Since the dawn of time, human beings have been always fascinated and envious of the flight of birds and human’s desire to fly has become stronger and stronger over the centuries. What child looking at a bird does not wish that they could take flight to chase it and view the at the landscape from above? The dream to fly is inside the human being since tender age, although the physical possibility does not belong to their nature. Nevertheless, humans have exploited the power of their brain and hands to get closer this dream.

Leonardo Da Vinci (1452-1519)1 was the first one that effectively studied the flight dynamics of the birds, developing several technical drawings and building flying machines between the XV and XVI century. However, for almost 400 years, no tangible developments occurred until the Wright brother’s famous flight (in 1903)2, which is the major milestone of the aeronautical world. From that far away event, enormous technological developments have been made so that a large number of apparatuses able to fly in the atmosphere and travel in Space have been developed.

One of the key technological sectors that has allowed the large growth of the aerospace industry is that of the materials. Specifically, the large use of composites materials and adhesives has allowed the design and building of modern airplanes, spacecraft and devices which are increasingly lighter, faster, stronger and tougher. Nevertheless, at the same time, the increase of aerospace performance by using new materials has raised the necessity to develop a technology which is able to provide information on the health state of the materials that composed the structures. This need is not only linked to the necessity to increase the safety but also to two other factors: to optimize the maintenance schedule of the aircrafts so that to save economical resources, and to increase our knowledge about the mechanical, damage and fracture behavior of the materials.

