Ultrasonic guided wave (UGW) detection with fiber Bragg grating (FBG)-based sensors has received increasing attention in the last decades due to the ability to perform non-destructive inspection (NDI) of large plate-like surfaces with a network of lightweight and multiplexed sensors. For accurate UGW measurements, several studies concluded that the ratio between the wavelength of the UGW and the length of the FBG should be above 7. However, shorter FBGs suffer from a lower FBG reflectivity and less steep slopes in the reflection spectrum. In this work we experimentally verified the effect of a passing UGW on the Bragg peak of FBG sensors of different lengths. By performing edge-filtering interrogation throughout the FBG’s reflection spectrum, we were able to reconstruct the FBG’s spectral response to a UGW in function of time. Our experimental findings are partially in line with those in the literature considering the UGW wavelength to FBG length ratio and the corresponding Bragg peak changes. We experimentally show for the first time that for shorter FBG sensors, the strain modulation is translated mostly into Bragg peak shifting, while for longer FBG sensors, Bragg peak deformation takes over as main mechanism. Despite the different mechanism for the latter, the UGW can still be detected by edge-filtering on the steepest slope, and with a much higher sensitivity.

Conventional contact sensors used for Lamb wave-based ultrasonic inspection, such as piezo-electric transducers, measure omnidirectional strain and do not allow distinguishing between fundamental symmetric and anti-symmetric modes. In this paper, we show that the use of a single fibre Bragg grating created in a dedicated microstructured optical fibre allows one to directly make the distinction between these fundamental Lamb wave modes. This feature stems from the different sensitivities of the microstructured fibre to axial and transverse strain. We fabricated carbon fibre-reinforced polymer panels equipped with embedded microstructured optical fibre sensors and experimentally demonstrated the strain waves associated with the propagating Lamb waves in both the axial and transverse directions of the optical fibre.

Composite materials are finding their way into aerospace applications thanks to their high stiffness-to-mass ratio. Nevertheless, composite components require frequent inspections because of their sensitivity to critical damage. Damage as small as barely visible impact damage (BVID) can grow as result of structural loading, with component failure as possible outcome. Optical fibre sensors (OFS) are considered excellent candidates for permanently installed structural health monitoring (SHM) systems, owing to their many advantages over electrical sensors. Current state-of-the-art BVID detection with OFS has so far however been limited to proof-of-concept demonstrations at low technology readiness levels. In this work, we equipped a total of 16 coupons, made of 5 different state-of-the-art composite materials, with aerospace compatible embedded or surface mounted optical fibre Bragg gratings (FBGs). We impacted each coupon at two locations and acquired the FBG reflection spectra before and after each impact. We first demonstrate how changes in the Bragg wavelength and in the Bragg peak shape can be quantified when the FBGs are exposed to the (non-)uniform strain field of BVIDs. Second, we show that this method was able to successfully detect the BVID in all considered scenarios and that in most cases, it was able to also locate the damage within an uncertainty of ±1 FBG location. Finally, we show the reliability of this method in terms of repeatability and considering the effects of temperature changes and on-ground airplane vibration. To the best of our knowledge, we are the first to use in-flight-compatible embedded and surface mounted FBG-sensors for the detection and location of BVIDs on aerospace-grade composite materials. These results motivate the use of FBG sensors as a permanent sensor network for cost-efficient damage detection in composite aerospace components for locally monitoring damage-prone locations.

Ensuring leak tightness and preventing emissions in bolted flange connections are critical for maintaining safety, reliability, and environmental compliance in industrial systems. This article offers a comprehensive review of techniques to improve installation processes and establish effective monitoring systems for bolted flange connections. The discussion on installation techniques focuses on achieving the appropriate gasket surface pressure to minimize leakage risks and enhance long-term reliability. The importance of proactive monitoring during installation is emphasized, as it enables the early detection and resolution of potential issues. The review also explores long-term monitoring strategies, spanning both traditional methods and advanced technologies. While direct observation is cost-effective, it lacks precision and fails to ensure safety. More advanced approaches, such as internal mass balance and real-time transient models, deliver superior detection and quantification capabilities, albeit with increased complexity and cost. External monitoring systems, such as cable-based sensors and hydrocarbon-sensing tubes, are highlighted for their effectiveness in broad-scale applications. Meanwhile, advanced techniques such as bolt load monitoring and fiber-optic sensing offer state-of-the-art solutions for identifying leaks and emissions. This work organizes these methodologies into distinct categories and provides a practical framework for applying them across various operational contexts. By emphasizing advancements in real-time monitoring and leak prevention, this study promotes safer and more sustainable industrial practices, laying the foundation for future innovations in bolted flange sealing technology.