Monitoring structural behaviors of prefabricated prestressed concrete structures is critical for ensuring their long-term safety and durability. Among these, beam-like structures with double-T cross-section are widely used in prefabricated structural systems due to their strength, long-span capabilities, and versatility, but exhibit nonlinear stress and strain fields due to complex geometry. Internal strain changes in different directions can provide valuable insights into structural behaviors, especially when structural issues such as crack opening or prestress losses occur, which can compromise the integrity or durability of the structure. Therefore, understanding how multidirectional strain responses develop under various conditions is essential for interpreting the mechanical behavior of such structural elements.
This study focuses on the multidirectional strain responses of a prefabricated prestressed double-T slab with embedded long-gauge fiber Bragg grating (FBG) sensors. The sensors are oriented in longitudinal, transversal, and angled directions (see Figure 1). The objective is to examine how elastic strain changes under applied prestress and external loadings reflect the structural behavior in different directions and their interrelationships. Measurements from long-gauge FBG sensors, combined with analytical modeling and numerical simulations, are used to interpret the multidirectional strain patterns and evaluate directional coupling effects. The main challenges include: (1) complex cross-sectional properties of the structure, which makes it not meet the Euler–Bernoulli hypothesis, and results in complex interactions between strain components in different directions, (2) thermal compensation, which requires reconstructing the temperature distribution along the double-T cross-section to identify thermally induced strains, and (3) measurements taken over a long term, which necessitates accounting for variations in material properties and boundary conditions.
The results demonstrate that the multidirectional embedded sensors capture variations in the strain field under applied loadings and during structural issues such as crack opening or prestress losses. Correlations among directional strains reveal elastic relationships indicative of the effective Poisson’s ratio and the geometric coupling within the double-T cross-section. These findings demonstrate the capability of multidirectional embedded sensing to enhance interpretation and validation of the mechanical response of prefabricated prestressed concrete systems with complex geometries.

The potential of Fiber Optic Sensors (FOS) networks for Structural Health Monitoring (SHM) on composite structures is considered high, as they can be easily integrated during the manufacturing phase, without appreciably altering their mechanical properties. However, the current State of the Art is difficult to implement in real aerostructures, as mostly present manual methods to integrate the sensor network in the composite during the manufacturing, even if current manufacturing techniques are automatic, such Automatic Tape Lay Up (ATL) or Fiber Placement (AFP). Additionally, the ingress-egress techniques to connect the embedded network are based on lateral connections, even if all composite structures are trimmed to nominal geometry after the manufacturing.

In this work, new automatic optical sensor lay up techniques and new connection methods through the thickness will be explored. First, the automatization of installation of optical fiber sensor network modifying an AFP head will be presented, including an additional optical fiber lay up. This method provides a precise determination of the position of embedded optical fibers. Additionally, it is presented a methodology to connect fiber optics through the thickness. Ultra high accurate fiber location (less than 5 micron) using phased-array ultrasonic inspection method using the phase signal is presented and validated. Experimental composite specimens will be manufactured with optical fiber sensors embedded at different depths and with various coatings and subsequently inspected using an Omniscan MX2 ultrasonic system. Finally, based on the results obtained, different repair and reconnection techniques for optical fibers embedded through the laminate thickness will be evaluated.

This study presents the embedding of Fiber Bragg Grating (FBG) optical sensors on an INVAR36 mold manufactured by L-WAAM[1] (novel DED[2] technique) intended for certain CFRP[3] of aeronautical sector. The integration of FBG sensors enables in-situ monitoring of both temperature and strain in critical areas of the mold, which can influence the surface quality and dimensional accuracy of the parts after autoclave curing processes.

The main objective of this research is to demonstrate the feasibility of combining additive manufacturing-based L-WAAM technology with fibre optic sensors to create a robust and smart metallic device capable of providing real-time information about its structural and thermal state.

FBG sensors were coated with nickel (Ni) using an electroplating process to provide mechanical protection and thermal resistance against the harsh L-WAAM embedding process. The electroplating parameters were optimised to achieve a smooth and continuous coating along the fibre, with varying thickness in the sensor regions. Before and after embedding the FBG sensors, a thermal calibration was performed to quantify possible changes in their sensitivity and to alleviate residual stresses induced during the coating process.