Although, during the decades, a number of inspection and monitoring technologies have been developed as such as piezoelectric, acoustic and ultrasound sensing, electrical gauges, fibre optic sensors, eddy current, comparative vacuum, interferometry, penetrating liquids, thermography, radiography (x-rays) and others, not one of these has proved to be applicable for the entire life cycle of the material (from the manufacturing, operating phase to the end of life) which provides in real-time enough information to effectively evaluate its health state.
Therefore, the here presented thesis aims to demonstrate the multi-sensing abilities and monitoring benefits of tilted Fibre Bragg gratings sensors (TFBGs) in order to fill a current technological gap in the previous technologies and to improve the state-of-art of the Structural Health Monitoring field. The research focused initially on the fundamentals, mathematical and numerical modelling, methodology and demodulation techniques of the TFBG sensors, successively the treatment is dedicated to the TFBG applications for simultaneous three-parameter monitoring embedded in composite material and silicone adhesive, respectively for aeronautical and space use. This selection of these materials was made from those commonly used in the aerospace industry. The application of TFBGs can be, from the very beginning, have a great impact in the scientific community and, also maybe, in industry.
Therefore, taking into account what was reported previously, the research presented in the following thesis was confronted with the aim to demonstrate that TFBG can be a promising sensor for structural health monitoring of aerospace materials, and are able to provide reliable and simultaneous multi-parameter measurements. The treatment begins with the first introduction chapter where composite materials, silicone adhesive and TFBG sensors are presented fro a historical perspective, and their current state-of-art regarding applications, issues and technological gaps in relation with the working load and environment, are reported. Then, the research questions and reasons that motivated the scientific investigation are introduced in the last part of chapter 1.
The starting point of the research can be considered chapter 2, where, first of all, the realization of a TFBG sensor customized for the desired application is confronted by providing a numerical model to simulate the spectrum based on the values of the parameters of the sensor Bragg structure. This is important to obtain TFBGs with a spectral signal usable for the simultaneous and separated measurements of different parameters before the manufacturing of the same sensor. It can be noted that, the simulation of the TFBG spectrum may bring other several benefits such as production time and costs savings. In fact, the determination, a priori, of the Bragg structure, and hence, the parameters of the TFBG manufacturing setup, allows the TFBG sensor to be obtained with the desired measuring characteristics without the waste of materials, testing and manpower time.
In chapter 3, the demodulation of the TFBG spectrum for refractometric measures was improved by developing a new technique based on the Delaunay triangulation of the datapoints that compose the spectral signal, which was demonstrated to be faster and more accurate than previous techniques. This technique is also compatible with the method to extract the strain-temperature variation from the spectrum, so that the methodology for multi-parameter measurement performed with a single TFBG is completed.
At this point, chapter 4 starts the beginning of the second part of the thesis, where the simultaneous thermomechanical measurements of the TFBG were used to determine the deformation effects of a thermal load profile on a glass-fibre/epoxy composite plate induced by heating lamps. A further step achieved in this chapter, was to compare the TFBG measurements with a classical strain sensing technique based on the use of a TFBG as standard strain gauge thermally compensated by a thermocouple. Furthermore, another comparison was performed between the experimental results and the Finite Element Model (FEM) analysis results obtained by modeling the composite sample with and without the embedded TFBG and applying a Gaussian thermal profile.
These comparisons show that the strains measured with the single TFBG are very close to the values obtained by using the classical approach and the full FEM model. The improvement regarding the single TFBG sensor can be obtained by increasing its thermal resolution which is the weak point of the simultaneous thermomechanical measurements. Nevertheless, it can be further improved by using an interrogation system with a finer wavelength scanning resolution or by designing a TFBG whose resonance peaks move in the spectrum much more with the temperature variation. The comparison of the empirical measured deformations with the strain values extracted by the FEMs analysis have highlighted the importance to model the optical fibre inside the composite to increase the accuracy of the model. In conclusion, the single TFBG sensor was able to monitor the thermomechanical trend inside the composite during the heating lamp exposure with a good accuracy as the values were close to those measured with the classical approach and full FEM simulation.
The conclusions drawn from the first and second part of the thesis are that the investigation on the multi-parameter sensing abilities of the TFBG sensors, also embedded in composite materials for a complete monitoring of their state from their manufacturing to the operating life, has given positive and promising results. Especially regarding the embedding of the sensor, each single TFBG sensor can be used in order to measure simultaneously thermomechanical and refractometric variations of the composite material where the sensor is embedded and in any step of the material life.
The second part of the thesis continues with chapter 5. Here, the TFBG sensors are embedded inside glass-fibre/epoxy resin composite material to monitor simultaneously the strain-temperature variations, the resin refractive index (RI) during the manufacturing process and the application of a thermal load profile. Regarding the TFBG monitoring performed during the composite manufacturing, several samples of different thickness were tested and sensorised. In these samples, the sensors were able to measure, simultaneously, the strain state induced in the material due to the manufacturing steps, the temperature profile, and the resin RI variations due to the crosslinking occurring during the curing. This allowed the evaluation, not only of the possible state of stress in the material during the production, but, even more interesting, the cure degree of the resin. Indeed, as anticipated already in previous research, the resin RI measured with the TFBG can indicate the curing degree matrix of the composites. The TFBGs were able to provide the entire RI profile of the resin during the curing in which three different behavior ranges were identified in combination with the temperature trend. The expected typical plateau was reached in the RI curve when the resin was considered fully cured. Furthermore, the refractometric sensing abilities of the TFBGs were also used to monitor the resin flow and speed during its infusion.
