Conference Agenda

The introduction of composite materials in aeronautics has brought numerous advantages, along with unique damage and failure modes. The structure health is currently ensured by a damage-tolerant design and non-destructive inspections. Among other techniques, Guided Wave-based Structural Health Monitoring (GW-SHM) has gained interest as a cost and time effective alternative to traditional non-destructive techniques. One of the main challenges for GW-SHM is the influence of environmental and operational conditions on the damage identification capability.

Aircraft structures undergo a broad range of mechanical load conditions, affecting the GW-SHM system. The study of load influence is meaningful in view of applications where the SHM system is expected to work under varying load conditions. Such applications may be in-flight monitoring, but also on-ground only usage and structural test monitoring.

A full‑scale CFRP door‑surrounding structure was instrumented with a GW‑SHM system and subjected to multi-axial dynamic mechanical loading. A hydraulic test rig applied a loading sequence resembling full‑scale fuselage fatigue tests. The resulting dataset contains 27 load combinations derived from the basic load cases. Although it does not capture the full complexity of the load conditions experienced by an aircraft, it provides the needed complexity to evaluate new approaches that address realistic load influences. The test was carried out in a hangar under uncontrolled temperature conditions, which produced a wide range of temperature variations during the experiment as well.

This study shows the effects observed by the GW‑SHM system under dynamic multi‑axial load conditions typical of fuselage structures. Finally, it presents a data‑driven approach based on Gaussian Processes to separate the influences of environmental and operational conditions, namely multi‑axial load and temperature, from those of damage.

Mode conversion-based full wavefield guided wave inspection is a promising approach for damage evaluation in complex structures because it enables physically meaningful discrimination between structural discontinuities and local damage. However, existing methods commonly rely on multi cycle narrowband excitation, which requires repeated tuning of the operating frequency for different structures and inspection conditions. To address this issue, this study proposes a mode conversion spatial tracking method using Gaussian pulse excitation. The proposed excitation contains broadband frequency content while exhibiting spatial compactness, making it well suited for mode conversion-based mapping methods. By tracking the incident S₀ mode in the spatiotemporal domain and capturing damage-related converted modes induced by local scattering, the method enables robust damage imaging without retuning the frequency. Experimental validation on aluminum honeycomb sandwich structures confirms reliable identification of both structural discontinuities and local damage, demonstrating the potential of the proposed method for guided wave inspection in complex structural configurations.

In recent years, the development of (reusable) cryogenic propellent tanks made of CFRP (carbon fiber reinforced polymers) for space and aeronautic application are continuously growing. These composite structures must be able to withstand extreme conditions without compromising safety and functionality and therefore structural health monitoring is beneficial. Typical production techniques for such tanks are filament or tape winding where pre-impregnated carbon fibers or tapes are winded around a Mandrel. Depending on the sensor design the integration of piezo sensors for SHM (structural health monitoring) during the manufacturing process could be done on the inner tank surface by placement in the mandrel or on the outer tank surface by overwinding with a protective layer.

In previous work the authors analysed three different sensors: foil type piezo sensors type smart layer® sensor and DurAct sensor for potential integration on the outer surface and AAC´s cylindrical piezo temperature sensor for integration on the inner surface with respect to its ability for impedance-based process monitoring and guided wave based structural health monitoring of composite structures. The applied fully coupled piezo-structural harmonic and fully coupled piezo-structural transient FE simulation showed a good agreement with the measured impedance curves and the voltage response of the different sensors caused by the guided waves. The simulation results of the wave propagation properties are important for the interpretation of the voltage response of the different sensors. The damage detection capability of the simulated defect depends on sensor design, especially on the size of the used piezo, and the actuation frequency. So far, all investigations were done at room temperature.

The aim of the presented work is to assess the effect of cryogenic temperatures and piezo sensor design on the excitation, reception, and propagation of guided ultrasonic waves in un-isotropic composites. Therefore, three nominally identical [0/90] cross-ply composite plates, already used in the previous work, equipped with five glued sensors located in the 0° and 45° direction of the above mentioned three types of sensors were used. Guided waves were actuated with a 3-sin burst signal in a frequency range between 40 kHz and 300 kHz and the response of the remaining sensors was measured. All measurements were performed at different temperatures from room temperature down to 77K in a temperature chamber and by fully immersion in liquid nitrogen. Out of the response of the four sensors the amplitude, arrival time, and group velocity of the present S0 and A0 mode were determined. The results show a much stronger temperature sensitivity and interaction with the slowly boiling liquid nitrogen of measurements performed at 77K of the asymmetric A0 mode compared to the S0 mode. The mode and temperature dependent non-monotonic amplitude depend on the changing material properties of the sensor and the CFRP materials, whereas the group velocities mainly depend on the temperature dependent material properties of the composite. Both properties are essential for the development of effective methods for temperature compensation.

The safety and service life of next-generation aerospace composite structures depend on the integrity of their complex geometries, such as curved C-sections. This paper presents a comprehensive methodology for Structural Health Monitoring (SHM) focused on the study of Mixed-Mode fracture in two curved composite demonstrators. The main objective is to develop and validate a Digital Twin (DT) framework capable of predicting the growth of delamination in real time under operational loads. The approach is to establish an accurate correlation between the Ultrasonic Guided Wave (UGW) data captured in-situ during fracture tests and the delamination status predicted by high-fidelity numerical models. This correlation is essential for an effective transition from traditional non-destructive testing (NDT) methods to intelligent SHM systems. The methodology is based on the continuous acquisition of UGW signals using a Micro-Fiber Composite (MFC) sensor network. Key UGW signal features are then extracted to estimate the experimental delamination size, which growth is correlated with Finite Element (FE) simulations and Reduced Order Models (ROMs), creating a direct mapping between the experimental features of the UGW signals and the numerical delamination progression. This validated Digital Twin allows the SHM system to use real-time UGW data to estimate remaining life and structural integrity under future load scenarios. In essence, the validation demonstrates the feasibility of an active UGW-based SHM system for continuous monitoring of Mixed-Mode fractures. This methodology, developed within the framework of the Horizon European GENEX Project, represents a significant step towards autonomous maintenance strategies for composite aircraft, improving safety margins and reducing inspection downtime.

