Smart Materials are those capable of responding, with change of their intrinsic properties, to an external stimulus (voltage, deformation, temperature, electric field, magnetic fields, etc …). Piezoelectric materials, by their application in sensor technology and capability of measuring different physical quantities (pressure, acceleration, deformation, etc …), are perhaps those with more widespread application in the context of engineering. However, the range of intelligent materials is not restricted to materials with piezoelectric characteristics and the intrinsic ability to generate chains when mechanically deformed or to change its shape (deforming) when an electric potential difference is applied to them. In addition, it is worth highlighting in the domain of intelligent materials: 1) materials (polymeric or metallic) with memory effect, whose deformations can be induced (or recovered) through the application of thermal field; 2) thermoelectric materials, whose main characteristic lies in the conversion of thermal gradient to electric current (and vice versa), an effect known as Peltier effect; 3) self-healing materials that allow the material to be “cured” by local regeneration (by intrinsic or extrinsic phenomena) of the fracture zone; and (4) materials with magnetocaloric characteristics which, when Exposed to an oscillating magnetic field, respond with a (reversible) change of the thermal field.

the range of intelligent materials is not restricted to materials with piezoelectric characteristics and the intrinsic ability to generate chains when mechanically deformed or to change its shape (deforming) when an electric potential difference is applied to them

In the more application domain of Smart Materials should be emphasized its application in obtaining structures/components with characteristics of self-sensing (Self-Sensing). The intrinsic characteristics of electrically conductive fibers (as carbon) and / or matrices doped with carbon nanofibers (CNF) allow the structures to be equipped with the capacity for self-sensing by reading the local variation of the electrical resistivity of the reinforcing fiber (or matrix itself), which variation is correlated with the local strain field generated in the component. The interest in the development of structures with self-sensing characteristics is evident because it becomes possible to obtain structures with an extremely high spatial sensing resolution without the need to install conventional sensor equipment, which would imply a number of sensors that would be physically limiting.

In the more application domain of Smart Materials should be emphasized its application in obtaining structures/components with characteristics of self-sensing (Self-Sensing)

Critical Materials S.A. has been developing work in the field of Self-Sensing materials for the detection, location and evaluation of the severity of structural damage caused by impact loads in aeronautical structures. In Figure 1 it is possible to visualize a prototype of self-sensed composite structure used for the detection and location of structural damage.

Protótipo de componente compósito

In the field of embedded sensing, it is worth highlighting the research that is being carried out in the sense of obtaining piezoelectric fibers that will allow, as already happens with the use of optical fibers (see Figure 2), deformation monitoring and, In addition, mechanically act on the structure in order to optimally adjust its geometric shape to the desired performance (structural morphing).

In the field of self-regenerating materials with extrinsic regeneration processes, it is important to emphasize the importance of the application of hollow fibers in the design of vascular structures transporting curing agents and reaction initiation, for an effective regeneration of the damaged zone. In Figure 4 the curing agent carrier fiber and its arrangement in the core of the fiber laminate is visible. It is important to note that the methods for obtaining a fiber reinforced thermoplastic or thermosetting composite material are almost exclusively based on extrinsic systems (vascular systems or capsule systems containing curing agent).

it is important to emphasize the importance of the application of hollow fibers in the design of vascular structures transporting curing agents and reaction initiation, for an effective regeneration of the damaged zone

In the field of materials with intrinsic self-regeneration process (usually with lower mechanical properties) it is important to emphasize the importance of reinforcing fibers to ensure a mechanical behavior compatible with service requests. I recently participated in a European project (www.selfhealingelastomers.eu/) whose main objectives were the creation and subsequent mechanical characterization (and numerical modeling) of an elastomer with intrinsic self-regeneration capacity (through the interaction of molecular chains with non- Covalent), where aramid fibers were considered for incorporation in the base material, in order to optimize (increase the initial modulus of elasticity and tensile strength) of the mechanical characteristics of the material, without significantly compromising its ability to self-regeneration.

the field of application of intelligent materials in engineering solutions is vast and, in my view, especially suitable for use in critical performance structures

As previously noted, the field of application of intelligent materials in engineering solutions is vast and, in my view, especially suitable for use in critical performance structures. In fact, the characteristics of an intelligent material allow, among other aspects, its self-sensing, morphing control and, in case of damage, its self-regeneration and consequent inhibition of a precursor of a Catastrophic structural failure. However, the incorporation of these materials into serviceable structures / components poses new challenges in terms of engineering. In fact, the multiphysical character of this family of materials implies an aggregated knowledge in materials science, numerical modeling and electronic engineering that is important to dominate, in order to promote an adequate use of this typology of materials in engineering solutions.

the multiphysical character of this family of materials implies an aggregated knowledge in materials science, numerical modeling and electronic engineering that is important to dominate, in order to promote an adequate use of this typology of materials in engineering solutions

References
Department of Aerospace Engineering, TMU. (2017). TMU-TMG-JAXA Join Project. Retrieved from http://aeronautics.sd.tmu.ac.jp/en/research_facilities/structures.html
Trask RS, W. H. (2007). Self-healing polymer composites: mimicking nature to enhance performance. Bioinspiration & Biomimetics, 2(1)
Zheng Wang, N. C. (2011). Piezoelectric fibers for sensing and actuation at ultrasonic and audio frequencies. Retrieved from http://www.rle.mit.edu/media/pr152/39_PR152.pdf