JEC COMPOSITES MAGAZINE - Issue #105 - June 2016 - 44

smart composites

ture in order to create symmetric arrays of transducers and thus a
highly symmetric distributed network. This approach can provide
the advantage of controlling the guided waves propagating into a
thin structure;
- limit the "cross-talk" between the different embedded elements.
A technical requirement has to be defined in order to set a design
rule concerning the minimum pitch between two electrical conductors or the minimum distance between two transducers;
- limit the thickness variations due to the piezoelectric inclusions. These inclusions inside the material inevitably modify the
thickness of the structure locally. This may be limited through the
use of thin piezoceramics (about 150 µm); moreover, electrical
connection based on conventional welding is not advisable, due
to the resulting excess thickness and the potential creation of a
wedge. Such a wedge could break the piezoceramics during the
manufacturing process, thus ruining the transducer. Therefore, a
specific connection technique was developed based on the use of
semi-finished products;
- achieve specific shaped structures (for instance, bi-concave
structures) to adapt to a wide range of real applications;
- achieve a robust link with the outside structure to provide
energy or modify the control law or the behavioural law in real
time.
Currently, the main research efforts focus not only on the development of solutions to solve the technical issues detailed above, but
also on the influence of exogenous elements in terms of durability,
reliability and maintainability of the host systems. The first prototypes were manufactured in the IRTES-M3M laboratory 12 years
ago and are still in operation. While this is not formal proof of a lesser
influence of these elements on a system's durability and reliability, it
is a good indication. The third point (maintainability) is more complicated. It is always possible to change an element embedded into a
structure with accurate machining operations and matter additions.
This is not exactly the objective of this type of structures, however,
and so considerable efforts are necessary to guarantee high durability
and reliability.
Another important point to take into account is the recyclability of
the end-of-life products. Of course, some smart composite structures
are manufactured with bio-based materials, as shown in figure 4.
This is an initial response. The integration of SAW tags could be a
solution to tag smart
composite structures.
Thus, it would be
possible to create a
specific recycling loop.
But the main, essential
issue, in particular for
mass application, is
the method used to
extract the exogenous
elements from the
Fig. 3: Close-up view of a smart material slice
composite material.

44 jec composites magazine / No105 June 2016

Fig. 4: Smart suitcase made of a bio-based material embedding three piezoceramic discs, an electric resistor and an LED

Currently, the question is still open. Basically, the compromise level
between efficiency and recyclability is a key parameter to prove the
industrial validity of this technology.

Potential applications
Once the manufacturing process and the design approach for smart
composite structures are mastered, it will be possible to consider lots
of applications in different fields. Four application fields are presented
as examples.
Vibration suppression
Mechanical vibrations can have a negative impact on systems [5]:
- failure can occur due to fatigue, excessive strain during transient
events or instability;
- vibrations and noise influence the customer's comfort;
- in precision engineering (optical systems, machining, etc.), vibrations limit the operating conditions of systems.
Smart composite structures can change their own properties (damping, stiffness, acoustic impedance, etc.) in order to modify their
vibration or vibroacoustic behaviour and thus manage the drawbacks
detailed above. For instance, it is possible to design a smart composite
structure for vibration isolation of sensitive electronic components
such as bulk acoustic wave resonators or inertial platforms.
Structural health monitoring (SHM)
Balageas [6] defined structural health monitoring as the ability to
diagnose, at any time during the life of a structure, the state of the
constituent materials, of the different parts and of the full assembly.
The embedded transducers network can check the quality or the
ageing of the host composite structure over a period of time. This
network can detect a fibre crack or other failures. Associated to a
micro-controller and specific algorithms, the generated data can be
used to locate flaws or to predict failure events.
Energy harvesting (EH)
Energy harvesting is a method used to extract energy from external
sources (base vibrations, wind, thermal gradient, sea waves, etc.) [7].
The development of EH devices is in progress for the different types
of external sources. In the piezoelectricity field, these devices and the

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #105 - June 2016