The development of large civil aircrafts is an engineering task of significant complexity, requiring, besides engineering skills, time and immense investment to be complete. Besides obvious economic motivation in the aerospace and airline industry worldwide, is the availability and affordability of modern, safe aircrafts with less emissions believed to be of global significance.

The airframe development is subject to constant innovation in various fields of aerospace. The required fatigue testing of large-scale aircraft structures has shown to be time-consuming in itself, sometimes up to the point where it determines the pace of development, not just by the eventual test results that testing provides for aircraft development and certification.

This work shows how Structural Health Monitoring can contribute to innovation in the way full-scale airframe structures are tested with the objective to reduce time and cost.
The use of Structural Health Monitoring in aerospace has foremost been pursued in the context of in-service aircraft maintenance with a replacement of conventional means like NDT in mind. SHM has been applied to structural tests in the past often with technologies and systems intended for in-service use. This work provides an overview on the motivation and associated requirements for use of SHM for airframe structural test, which are the basis to then define a conceptual framework and assess different SHM technologies with the intent of holistic and efficient application.

This work focuses thereafter on Guided Waves (GW) as the selected SHM technology for development, test and assessment, which was applied on a large scale composite aerospace panel [1]. The GW SHM system, previously designed for in-service application, was adapted in terms of data acquisition and data management and integrated with the overall test stand and a dedicated data management and data processing environment.
That enabled a novel dedicated signal processing framework, as it provides damage detection capability under variable environmental and operational conditions, as well as the transient loading condition of a fatigue test.
The test set-up was representative for fuselage tests and solely available for the purpose of the research project. That allowed us to define and execute an experimental test of the GW SHM system with test conditions representative for typical fuselage fatigue tests.

The work concludes with an analysis of the test results with respect to the requirements and a reflection of their significance for the conceptual framework of SHM for structural test.

[1] Moix-Bonet, Maria und Schmidt, Daniel und Eckstein, Benjamin und Wierach, Peter (2024) A Composite Fuselage under Mechanical Load: a case study for Guided Wave-based SHM. In: 11th European Workshop on Structural Health Monitoring, EWSHM 2024. NDT.net. EWSHM 2024, 2024-06-10, Potsdam, Deutschland. doi: 10.58286/29842. ISSN 1435-4934.

Hydrogen, as an energy carrier, is finding its way into more and more areas of application, such as on-board tanks for passenger cars, trucks and buses, high-pressure buffer storage at refueling stations, stationary and backup power systems, emerging rail/ship/offshore demonstrators, and industrial/laboratory gas supplies. Safe, available, and cost‑effective hydrogen storage demands monitoring solutions that operate continuously without disrupting service. We present an application‑driven Acousto‑Ultrasonic (AU) concept for Type IV pressure vessels that integrates a dense network of piezoelectric transducers into the outer composite structure via a low‑temperature secondary lamination process. The approach targets guided‑wave interrogation across the cylindrical wall to detect and localize damage relevant to hydrogen service while fitting existing manufacturing and maintenance workflows.

A 9 L, 300 bar Type IV vessel (PET liner, carbon fiber overwrap) serves as the demonstrator. The sensing network comprises three circumferential rings of piezoelectric transducers with triangular spacing (typically 3–4 nodes per ring). Signal routing is realized on a thin, conformal multilayer flex PCB that is co‑bonded to the tank and fully encapsulated under glass‑fiber/epoxy cover plies, yielding a total buildup below ~1.5 mm and preserving aerodynamic and handling requirements. The piezoelectric sensors themselves are soldered to the PCB, ensuring a reliable connection. A single external interface—implemented as a low‑profile, keyed, magnetically latched pogo‑pin connector—enables rapid, tool‑less attachment of AU instrumentation while maintaining a sealed, snag‑resistant surface during operation.

Process development focused on field viability: vacuum‑assisted hot‑bonding below 90°C avoids thermal over‑exposure of the polymer liner; surface preparation and layup sequences were optimized to minimize voids and ensure adhesion on curved substrates. Trials on representative coupons, metal cylinders, and composite tanks confirmed reliable conformity of 200–350 µm interconnects, robust anchoring of local steel backings for the removable connector, and repeatable lamination with minimal porosity when surfaces are abraded and vacuum levels ~130 mbar are maintained. The measurement concept uses a pitch–catch scheme—one actuator, all others as receivers—with comprehensive shielding and a star‑routed ground to enhance signal integrity over longer runs and reduce the number of required bulkhead connections.

The primary impact of this work is twofold. First, it demonstrates a retrofit‑compatible path to deploy guided‑wave SHM on certified vessels without altering the original winding or requiring autoclave processes. Second, it provides a manufacturable blueprint—sensor placement rules, interconnect architecture, connectorization, and a validated lamination window—that scales to larger vessels and higher sensor densities. Ongoing testing includes AU baseline stability under pressure/temperature, defect detectability in the hoop regions, and durability. The results support condition‑based maintenance of hydrogen vessels, enabling reduced inspection downtime, expanded coverage of otherwise inaccessible regions, and improved fleet safety at manageable integration cost.