From chapter 6 on, the treatment is focused on the TFBG application study on the space qualified silicone adhesive, which regards the third part of the thesis. Silicones are strategic and widely used materials in many engineering fields, but mainly their use finds great importance in space industries, especially for electronic and structural components. Nevertheless, some parts of the spacecraft where silicone elements and adhesives are used undergo a direct exposure to the space environment, which represents harsh operating conditions and is strongly degrading for any material. In fact, perturbations as severe temperature gradients, ultra-high-vacuum, Ultra-Violet (UV) and ionizing radiations (x-rays, γ-rays, etc…), extreme thermal range and cycles, thermal shock, micro-gravity, atomic oxygen (ATOX), high accelerations, vibrations and space debris are characteristic of the space operating environment. Furthermore, it has to be also considered that these elastomers have to maintain their original mechanical and physical properties during their operational life in space, which is a challenging target. In this context, TFBG sensors embedded inside silicone adhesives, through their simultaneous thermomechanical and refractometric measuring abilities, may offer fundamental information to evaluate the internal mechanical and chemical state of the elastomers during the use in space environment. This would offer the possibility to have a technology able to provide a general view on the degradation state of the adhesive in real-time during the laboratory testing or operational life, which may improve the evaluation of its health and performance trend along the exposure time. Nevertheless, as for the composite, the monitoring technology should be always minimally intrusive, light and not affecting the material performance and properties. Hence, in order to match these requirements, each embedded TFBG sensor has to be able to perform the thermomechanical-refractometric measurements as single sensor without to be affected by the exposure to the space environment. The demonstration of a working TFBG sensor in a simulated space environment may be an interesting and promising starting point for the future development of a sensing technology able to real-time detect degradation and damages in spacecraft structures and components during the space missions.
In chapter 6, since studies in literature about the compatibility of optical fibre sensor layers in vacuum were not found, initially, the waveguide containing the TFBG was subjected to an outgassing test. Once the compatibility was verified, a TFBG was then embedded inside a space qualified silicone used as adhesive to join two micro-sheet cover glasses in order to compose a sandwich. This TFBG sensorised glass sandwich was tested inside a vacuum chamber and exposed to high-vacuum (around 10-6 mbar) and loaded with a thermal cycling profile. Hence, in this first approach, the experiment was planned to simulate the degradation on the silicone adhesive of the TFBG sensorised sample generated by outgassing (due to high-vacuum) and thermal cycling loads, which are perturbations constantly present in space environment.
The measurements of the single multi-parameter TFBG sensor, acquired during the experiment, were compared with the values obtained by using the classical sensing approach consisting in the thermal compensation of the TFBG (used as strain-gauge) through a thermocouple. The comparison highlighted the inaccuracy of the classical sensing method due to the different location of the sensors in the silicone which brings serious mistakes in the measurements. In fact, due to the absence of an atmosphere in the space environment, the transfer of heat inside a body depends only from the thermal conductivity of the material which accentuates the issue of the classical sensing approach linked to the different location of the sensors. While, the self-compensated TFBG can overcome this gap as it is thermally self-compensated. Furthermore, for a complete evaluation of the health state of an organic material such as a silicone adhesive, temperature and deformations may be not sufficient as it is not easy to obtain information on the chemical state of the material, such as the variations of the silicone RI. The embedded TFBG sensor, simultaneously sensitive to temperature and deformations, was able to provide measurements of the silicone RI that was changing due to hardening and chemical evolution induced by the thermal cycles in high vacuum environment. The chemical variations were checked also by testing a sample of pure cured silicone adhesive via the Differential Scanning Calorimetry, which showed chemical variations due to the realising of volatiles during the heating-up and cooling-down phases of the test. These results make the TFBG a promising sensor able to provide a complete evaluation of the silicone state that comprises the thermomechanical and refractometric measurements while working in space environment.
By exploiting the experiences achieved in the previous chapters, chapter 7 treats of the research focused on the detection of the UV effects induced in the silicone adhesive working in space simulated environment. This topic may be interesting for polymers used in a space working environment as long exposure to UV radiation in vacuum can cause severe damages and bring the component to failure together with the structures of a spacecraft. The reason lies in the absorption of these light wavelengths by the silicone, whose organic chemical composition is photochemically susceptible to these light wavelengths. As a consequence, the photochemical reactions cause severe degradation of the material with deterioration of the original properties and efficiency and shorter working life. Then, it is easy to understand that a sensing technology able to monitor the degradation of the silicone working in the previously described conditions, may be really useful to evaluate the state of the material during its use, but also, in the phase of laboratory testing to better detect the material behaviour and its changes. Therefore, several and different TFBG sensorised samples were tested in a high vacuum chamber provided of UV lamps which emitted a high radiation level for a certain exposure time. Hence, the acquired spectra were demodulated and used to measure the thermomechanical and refractometric variations induced by the UV exposure inside the silicone in correlation with the equivalent exposure solar hours. This allowed an analysis of the different degrees of degradation of the silicone based on the sample configuration and exposure time. In conclusion, the achievement of this research was the demonstration of a minimal intrusive sensor as the TFBG is not only compatible with operating in a space environment, but is also able to provide in-situ reliable multi-parameter sensing for the monitoring of the thermomechanical and refractometric state of the material during its operations in space environment.
The conclusions of the thesis are reported in chapter 8 where the outcomes of the conducted researches highlighted the potentiality of the TFBG as a single three-parameter sensor to monitor the state of materials for aerospace industry. As consequence, these results may induce the raising-up of the SHM concept by developing a sensing technology based on the TFBGs that are able to provide an overall view of the material state, from the manufacturing to the operational life, also working in harsh environmental conditions as those in space.@en

