Fibre metal laminates, such as GLARE, are used in aviation and can be exposed to operating temperatures from −55°C to 70°C. During the recurring on-site inspections, temperatures ranging from −5°C to 55°C may occur depending on the geographical region. The material properties of the constituents, and especially the matrix material of fibre reinforced polymer laminates, are highly temperature-dependent and behave increasingly non-linearly the closer they get to the glass transition temperature. Also, due to the different thermal expansion coefficients of the constituents, additional thermal stresses can arise in the laminate. Temperature-induced changes in the material properties and thermal stresses influence the wave propagation behaviour of guided ultrasonic waves, which are widely used for structural health monitoring. The wave field in fibre metal laminates is generally complex due to the heterogeneity of the material and structure, making the guided ultrasonic waves application for structural health monitoring difficult. However, both wave propagation velocities and attenuation behaviour are influenced by temperature. Prior knowledge of the wave propagation characteristics of the structure is therefore essential for conducting structural health monitoring utilising guided ultrasonic waves.

Information on the thermo-mechanical behaviour of GLARE in terms of crack propagation and fatigue is available in the literature. Nevertheless, there are only a few studies on the influence of temperature on the wave propagation behaviour for GLARE. Temperature-related changes in the wave field must be considered to make reliable statements about the health status of the material.

This work numerically investigates the temperature-dependent behaviour of GLARE by calculating the dispersion diagrams for the group and phase velocities of the A0 and S0 modes, respectively. For this purpose, the thermo-mechanical properties of GLARE are determined based on literature data for a wide temperature range. The numerical model is then validated with experimentally determined dispersion diagrams of the group velocities of the S0 mode for different temperatures. Furthermore, these experiments provide information about the temperature-dependent damping behaviour. Thermally induced stresses are not considered in this work.

The data obtained in this study can be used to improve the reliability of structural health monitoring systems under real-world conditions.

The detection and quantification of defects plays a crucial role in several industries. Assessing the severity enables to schedule maintenance appropriately, optimizing costs and reducing risks. Elastic guided waves (GW) are particularly suitable for Structural Health Monitoring (SHM) applications on thin structures (such as plates, tubes or tanks) thanks to their long range and high sensitivity.

Working in an SHM framework requires robust methods under variable environmental and operational conditions (EOCs). As GW methods use the material properties of the structure as an input data, the modification of the EOCs affects the results quality through the estimation of the dispersion curves. In this work, we focus on temperature variations involving variations in material properties. An experimental study is conducted in a climatic chamber to get GW data for a range of temperatures from -20°C to 50°C and for a range of frequencies from 30 kHz to 100kHz. We propose a method that assesses effective material properties based on the GW signals obtained from a distribution of sensors surrounding an area that we want to monitor. Each signal between a couple of sensors is represented in the time-frequency domain thanks to a reallocated spectrogram which enables to compute a fitting of an explicit equation given by Mindlin-Reissner theory on the power spectral density given by the time-frequency representation. We assume that temperature affects mostly the Young’s modulus which is thus the fitted parameter. A strategy is also being implemented to remove contributions that are affected by a potential defect. We show that effective material properties estimated by the method depend on frequency (see the figure attached to this abstract).

Our group works since several years on an original SHM approach for the detection and quantification of corrosion in pipes called "passive guided waves tomography". This technology is based on the combination of GW tomography algorithms and a passive method such as the so-called ambient noise cross-correlation. It allows to obtain absolute and precise maps of the thickness of an area surrounded by a distribution of sensors without emitting waves, simply by analysing the elastic noise that exists naturally in the structure (due to vibrations, turbulences of the moving fluid, etc.). The type of corrosions imaged by this technique are thickness losses on extended areas of size often higher than the centimetre. The method, presented before, estimating effective material properties from GW data is applied to GW tomography imaging for the same range of temperatures and frequencies. For the sake of experimental simplicity, this study is performed in active. It shows that GW tomography gives better imaging results when it uses dispersion curves computed for effective material properties given by the dynamic estimation performed at the working frequency of tomography instead of the nominal (theoretical) value at such frequency.

In structures equipped with piezoelectric transducers, the quantification of the efficiency of the energy conversion between mechanical and electrical domain (and vice versa) was already demonstrated to be a reliable feature for determining the presence of a structural damage. This monitoring approach was demonstrated not only to be sensitive to damage presence, but also to its location. Furthermore, low sensitivity to temperature was demonstrated analytically and through experimental tests. The efficiency of energy conversion is determined by means of vibration measurements to derive the value of the modal electro-mechanical coupling coefficient, which is the damage feature employed to calculate damage indexes. For a given piezoelectric transducer (either patch or stack) attached to a structure, the modal electro-mechanical coupling coefficient is estimated at a given eigenmode by deriving the corresponding eigenfrequencies with the transducer short- and open-circuited, implying the need of performing two operational modal analyses in two different configurations. It is possible to demonstrate that method sensitivity to damage is similar to those of methods based on estimation of either eigenfrequencies or mode shapes, but with the following advantages: the modal electro-mechanical coupling coefficient is less sensitive to temperature compared to eigenfrequencies, and its value is correlated to mode shape even if its estimation does not require any dense measurement mesh (which instead occurs for mode shape estimation).

