Composites can be manufactured using either resin transfer ...

[23, 37]. A higher voltage helps to generate stronger Lamb waves with a larger signal to noise
ratio but at the cost of high power consumption [38]. High power consumption is not
recommended as SHM systems typically have requirements of low power. A voltage of 5-10
V has been found to be optimum for most SHM applications. A signal with a larger number
of peaks has a higher energy associated with it. This aspect helps in long distance propagation
as it compensates for any attenuation due to damping. However, increasing the number of
peaks increases the pulse width and, in turn, the interference [39, 40]. A three to five cycle
Hanning modulated tone burst has been shown to be an optimal form of actuation because
this method allows separating the anti-symmetric and symmetric modes and allows
separating the reflection from the edges or from cracks [41]. Diamanti and Soutis [42] studied
the use of low frequency for composite damage detection. Lower frequencies excite a smaller
number of modes making them more distinguishable. The first order anti-symmetric (A0)
mode has a smaller wavelength for a given frequency compared to the first order symmetric
(S0) mode making A0 waves more sensitive to small levels of damage. In addition, S0 modes
undergo higher attenuation during propagation [24, 40]. Given these characteristics, a five
cycle tone burst of 1 kHz center frequency and an amplitude of 6 V to generate A0 mode
Lamb waves was considered in the testing protocol. The excitation input signal is shown in
Figure 2.2.
Figure 2.2. A typical PZT excitation input signal for the testing scheme used in this study.
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