Piezoresistivity is an electromechanical effect characterized by the reversible change in the electrical resistivity with strain. It is useful for electrical-resistance-based strain/stress sensing. The resistivity can be the volumetric, interfacial or surface resistivity, though the volumetric resistivity is most meaningful scientifically. Because the irreversible resistivity change (due to damage or an irreversible microstructural change) adds to the reversible change that occurs at lower strains, the inclusion of the irreversible effect makes the piezoresistivity appear stronger than the inherent effect. This paper focuses on the inherent piezoresistivity that occurs without irreversible resistivity changes. The effect is described by the gage factor (GF), which is defined as the fractional change in resistance per unit strain. The GF can be positive or negative. Strong piezoresistivity involves the magnitude of the fractional change in resistivity much exceeding the strain magnitude. The reversible effect of strain on the electrical connectivity is the primary piezoresistivity mechanism. Giant piezoresistivity is characterized by GF ≥ 500. This critical review with 209 references covers the theory, mechanisms, methodology and status of piezoresistivity, and provides the first review of the emerging field of giant piezoresistivity. Piezoresistivity is exhibited by electrically conductive materials, particularly metals, carbons and composite materials with conductive fillers and nonconductive matrices. They include functional and structural materials. Piezoresistivity enables structural materials to be self-sensing. Unfortunately, GF was incorrectly or unreliably reported in a substantial fraction of the publications, due to the pitfalls systematically presented here. The most common pitfall involves using the two-probe method for the resistance measurement.

A critical review of piezoresistivity and its application in electrical-resistance-based strain sensing

In recent years, Carbon Fiber Reinforced Plastics (CFRP) or Glass Fiber Reinforced Plastics (GFRPs) have become a very common material for aircraft and wind turbine structures. These structures are often protected from lightning strikes using conventional metal-based protective films/foils. Non-conventional, non-metallic lightning strike protection (LSP) technologies have not yet been fully realized, but research on non-conventional LSP systems for CFRPs has gained momentum in the last few years. The discovery of new structural conductive materials and improvements in the processing of carbon nano-filler based composites have challenged the conventional metal-based LSP systems by providing the potential for lightweight, non-metallic alternatives. However, a major challenge in using non-conventional LSP is the complex nature of a lightning strike event and its complicated thermal and mechanical impact on CFRP structures. Understanding the direct effect of a lightning strike on a CFRP structure requires understanding multiple transient loads, such as electrical, thermal, magnetic, acoustics, shock, and inertia. This review article focuses on new findings and discusses the complex direct effects of lightning on CFRPs. The focus is to find the important factors that regulate and control the damage to FRPs and they are classified and discussed with the help of available literature based on experimental results. Possibilities and limitations to these new findings are also discussed.

Factors affecting direct lightning strike damage to fiber reinforced composites: A review

In this paper, reduced graphene oxide (rGO), decorated with immobilized carbon nanotubes (CNTs), was introduced into a thermoplastic polyurethane (TPU) matrix to obtain a conductive nanocomposite with 2 wt% nanofillers content. Hot-pressed composite sheets were then biaxially stretched into films to efficiently fabricate flexible strain sensors. The biaxial stretching process was shown to promote secondary dispersion and parallel alignment of the nanofillers, resulting in the sensors exhibited significantly higher sensitivity (GF = 150 for the rGO/TPU4×4 sensor with a stretching ratio of 4 × 4 relative to GF = 3.5 for the rGO/TPU1×1 sensor at 30% strain). Furthermore, due to the synergy of the CNTs and rGO nanoparticles, the rGO/CNT/TPU4×4 sensor had a lower resistivity (5.62 × 104 Ω·m), wider monitoring range (0.3 ∼ 400% strain), and higher stability compared with the rGO/TPU4×4 sensor. The ability of the sensor to recognize body movements and physiological activities was also shown.

High-performance flexible strain sensors based on biaxially stretched conductive polymer composites with carbon nanotubes immobilized on reduced graphene oxide

Carbon nanomaterial-coated piezoresistive fiber sensors are finding applications in many industries for in-situ process and structural health monitoring of composites. These nanomaterials are embedded within the composite in two different ways; either by incorporating them in the matrix, or by depositing them on fibrous reinforcements. This review highlights the utility of carbon nanomaterials as deposition materials for fiber reinforcements and turning them into sensors for process monitoring during manufacturing and structural health monitoring during in-service life. A number of different strategies to coat carbon nanomaterials on fiber reinforcements are also discussed. A review of various monitored parameters during composites manufacturing such as reinforcement compaction response, flow-front tracking, and resin gelation and cure, as well as damage detection of finished composites using nanomaterial-coated in-situ sensors is also presented. Finally, current and future challenges are discussed where new types of 2D materials and their hybrids for next generation smart sensors are highlighted.