Nevertheless, in some cases, the estimation of this damage feature with vibration measurements can be challenging because of different reasons, such as, e.g., non sufficient external excitation to the system, low values of the modal electro-mechanical coupling coefficient which makes its estimation uncertain and implies the need of shunting the piezoelectric elements with tailored electrical impedances for improving estimation, significant temperature variations during the modal tests when they are long.

The present paper aims at showing that similar damage indexes can be estimated also by measuring capacitance trend as function of frequency for each piezoelectric element in the structure. This allows monitoring the structure by means of quick tests not affected by environmental changes, without any need of vibration measurements and addition of external electric circuits used to shunt the piezoelectric elements, with significant simplification of the estimation procedure.

Three different damage indexes are proposed, evidencing which of them are the most reliable for detecting the presence of the damage and getting information about its location. The method and the indexes are studied through numerical analyses and validated by means of an experimental campaign on tailored set-ups.

Precise and reliable fuel gauging (i.e. estimation of the remaining fuel mass) is essential to most transport industries, especially in the aeronautics sector. The measurement of liquid level and density through conventional sensors placed inside a fuel container implies complex and costly inspection and maintenance operations of said sensors, whereas sensing methods using external instrumentation could considerably simplify those operations. To that extent, ultrasonic guided waves propagating within the tank walls offer a suitable alternative, as they are known to be sensitive to the presence and properties of a liquid and can be emitted and sensed indistinctly on both sides of the walls. In this study, we propose a liquid level sensing method using piezoelectric transducers placed on the outer walls of a container, that combines ultrasonic signals from various paths in order to compensate for the effects of temperature. It is based on both time of flight and phase of those signals, which allow for a robust and precise level estimation. Experimental tests are carried out on an aluminum tank with various liquids (water, fuel) on a large temperature range, and levels estimated by the method show a millimeter accuracy. A second series of experiments is also presented, where a larger set of liquids is tested (multiple fuels and oils) and a density estimation method based on the same signals shows a precision of around one percent. These results open the way for a new kind of non-intrusive, accurate gauging systems that suit the needs and constraints of the aeronautics industry.

Since the first successful orbit insertion in 1957, space has evolved into a major political, military and scientific issue, and subsequently, an economic concern. However, the high development and production costs associated with the non-reusability of expendable space launch vehicles restrict their use. In recent years, several developments have been made to enable the reuse of the first stage of launch vehicles, which account for 70% of the total cost. Nevertheless, due to the limited knowledge of operational environments and risk of in-flight events that could damage the structure, the vehicle's flightworthiness must be systematically validated prior to take-off. Therefore, the cost savings achieved with reusable launch vehicles (RLVs) depend on minimizing the cost of reuse.

To that end, non-destructive testing (NDT) methods can be used to inspect a structure without compromising its integrity. However, NDT inspection techniques can be cost-prohibitive in areas with limited access and require scanning the zone of interest, which is time-consuming especially for large structures. Structural health monitoring (SHM) systems, which consist of the permanent integration of sensors, avoid exhaustive non-destructive testing, enable reductions in downtime thus reducing costs, while simultaneously enhancing the reliability and durability of RLVs through real-time assessment. Nevertheless, implementing SHM in space applications faces important challenges. One of these concerns the degradation of the SHM system due to thermomechanical aging resulting from exposure to extreme operational conditions experienced by RLVs, which results in reduced reliability of the system to assess the state of the structure. In this context, this work addresses the revalidation of a composite cryogenic tank between two flights, using ultrasonic guided waves which are generated and received by bonded piezoelectric transducers.

The present study investigates the effects of aging on the detection and localization of a delamination damage on a composite tank with a Finite Element model. For this purpose, a numerical model representing both the SHM system and the composite tank, whose dimensions are approximatively of 1,2 m length and 24 cm of diameter, has been developed using the finite element method (FEM). In order to keep a reasonable computational cost, the composite structure is modelled using the Equivalent Single Layer (ESL) approach associated with shell-type elements, as the structure has large dimensions with relatively small wavelengths (about 10 millimeters) and time steps (about 10 µs). First, the undamaged tank model is compared with experimental data acquired from wave propagation measurements performed on a full-scale composite cryogenic tank using a laser Doppler vibrometer. Then, damage detection and localization tests are performed numerically using different damage indexes (DIs) and localization algorithms such as RAPID (Reconstruction Algorithm for Probabilistic Inspection of Damage) and DAS (Delay and Sum) under various aging states of the SHM system.

As a result, the study demonstrates that aging significantly compromises the damage detection and localization performance of an SHM system, emphasizing the importance of ensuring its reliability throughout the operational lifespan of reusable launch vehicles.