Tilted fiber Bragg grating (TFBG) sensors were demonstrated to simultaneously measure the material thermomechanical and refractometric state in which they are embedded. In this work, for the first time, TFBGs are investigated for three-parameter monitoring of space-qualified NuSil® CV16-2500 silicone operating during high-vacuum ultraviolet (UV) exposure. The first part of the work is focused on the ultraviolet effect on the TFBG spectrum when the sensor is 1) directly exposed to the radiation, 2) covered by a thin cover glass, and with a Kapton layer on top. Successively, the silicone is used as an adhesive in a sandwich structure in which the TFBGs are embedded and exposed under high vacuum to various UV/vacuum UV intensity radiations and durations. The sensors’ spectra were acquired and demodulated to detect the silicone strain-temperature-refractive index variations and correlate the silicone refractometric changes with the equivalent exposure solar hours. The second part of the paper is on silicone degradation state evaluation using the same sensor but during a direct exposure of the adhesive to the radiation. This allowed the UV effects on the silicone to be enhanced but needed a method to compensate for the damaging effect of UV radiation on the TFBG spectrum.

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In this research the ageing of a silicone adhesive in a simulated space environment is monitored through an embedded three parameter tilted fibre Bragg grating (TFBG) sensor. Here, the silicone is used as an adhesive between two thin cover glasses, and the space environmental ageing is simulated by thermal cycles in high vacuum conditions (better than 10-5 mbar). These operational conditions can induce variations in the silicone adhesive with respect to its original properties such as dimensional stability, chemical composition, generated contaminants, discoloration and, mechanical or optical degradation. Therefore, surrounded by the adhesive, in the centre of the cover glass sandwich, a weakly tilted FBG sensor was placed to obtain information from its spectra on the state of the polymer during the test. Specifically, the temperature, strain and refractive index (RI) of the silicone can be, simultaneously and separately, measured from the spectrum of a single TFBG from selected resonance peaks. These parameters can be used to evaluate the 'health' state of the silicone during the vacuum thermal cycles. The simultaneous TFBG thermomechanical measurements gave a solution to the non-localized measuring issues when using classical fibre optic or electrical strain-gauges and a thermocouple to compensate the temperature and to better understand the material behaviour. The trends of the measured parameters are reported during the entire testing time, and at the end of the test, the optical fibre sensor measured a negative strain of ∼100 μϵ and a positive RI variation of ∼0.002.

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The unique sensing features of the tilted Fibre Bragg Grating (TFBG) as a single three-parameter optical sensor are demonstrated in this work, to monitor the manufacturing process of composite materials produced using Vacuum Assisted Resin Transfer Moulding (VARTM) process. Each TFBG sensor can measure simultaneously and separately strain, temperature and refractive index (RI) of the material where the optical fibre is embedded. A TFBG embedded in a 2 mm glass-fibre/epoxy composite plate was used to measure the thermomechanical variations induced during the curing process. At the same time, the RI measurements, performed with the same TFBG sensor, can estimate the degree of cure of the resin. The TFBG sensor shows to be a valid and promising technology to improve the state of art of sensing and monitoring in composite material manufacturing.

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To perform active structural health monitoring (SHM), guided waves (GW) have received great interest as they can inspect large areas with a few sensors and are sensitive to barely-visible structural damages. Fiber Bragg grating (FBG) sensors offer several advantages such as small size, low weight and ability to be embedded but their use has been limited for GW sensing due to their limited sensitivity while using spectrometers. FBG sensors in the edge-filtering configuration have overcome this issue with reasonable sensitivity and there is a renewed interest in their use. It is well known that when subjected to a transverse strain, the circular cross-section of the fiber deforms into an elliptical shape generating the birefringence phenomenon. This deformation, influences the coupling mode of the light inside the FBG and hence, modifies the resulting reflectivity spectrum. This paper investigates how controlled changes in the reflectivity spectrum can be introduced using different transverse loads. The effect of the modified spectrum on the sensitivity of the FBG for GW measurements is then studied. The study also investigates the effect of the transverse strain on the coupling of the GW from the structure into the fiber.

