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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Conventional resistance strain gauges are affordable due to wide use and inexpensive parts from their simple functioning principle; they allow accurate and repeatable measurements from micro-strain to several percent strain and can operate in both static and dynamic regimes (Giurgiutiu, 2016; Glisic, 2022). Theses discrete sensors provide local point-based strain measurements and are usually manually bonded at hot-spot areas onto a structure with an adhesive. However, the implementation of these resistance strain gauges is limited by the adhesion with the structure, extensive cabling, and the adverse effect of electromagnetic interference and temperature changes (Giurgiutiu, 2016; Glisic, 2022). Their commonly rigid design strongly limits scalability and integration, along with bonding quality reducing accuracy and durability. To address these limitations, recent research has explored the use of direct-write technologies to fabricate lightweight, flexible sensors directly onto structural surfaces or within the structure (Dumstorff and Lang, 2017; Shen et al., 2017; Zhao et al., 2012). These direct-write methods, such as inkjet printing, aerosol jet printing, and screen printing, enable the deposition of functional materials like conductive inks and piezoelectric polymers with high spatial resolution and design freedom. Directly written sensors offer several advantages, including reduced weight and profile, improved mechanical coupling, and compatibility with complex geometries (Philibert et al., 2022). These printed sensors can be tailored in geometry and material composition to optimize sensitivity, durability, and integration with the structures to monitor. In parallel, printed strain gauges have emerged as promising sensors for SHM. By leveraging direct-write techniques, strain gauges can be fabricated using conductive or piezoresistive inks on flexible substrates. Aerosol-jet printed strain gauges made with silver nanoparticles ink were successfully integrated inside composite structures with almost no mechanical degradation thanks to a carbon fibre prepreg pre-cure protocol, and achieved a reliable GF of about 2; the resistivity of the printed traces was about 5 μΩ·cm (Zhao et al., 2012). Capacitance-based strain sensors with wireless sensing ability were screen-printed on flexible polyimide film using silver-based conductive stretchable ink for the interdigitated electrodes and carbon-based piezoresistive ink; they exhibited high sensitivity during bending test when embedded into a glass fibre-reinforced polymer composite (Mahmoud et al., 2025). A smart printed sensor was inkjet-printed for monitoring curing and damage of composite scarf-repair patch using sensing traces that would indicate damage when broken along with interdigital sensors monitoring capacitance changes (D. Bekas et al., 2019). The sensing traces are 1 mm wide and made of conductive silver nanoparticles ink; it was required to print 5 layers of ink on top of each other to achieve a resistivity of 10 μΩ·cm. Similar work using inkjet printed interdigital capacitive sensor was done to monitor the bondline of a single lap of a CFRP composite joint, evaluating the curing process and assessing the structural integrity of the adhesive epoxy (D. G. Bekas et al., 2019). While inkjet or aerosol printing offer high resolution, it requires expensive equipment. Screen-printing technology is cheaper for batch production but implies lower resolution and high ink waste. Besides low scalability, direct-write printing offers adaptability and good resolution, and is hence well-adapted for optimising printed strain sensors. Some recent advancements in printed strain sensors using Voltera materials dispensing printers are summarised in the following. Wearable glove with sensing traces on each finger was developed for remote robotic hand control application, and showed sensitivity to finger bending (Voltera Inc., 2022). The sensing traces were 1 mm wide and made with stretchable conductive silver ink on TPU and then heat-laminated onto garden glove. Printed strain sensors were patterned in a 11 cm long serpentine shape to successfully perform small deformations detection with a resolution of 0.05% (Hassan et al., 2025). The sensing traces were 0.8 mm wide, 124 µm thick, and made with stretchable conductive inks, silver-based and a carbon-based section over 5 cm, on 50 µm polyimide film. These robust sensors achieved a GF of 2.45 within a strain range of 0-0.25% and proved good durability and repeatability with minimal resistance change (0.01%) after 1000 bending cycles. Fatigue crack detection was performed on aluminium sheet during tear test using a series of traces perpendicular to the crack that would indicate crack progression when broken (Smith-Lewis and Downey, 2023). These sensing traces were made with flexible conductive silver on polyimide film and connected to a multiplexer and wireless microcontroller. These studies highlight the potential of printed strain sensors for SHM, motivating further exploration of materials, designs, and applications. This paper explores the design, fabrication, and evaluation of printed strain gauges made with silver inks on flexible substrates for SHM applications. The goal is to develop reliable, scalable printed sensors for integration into aircraft composite structures. Various ink-substrate combinations, printing parameters, and sensor geometries are investigated to assess electrical performance, strain sensitivity, and consistency. The printed strain gauges are validated through electrical and mechanical testing.