Carbon coated piezoresistive fiber sensors: From process monitoring to structural health monitoring of composites - A review

The development of sophisticated telecommunication equipment and other electro-electronic devices resulted in a kind of electromagnetic pollution that affects the performance of other equipment as well as the health of human beings. Intrinsically conducting polymers (ICP), mainly polyaniline and polypyrrole, have been considered as promising candidates for applications in efficient electromagnetic interference shielding (EMI) due to their ease of preparation, light weight, good conductivity and corrosion resistance. One of the important advantages of these materials is the capability to interact with the EM radiation through both absorption and reflection mechanisms thus enlarging the field of application. In this context, this review article describes a recent overview of the existing methods to produce intrinsically conducting polymers and their blends for electromagnetic shielding application. Additionally, it highlights the relationship between preparation methods reported in the literature with the structure and properties, such as electrical conductivity, electromagnetic shielding effectiveness (EMI SE), complex permittivity and permeability of these materials. Furthermore, a brief theory related to the electromagnetic mechanism and techniques for measuring the microwave absorbing properties are also discussed.

https://www.mdpi.com/2504-477X/5/7/173/pdf

Conducting Polymeric Composites Based on Intrinsically Conducting Polymers as Electromagnetic Interference Shielding/Microwave Absorbing Materials—A Review

Conductive layer-based multifunctional structural composites for electromagnetic interference shielding

Multi-functional fibre-reinforced polymer (FRP) laminates are the requirements of advanced aircraft structures. FRP laminates are the primary design to carry structural loads. However, improved/high electrically conductive FRP laminates can further use for multiple non-structural functions, including the lightning strike protection (LSP), self-sensing strain and damage sensor, electromagnetic interference (EMI) shielding, and other future opportunities like signal communication, non-destructive inspection (NDI), actuation of structures. This review article discussed the various research articles that modify and improve the FRP laminates' electrical conductivity for non-structural applications like lightning strike protection (LSP), EMI shielding, and strain/damage sensing. Different approaches like metal tufting/stitching, conductive resin, coating, and interleaves are discussed for LSP, whereas conductive resin, interleaves, and coating techniques were discussed for EMI shielding and strain/damage sensing. These approaches have their advantages and challenges, influencing the large-scale fabrication of the resulting composites. Therefore, it is crucial to understand these materials' properties and fabrication processes for further development. For LSP developments, it is summarized that metal-tufting and PANI-based conductive resin can sufficiently resist lightning damage at high peak current. Additionally, PANI-based resin has proven theirs performs well in strain sensing and EMI shielding.

A brief review of modified conductive carbon/glass fibre reinforced composites for structural applications: Lightning strike protection, electromagnetic shielding, and strain sensing

Strain sensing behavior of multifunctional polyaniline-based thermoset polymer under static loading conditions

Polyaniline-based multifunctional glass fiber reinforced conductive composite for strain monitoring

Abstract Electrical impedance characteristics of polyaniline (PANI)-dodecylbenzenesulfonic acid (DBSA)/divinylbenzene (DVB) composite were evaluated as a function of PANI content in the frequency range of 102–108 Hz. Polymer composites were prepared by a one-step thermal process where doping of the PANI and curing of the polymer matrix DVB have occurred simultaneously in the presence of a strong protonic acid, i.e. DBSA. The alternating current (AC) conductivity value with respect to frequency of the PANI-DBSA/DVB composite shows a direct current (DC) plateau up to an extensive frequency range. Almost metal-like behavior is obtained with such highly conductive plastic material. In all frequency regions, composites with higher PANI concentration showed a frequency independent behavior, while in the case of neat composites without PANI, a frequency dependent behavior is observed. A plot of the real part of complex impedance vs. frequency and a plot of AC conductivity of the composites indicate that the high electrical conductivity in high PANI concentration composites is because of the direct contacts between filler PANI particles. In this work, AC conductivity behavior and dielectric properties of very highly conductive thermoset composites are presented, and it is shown that conductivity is frequency independent for a very high frequency range (up to 108 Hz).

Sukanta Das

Papers

Conductive layer-based multifunctional structural composites for electromagnetic interference shielding

A brief review of modified conductive carbon/glass fibre reinforced composites for structural applications: Lightning strike protection, electromagnetic shielding, and strain sensing

Strain sensing behavior of multifunctional polyaniline-based thermoset polymer under static loading conditions

Polyaniline-based multifunctional glass fiber reinforced conductive composite for strain monitoring

Frequency independent AC electrical conductivity and dielectric properties of polyaniline-based conductive thermosetting composite