700 kHz. The sources were classied according to the
criterion that 70% of the signal power had to lie within either frequency
band. If not, the source was expected to be debonding. Similar results
were presented in a paper by de Groot et al. in which the frequency spectrum
between 50 to 600 kHz was analyzed. 49
They determined that matrix
cracks emit frequencies between 90 to 180 kHz, ber pull-out release frequencies
between 180 to 240 kHz, debonding produces frequencies between
240 to 310 kHz, and that bre breakage gives AE with frequencies above