This contribution presents an integrated static–dynamic monitoring study for civil applications using long-gauge Fiber Bragg Grating (FBG) strain sensors. An experimental campaign was conducted on prestressed concrete beams with a total length of 8 m, simply supported over a clear span of 7 m, which are representative of highway bridge girders. Two dedicated inspection windows were opened along the beams to expose the steel tendons, which were progressively cut to simulate distinct structural defect scenarios.

A bonded prototype was instrumented with four long-gauge FBG sensors sensing four different zones along the beam span, each measuring the average strain over a 0.60 m length. Zones 1 and 3 were located in undamaged regions and served as references, while zones 2 and 4 were positioned in regions affected by the progressive tendon cuts within the inspection windows. In particular, zone 2 was placed at approximately L/3 and included two inspection windows, while zone 4 was centered at mid-span with a single inspection window. This layout enabled tracking the evolution of longitudinal strain in both damaged and undamaged parts of the beam, and investigating how local prestress loss redistributes strain under service and ultimate loads.

Quasi-static tests included repeated service level load cycles and loading up to ultimate conditions. Under static conditions, the FBG data provided continuous measurements of the average strain field along each 60 cm gauge zone. The results revealed clear strain concentrations and a gradual increase in tensile strain in zones 2 and 4 as strands are cut, while zones 1 and 3 exhibited nearly elastic behavior. These static measurements allowed quantification of local stiffness and prestress losses and helped distinguish between global deflection and highly localized damage in the tendon region.

Simultaneously, the same FBG sensors were used as dynamic transducers to perform Operational Modal Analysis (OMA) directly based on strain response. Ambient and shaker forced-vibration tests were carried out after each damage step, and standard OMA techniques (such as frequency and time domain methods implemented in a Python toolbox) were applied to the FBG strain time series to study the evolution of natural frequencies, damping ratios, and strain mode shapes with increasing damage levels. A detailed finite element model of the prestressed beam, including tendon layout and strand cuts, was developed to compute numerical strain mode shapes along the FBG gauge lines. Experimental and numerical strain mode shapes were compared using standard correlation metrics, allowing for a quantitative model validation and demonstrating the sensitivity of FBG-based OMA to tendon damage.

The combined use of long-gauge FBG sensors for static and dynamic monitoring confirmed that these transducers can offer a powerful and compact sensing strategy for detecting and localizing strand-level damage in prestressed concrete bridge components.

Structural Health Monitoring (SHM) of composite structures requires sensing technologies that are compact, immune to electromagnetic interference (EMI), and robust challenging environmental conditions (high temp, pressure or chemical environments).
Conventional optical fibre sensing solutions (such as Fibre Bragg Grating (FBG) sensors and Distributed fibre sensing solutions, provide strain and temperature data) are limited in multi-parameter capability and spatial resolution.
This work presents a new class of integrated photonic circuit (PIC)-based multisensors, leveraging all the advantages of photonic sensing (EMI-free, robustness in challenging environments), as well as the advantages of PICs, namely highly miniaturised, ability for point measurements, allowing flexibility in sensor design, and suitable for high scalability (high multiplexing and CMOS wafer scale fabrication).
This new class of sensors is fully compatible with existing photonic based solutions in SHM, to the point that the same interrogator may be employed for and FBG and PIC sensors.
In this work we will present our approach for silicon photonic PIC multi sensors for point-wise monitoring of key physical SHM quantities such as temperature, refractive index and vibration, as well as results on these sensors deployed in aerospace grade composites.
The sensing element is fabricated on a 220 nm silicon-on-insulator platform, employing corrugated waveguides that act as phase-shifted Bragg gratings. The PIC is optically coupled to a single-mode fibre (SMF) through a micro-ball-lens interface, resulting in a miniature, passive device that operates purely with light. Its spectral response demonstrates a temperature sensitivity of 85 pm / °C, high linearity (R² = 0.97), and repeatable performance up to 180 °C. The packaged sensors have been embedded in representative composite materials and structural coupons, validating compatibility with aerospace manufacturing and service environments, while also a repeatability study will be presented.
The integration of multiple sensors from a single PIC (size < 1 mm2), in cooperation with the established fibre sensing technologies, enables acquisition of multiple heterogeneous parameters from within the material volume. This effect is further enhanced considering the multiplexing capabilities of these sensors both in time (readout speed for each PIC at kHz range) and wavelength (utilising the entire 1530-1565 nm range (C-band in telecoms)).
Ongoing work focuses on extending the concept to hybrid PIC and other photonic solutions, gathering data from all different modalities to implementing data-driven processes, both for SHM during operation as well as during manufacturing of composite aerospace grade parts.

