Abstract
Structural health monitoring (SHM) is an emerging paradigm of real-time in situ structural evaluation for the detection of damage and structural degradation. This is achieved while the structure is kept in-service as against traditional non-destructive evaluation (NDE) techniques which require scheduled interventions while the structure is kept offline. SHM offers great advantages over traditional regimens of condition monitoring (CM) by improving structural reliability and safety through timely detection of structural defects also known as “diagnosis”. Polymeric composite materials offer the unique opportunity of integrating different phases for designing self-sensing smart systems capable of self-diagnosis. Polymers are unique in the sense that they can be designed in various configurations as they generally have facile manufacturing procedures. Among other properties, piezoresistance is the one that can be detected in composites in real-time as a function of strain. Conductive polymers including intrinsic and extrinsic conductive polymers can be used to induce piezoresistivity in composites. Careful design procedures can be adopted to maximize the sensitivity of these piezoresistive composites in order to fully exploit the potential of this property for SHM. Various manufacturing/integration strategies can be employed to effectively use piezoresistance in composites for structural health monitoring. These include self-sensing in carbon fiber-reinforced composites, use of surface deposited/mounted sensing films and patterns, integration of filaments and yarns during reinforcement manufacturing or lay-up and impregnation of reinforcements with piezoresistive matrices. A comprehensive review of these techniques is presented with the view of their utility in the SHM of composites. A selection criterion for these techniques is also presented based on sensitivity, manufacturing method and detection capability.
Highlights
Composites are defined as multiphase materials with certain complementarity in their properties arising from the synergistic effect of their constituents on one another
The conductive networks formed by the conductive filler inside the polymer matrix called the percolation networks, are responsible for charge transport mechanisms. The conductivity in these networks is either due to electron hopping from one conductive filler to another if the distance between the neighboring particles is less than a certain minimum value, or the electron tunneling, whereby the electron can jump from one conductive particle to another even if the kinetic energy of the electron is lower than the potential energy of the barrier
The reported self-sensing mechanisms in polymer composites are classified into four categories on the basis of integration methodology, manufacturing route adopted and strain sensing/damage monitoring capability
Summary
Composites are defined as multiphase materials with certain complementarity in their properties arising from the synergistic effect of their constituents on one another. The compliant polymers offer superior flexibility whereas exceptionally high electromechanical coupling of piezoelectric ceramics gives them mechanical actuation and energy harvesting properties [28] These piezoelectric composites are designed in so-called 1–3 or 0–3 configurations [29]. Self-healing composites are categorized among smart composites since they have the ability to recover some of their mechanical properties once the healing cycle is allowed to proceed either with or without the intervention of some external trigger [34] In the former case, they may be termed as non-autonomous self-healing systems requiring heat or UV light for healing to take place. A smart structure is able to interact with its environment by acquiring information about itself and its environment including the loading conditions This results in greater reliability and safety and helps develop operational history from the real-time data acquired from the suites of sensors. These sensors are usually applied as external sensing elements or as integrated sensors in various configurations during the manufacturing of the aircraft components
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