Conference Agenda

One of the current challenges in Structural Health Monitoring (SHM) is the validation of new processing algorithms and the development of new methods. To achieve this, it is relevant to have complete control over the entire chain, from instrumentation to processing algorithms.

To address this challenge, CEA List has developed an opto-electronic acquisition system using Fiber Bragg Gratings (FBGs) to measure ultrasonic waves. This system called “Optogero” has seven optical channels to enable simultaneous interrogation of multiple gratings. (cf. Figure 1).

In several SHM applications, FBGs offer numerous advantages over the more commonly used piezoelectric transducers (PZT). For instance, they can operate at much higher temperatures (above 1,000°C) than conventional PZT (around 200°C). Optical fibers can also be integrated directly into a structure with very limited intrusiveness. However, the interrogation of FBGs presents several technical challenges, and their sensitivity currently remains lower than that of PZTs.

The Optogero optoelectronic interrogation system is based on the edge-filtering measurement method. It can scan up to 7 Bragg gratings in parallel to identify their optimal operating point in order to maximize their sensitivity to ultrasonic waves. Real-time control of the laser source wavelength eliminates the consequences of varying environmental conditions, such as quasi-static strain and temperature changes in the structure. These effects would otherwise alter the response of the FBG and drive it away from the optimal operating point of the system. Various analog and digital processing techniques are integrated into the system to improve the signal to noise ratio. The Optogero system also includes a middleware architecture that allows users to easily embed data processing algorithms (imaging, artificial intelligence) for specific applications.

The system has been tested over a two-year period in the laboratory to validate its operation. Measurements were done on various materials such as aluminum, polymer-based composites or concrete, using several different testing methods (acoustic emission measurements, guided wave imaging, at room temperature and high temperatures). We will present the operating principles of Optogero and the system's performance in a few representative use cases.

In Austria, a new railway bridge with a composite design has been constructed in an area characterized by complex geological conditions. To obtain detailed insights into the bridge’s structural behaviour from the initial state on, an integrated monitoring concept based on distributed fibre optic sensing (DFOS) was implemented during the construction process. The bridge features a steel truss superstructure combined with a reinforced concrete deck. Fibre optic sensors were embedded directly within the composite joint, in the concrete slab, and along the steel framework to enable spatially continuous, high-resolution measurements of strain and temperature throughout the construction phase and ongoing operation. The initial measurements covered key stages starting from concreting to proof load testing. Subsequent periodic measurements are planned to capture long-term changes in the internal strain state of the bridge, including longtime effects of the structure and surroundings.

The chosen monitoring approach enables spatially continuous detection of both strain and temperature in the different bridge components. To ensure reliable strain evaluation, a temperature compensation model was developed through laboratory calibration using climate chamber tests on representative steel and concrete specimens with embedded fibres. These tests facilitated the derivation of material-specific temperature–strain relationships and allowed for the separation of thermally and mechanically induced strain components in the field data. The resulting compensation model is applied to the distributed measurements, ensuring accurate assessment of structural strain under varying environmental conditions.

This paper presents the overall concept and exemplary results obtained during the construction phase, demonstrating the feasibility and performance of the monitoring system under real-world conditions. The recorded data provide valuable insights into the evolution of the structural response during key construction stages, including concreting, composite activation when removing the auxiliary supports and proof load tests just before starting the operation. Initial findings show a clear correlation between measured strain patterns and construction activities, validating both the sensor placement and the overall measurement concept. The presented results represent an important step toward the practical application of distributed fibre optic sensing in large-scale composite bridge structures under complex site conditions. The insights gained will serve as a foundation for further evaluation during the operational phase, supporting continuous structural health monitoring, early detection of potential damage, and an improved understanding of the long-term behaviour of steel–concrete composite systems.

Measurement of guided waves at high temperatures is a major challenge
in SHM for several critical applications in aerospace, nuclear and other industries.
Classic piezoelectric sensors are based on lead zirconium titanate
which have a Curie temperature around 350 °C preventing the use of these
transducers at higher temperatures. Optical fiber sensors, based on fused
silica, are intrinsically more stable at high temperatures. Particularly, regenerated
and femtosecond microvoid-based Fiber Bragg Gratings (FBGs)
were demonstrated to operate at temperatures exceeding 1000 °C.
Ultrasonic wave measurement with FBGs can be realised using the edge filtering
method, consisting of placing a narrow-band light source on the edge
of the reflection peak of the grating. Strain applied to the FBG will modify
the grating reflection spectrum and hence change the reflected power at the
source wavelength. This power is measured and constitutes the ultrasonic
signal. Among the advantages of this method, it provides a high-enough
sensitivity to measure sub-micron strain and it enables high sampling rates,
both being required to measure guided-waves. As FBGs are also sensitive
to quasi-static strain and temperature changes, it is advantageous to use a
tunable laser with a tracking algorithm to lock the laser on the edge of the
grating. A home-made interrogation based on the aforementioned principles
was developed.

To this day, high temperature measurements of ultrasonic waves remains
challenging. Particularly, the sensor’s lifespan will depend on the amplitude
loss and wavelength drift of the grating, but also on its attachment especially
on structures with high thermal dilatation. Hence, coupling strategies
for the fiber will be discussed: indeed careful choice of the adhesive should
be made. Removing the coating of the fiber can also be considered as offthe-
shelf coatings have temperature limits below the one of the fiber. In
this contribution, we will present measurements of ultrasonics guided waves
performed at high temperatures (up to 1000 °C) in a tubular oven (cf. attached figure) with femtosecond microvoid FBGs, using the edge-filtering based
interrogation system. Especially, we will illustrate its ability to measure
acoustic emission signals in severe conditions hardly achievable by current
monitoring solutions.

Structural health monitoring (SHM) has a potential 13.1 times return on investment as it allows reduction in maintenance costs as well as increased service life. Several different SHM techniques have been developed trying to detect damage in the structure making use of different damage sensitive features. Based on the application demand, different sensor systems have been employed for the collection of the data. Each sensor system brings their own set of advantages and disadvantages.

In the last 20 years, immense work has been carried out using optical fiber sensors (OFS) for the SHM applications. OFS are light in weight, easy to multiplex, electro-magnetically neutral and may be embedded in structures. Hence are seen as ideal sensor for structural health monitoring (SHM). Fiber Bragg grating sensors have been utilized for guided wave sensing in the edge filtering configuration, they offer several interesting opportunities such as acoustic coupling, mode filtering, and self-referencing. But they suffer from drawback such as directional sensitivity, and limited bandwidth (upto few hundred kHz). To overcome these shortcomings, a new class of OF sensors based on the micro ring resonator (MRR) is proposed. The MRR works on the principle of constructive interference where integer multiples of particular wavelength of light constructively interfere to give large reflection peaks (similar to that of the FBG sensors). The key difference, is that the FBG sensors are typically of the order of few mm (typically 10mm) while the MRR is typically about 200 times smaller (45-50 µm). This makes the MRR a really point sensor thus allowing to overcome the directional sensitivity, as well as increase the bandwidth by about 100 or so times. The MRR response under different excitations frequencies for guided wave sensing is investigated in this study.