ABSTRACT: 60s economic boom led to spread construction of large transport infrastructures. Many of these steel reinforced concrete structures attain their end of lifespans on this and next decade. With no major renewing plan, repairing and retrofitting are explored alternatives. A good example is Milano's ring-road viaduct; while already repaired and its concrete deck enlarged, SHM begins nowadays. Monitoring of thermal and mechanical induced strain, static and dynamic, brings access to permanent strain, thermal expansion, eigenmodes of each single road span and better understanding of the whole structure dynamic behavior. Three trucks moving at 30 km/h load dynamically the enlarged deck, while real traffic is used for modal analysis. Often, this kind of comprehensive monitoring requires combining various measurement technologies, making their installation time-consuming and expensive. Thus, the number of sensors may be undercut, and measurement campaigns duration reduced, which may result in poorer monitoring results and mismatching between experimental results and model's ones. FEBUS SHM solutions based on DAS (Distributed Acoustic Sensing), DSS (Distributed Strain Sensing) and DTS (Distributed Temperature Sensing) provide quick instrumentation and easy monitoring. With long-range devices to address tens of km of infrastructure instrumented in a row, up to 400 kHz continuous monitoring, state-of-the-art DAS repeatability threshold of only 2 picoStrain/SquareRoot(Freq), FEBUS DAS solution brings values for every node of the structure, remote monitoring and mastered opex and capex.

Having the appropriate contact force between a pantograph of train and the overhead contact line is crucial for the lifetime of the overhead contact line and the contact strip of the pantograph as well. Conventionally the contact force is measured with a pantograph, e.g. by using fiber Bragg grating (FBG) sensors. Commercial trains are typically not equipped with pantographs which measure the contact force. Even if trains of railway undertakings measure the contact force, the infrastructure manager does not receive this information and hence cannot make counter actions in case of too high forces.

We present an alternative approach where a section of an overhead electric line is equipped with distributed fiber optic sensing cables. In a pilot study, the speed of a test train was stepwise changed and the impact on the contact force was measured with Optical Frequency Domain Reflectometry (OFDR) and Distributed Acoustic Sensing (DAS) methods. We demonstrate that a change of the contact force when the train is static can be accurately determined by the high-resolution Distributed Strain Sensing (DSS) measurements of OFDR. However, with increasing train speed the measurement results become incomplete as the OFDR interrogator cannot follow the large strain changes anymore. On the other hand, DAS, although having a coarser spatial resolution, can detect the contact point of the pantograph and speed of the train even at high speeds. Deriving the contact force is more complicated as DAS records strain rates and not strain.

Overall, equipping sections of electric overhead railway lines with DFOS has the advantage of assessing every train that passes by. Thus, the infrastructure manager obtains a complete picture of contact forces and can initiate counter actions if limits are exceeded.

The performance of SHM systems relies heavily on the configuration of the sensor network. Strain-based monitoring using fibre Bragg grating (FBG) sensors can provide enhanced damage localisation capabilities compared to traditional vibration-based methods, yet optimising FBG placement is a complex, multi-objective, and constrained task. This paper presents a novel framework for optimising FBG cable routings on aircraft wings by leveraging data from ground vibration tests (GVT). The proposed method aims to maximise sensitivity to damage by considering changes in dynamic strain modeshapes. Notably, the framework adopts a probabilistic approach constructed using Gaussian process regression to account for uncertainties present in modal test data. The methodology is demonstrated via an experimental case study utilising data from a Hawk T1A aircraft, illustrating how standard GVT procedures can be repurposed to extract further value for in-flight monitoring system design. The principles underlying this uncertainty-aware framework are applicable beyond aerospace to other sectors, including civil infrastructure and offshore wind.