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A simultaneous two-parameter single sensor based on weakly tilted Fibre Bragg Grating (TFBG), embedded in a 1 mm glass fibre/epoxy composite plate, is demonstrated to measure independently the temperature and strain variations induced in the material by the exposure to heating lamps. The spectrum of weakly TFBGs is composed of several peaks that can be used for different sensing purposes. Here, the shifting of the Bragg and the Ghost peaks are considered to calculate the strain and temperature variations through thermomechanical sensitivity coefficients of the selected peaks. To prove the reliability of the TFBG measurements, the resulting strain values were compared with the strain measurements obtained from the TFBG when compensated by a K-thermocouple embedded close to the optical fibre sensor. Furthermore, the numerical simulation of the full Finite Element Model (FEM) (composite + TFBG) and partial FEM (composite only) models were carried out by assuming a 3-D Gaussian temperature profile. This allowed the TFBG experimental measurements to be compared with the simulated results. A study focused on the strain deviation showed a good match between the full FEM and the TFBG measurements with an average error of ~5% in the case of the dual-parameter sensor and ~2% for the compensated TFBG.

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PowerPoint presentation: Dutch initiative of innovative companies and knowledge institutes combine and develop knowledge and experience in inspection, production, repair and maintenance of composites.@en

Fibre Bragg gratings (FBGs) are obtained through a permanent and periodic refractive index modulation in the core of the single-mode optical fibre. For many years, they have been employed in telecommunication industry as a passive device for wavelength division multiplexing and dispersion compensation components, or in laser apparatus for laser fibre stabilization, Erbium amplifier gain flattening device and amplifier pump reflectors. In aerospace structures, FBGs are used as sensors for structural health monitoring of composite materials as they are able to perform measurements of several parameters inside the material in an elegant and low intrusiveness way. Based on the Bragg and optical fibre structure many kind of customizations can be applied on FBG sensors during the manufacturing process. Each of them gives to the FBG sensor different proprieties and sensing abilities. In this work, we address the numerical simulation of the reflected spectrum by a special FBG sensor called a tilted FBG (TFBG), in which the core refractive index modulation is performed in way to obtain a tilted Bragg super-structure. By considering the classic Coupled-Mode theory for weakly-guided waveguides, we solved the mode propagation equations with the Transfer Matrix method (TMM) obtaining the TFBG reflectivity for different tilt angles.

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Reflective Tilted Fiber Bragg Grating (TFBG) sensors have intriguing sensing capabilities due to the resonance-peaks present in their transmitted spectrum. Previous works measured the external refractive index (ERI) in which the TFBG sensor is placed, by considering the wavelengths or the envelope of the cladding-modes resonances. In this paper, primarily, we demonstrate the effectiveness of an alternative global technique, based on Delaunay triangulation, to analyze the TFBG spectrum for refractometer purposes. Hence, we performed the correlation between the area subtended by the upper and lower cladding-modes peaks and the ERI. An investigation on the goodness-of-fit correlation functions is also presented for TFBG sensors written in standard- and thin-optical fibres and considering different values of the fundamental triangulation parameter α.

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PowerPoint presentation@en

Hovering flapping wing flight is intrinsically unstable in most cases and requires active flight stabilization mechanisms. This paper explores the passive stability enhancement with the addition of top and bottom sails, and the capability to predict the stability from a very simple model decoupling the roll and pitch axes. The various parameters involved in the dynamical model are evaluated from experiments. One of the findings is that the damping coefficient of a bottom sail (located in the flow induced by the flapping wings) is significantly larger than that of a top sail. Flight experiments have been conducted on a flapping wing robot of the size of a hummingbird with sails of various sizes and the observations regarding the flight stability correlate quite well with the predictions of the dynamical model. Twelve out of 13 flight experiments are in agreement with stability predictions.

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Loads applied transversely on the external surface of waveguides change their circular cross-sectional geometry generating birefringence. Due to this effect the reflected spectrum of a Fibre Bragg grating (FBG) undergoes a splitting of the single peak of the Bragg wavelength. In this work, we employed the Transfer Matrix Method (TMM) for x- and y-polarized wave-modes to model the uniform FBG reflection spectra for uniform and non-uniform transverse loads. We also performed experimental measurements for two different transverse load scenarios. The load profiles chosen for these experiments were applied on the FBG sensor through a block of steel and a roll bearing pin. Then, the modelled and experimental results were compared resulting in good agreement of 85% (on average). Finally, during the roll bearing pin loading test, different responses were observed depending how the FBGs were surface mounted. To investigate this, the glue layer influence on the reflected spectrum was further studied experimentally.

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