Preliminary embedding tests were performed using INVAR36 walls fabricated by L-WAAM on tensile specimens to determine the minimum coating thickness and process parameters that would ensure the survival of the sensor without compromising the integrity of the structure. The thermal and mechanical characterisation of these specimens with the embedded FBG sensors contributed to optimise the process and check the functionality of these embedded sensors.

As a final validation, two FBG sensors were embedded in a full-scale INVAR36 demonstrator mould. The number and location of the sensors were defined in coordination with the end-user, targeting previously identified defect-prone regions critical for structural health monitoring of the mould. Given the proof-of-concept nature of the study (TRL5) and the need to avoid re-machining or significantly altering the mould geometry, a limited number of sensing points were selected to validate embedding feasibility while ensuring minimal impact on structural integrity. The sensors successfully provided temperature data during the curing cycles and remained fully functional after several tests.

In conclusion to this work, it was demonstrated that the integration of FBG sensors into metallic structures using L-WAAM can be a viable approach in the development of smart metallic tools. The combination of fibre optic sensing and additive manufacturing technologies enables real-time monitoring and lifetime assessment of high-performance tools and structures in the aerospace industry.

[1] L-WAAM: Wire Arc Additive Manufacturing assisted by Laser

[2] DED: Directed Energy Deposition

[3] CFRP: Carbon Fiber Reinforced Polymer

In the last few decades, optical fiber sensors (OFS) have gained attention due to their immunity to electromagnetic interference, remote sensing capability, light weight, compact size and durability. A significant number of innovative sensing systems based on OFS have been developed for continuous measurement and real-time assessment of various systems. In our study, we propose a multimodal sensing platform based on multicore fibers using two-photon polymerization. Exploiting the benefit of having several cores in one fiber, it is possible to 3D print more than one sensor geometry and separate the effect of temperature and pressure as an example. To do so, different microsensors were fabricated on top of multicore fibers with two-photon additive manufacturing.
One geometry of the proposed sensor consists of two 45° angled prisms on top of two cores with a core-separation of 40 µm which creates an air-cavity Fabry-Perot (sensor-1) in between them. On the other two cores of the multicore fiber, we can fabricate a short-length corrugated grating along with the same type of two prisms (sensor-2). These two types of structures can sense different parameters simultaneously. In the figure below, demonstration for sensor-1 has been shown along with the recorded reflected spectrum.

Nuclear fusion is a clean and virtually inexhaustible source of energy and is currently humanity's hope for meeting its future energy needs. One promising method for obtaining energy from nuclear fusion is magnetic confinement of hot plasmas by strong magnetic fields. Of the various nuclear fusion reactor concepts, compact spherical tokamaks are the most advanced, achieving the highest power densities.

The recent development of high-temperature superconductors (HTS), which can operate at 20 K in magnetic fields of up to 20 T with high current densities (up to 400 A/mm²), has transformed the international nuclear fusion landscape. This development paves the way for even more compact, efficient and economical fusion reactors.

Cryogenic magnets in general, and HTS magnets in particular are complex and expensive systems, and In general, cryogenic magnets, and HTS magnets in particular, are complex and expensive systems. Therefore, it is imperative that they are protected against accidental damage during operation. Magnets are susceptible to quench events, whereby a region of the coil ceases to be superconducting, causing the current to encounter non-zero resistance. This transition leads to rapid local heating that could potentially cause catastrophic damage to the system. Therefore, reliable quench detection with advanced monitoring strategies is critical to ensuring the structural integrity of the magnets and stable reactor operation, which is required to maintain plasma confinement conditions.

Conventional quench detection techniques based on electrical and thermal measurements are well established for low-temperature superconductors (LTS) systems. However, they are less effective in HTS systems due to slower quench propagation, smaller voltage signals and the delayed response of point-based thermal sensors. These challenges highlight the need for more sensitive, distributed monitoring solutions, such as fibre Bragg grating (FBG)-based optical sensing systems. Compared with conventional electrothermal sensors, FBG sensors offer several advantages, including immunity to electromagnetic interference, the ability to multiplex, and minimal thermal perturbation.

This study explores the integration of an FBG-based fibre optic sensor network into HTS tapes for real-time temperature monitoring and quench detection in tokamak reactors. Owing to the limited space available for sensor integration within HTS tapes, the effect of reducing the fibre coating thickness on FBG sensitivity at a cryogenic temperature of 20Kk was studied experimentally at a laboratory scale prior to Tokamak integration, with the aim of minimizing the coating thickness without compromising performance.