Composites can be manufactured using either resin transfer ...

making them susceptible to electromagnetic interference. Similar to FBG, another
disadvantage of a strain gauge is that the sensitivity covers a small area and thus a large
number of sensors are required for a large structure. This characteristic increases the
complexity of any system relying on strain gauges [31]. In addition, the temperature range
and fatigue life of strain sensors are somewhat limited.
Magnetostrictive sensors are based on the principle of converting mechanical energy
into magnetic energy and vice versa. They are light weight, durable, and can act as both
actuators and sensors. For these reasons, they are suitable for active health monitoring [32].
However, they are temperature sensitive, expensive, and several aspects regarding their use
are being researched. In addition, embedding magnetostrictive sensors can degrade the
quality of a structure.
Piezoelectric transducers are light weight and are sensitive to small amounts of strain.
They generate relatively high amplitude output signals compared to other sensors. One of the
biggest advantages of lead zirconate titanate (PZT) sensors is the ease with which the sensors
can be used in different detection techniques - ultrasonic Lamb waves, acoustic emission, and
strain-based methods [16]. In addition, the same PZT transducer can act as either an actuator
or a sensor at any time. This allows for baseline-free detection and gives the flexibility to
monitor proper sensor operation. Several authors have proposed different methods such as
residual measurement and modal factors for validating sensor operation [33, 34]. Given these
advancements, in this thesis, piezoelectric transducers coupled with Lamb waves were used
for detection of a crack in composite materials.
2.4 Piezoelectric transducers and principle of operation
Surface mounted PZT actuators are commonly used to generate Lamb waves in
composite materials [35]. Piezoelectric transducers operate on the principles of both the
piezoelectric and the inverse piezoelectric effect. The crystalline property of a piezoelectric
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