PSI - Issue 80

Marilyne Philibert et al. / Procedia Structural Integrity 80 (2026) 65–76 Author name / Structural Integrity Procedia 00 (2019) 000–000

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continuous and autonomous monitoring, improving safety and reducing maintenance costs and time. This requires sensors to be permanently installed on the structure, performing passive and/or active sensing. Passive sensing implies continuous measurement of parameters (strain, acoustic emission, temperature, etc), while active sensing implies use of actuator interrogating the structure (guided ultrasonic waves). The increasing use of carbon fibre reinforced polymer (CFRP) composites in aerospace structures and the progressive failure characteristic of CFRP have driven the need for advanced SHM systems capable of detecting damage early and reliably (Philibert et al., 2022). In aerospace applications, the main requirements for selecting sensors are the price, weight, and intrusiveness (Giurgiutiu, 2016). These sensors must operate under various environmental conditions, hence, often require temperature compensation methods. Moreover, the bonding quality strongly affects sensing accuracy and durability. Among the different SHM techniques, strain gauges have been widely studied for their effective and straightforward monitoring abilities, as strain is directly linked to stress and deflection. In civil engineering, strain-based sensing has been used for a long time with first deployed onsite application in the 1920s (Glisic, 2022). The principle of conventional resistive strain gauges is based on the change in the electrical resistance of a conductor or semiconductor when it is stretched (tensile strain) or compressed (compressive strain). When strained, the length, , and cross sectional area, , change, which is the geometric effect. The inherent resistivity of the material, , may also change due to change in electron mobility (inter-atomic spacing affecting bandgaps), which is the piezoresistive effect. In metals such as for traditional foil strain gauges, the change in resistance is purely due to the geometric effect, while in semiconductors or composite-designed strain gauges, the piezoresistive effect is significant and the geometry effect accounts for less than 2% in the resistance change (Hoffmann, 1989). These combined effects alter the electrical resistance, , according to: = " ! (1) The relative change in resistance, ∆ , is proportional to the strain, , experienced by the strain gauge with unstrained resistance # and can be quantified with the gauge factor, defined as: = ∆%/% ! ' (2) For most metallic strain gauges, the GF is approximately 2, whereas piezoresistive strain gauge designs can achieve significantly higher sensitivities with GF improved by up to three orders-of-magnitude, enabling measurement of very small strains (Hoffmann, 1989; Weng et al., 2025). However, compared to metal strain gauges, the semiconductor strain sensors are made with more expensive parts and limited by brittleness, non-linear responses, and severe temperature, humidity and contamination sensitivity (Hoffmann, 1989). To address these challenges, modern piezoresistive strain sensors are usually made of flexible composites containing an active layer (for electrical properties, in particular micro or nano fillers), polymer substrate, and electrode materials (Weng et al., 2025). Material combinations can use for instance graphene, carbon black (CB), carbon nanotubes (CNTs), silver nanowires, or silver nanoparticles as active layer in thermoplastic polyurethane (TPU) or polydimethylsiloxane (PDMS), allowing GF up to 70’000, strain range from 5% to 2770%, enhanced durability, and excellent integration (Weng et al., 2025). Complex mechanisms behind flexible piezoresistive strain sensor have been extensively studied, however, in-service performance degradation, nonlinearity, and complex fabrication remain challenges in deploying these sensors (Gao et al., 2022). Conventional and widely used metal-based resistive strain gauges are typically made of thin metal wire (about 5 μm) or etched metal foil, usually Constantan (copper-nickel alloy), nichrome or Karma alloy, patterned into a measuring grid (zig-zag pattern of parallel lines) onto an insulating carrier material, such as epoxy or phenolic resin or polyimide (Hoffmann, 1989). The measuring grid pattern increases the effective length of conductor in the direction of strain, improving resistance measurement accuracy while maintaining a compact device. The resistance change is usually measured using a Wheatstone bridge circuit to enhance resolution and compensate temperature-induced errors (Giurgiutiu, 2016). Along with the well-known resistance-based strain sensors, fibre optic strain sensors are also a mature and reliable technology available in the market and widely applied in strain-based SHM studies (Glisic, 2022). Distributed fibre optic sensors allow for monitoring over large areas and long distances for improved spatial coverage.