Peter Wierach’s research while affiliated with Technische Universität Braunschweig and other places

A full-scale composite door surrounding aircraft structure was instrumented with a GW-SHM system and subjected to three representative quasi-static load cases using a hydraulic test rig. The test was performed in a hangar under uncontrolled temperature environment, resulting in broad temperature variations throughout the experiment. This work focuses on differentiating between benign environmental and operational conditions and barely visible impact damage. A data-driven approach based on Gaussian Processes is used to detect barely visible impact damage introduced during the test campaign, differentiating between benign environmental/operational conditions and barely visible impact damage.

Available energy is the most decisive factor for a successful space mission. The electrical energy provided, determines the duration of the mission and thus the amount of scientific data obtained. A lot of research is currently done to develop powerful and efficient batteries. Due to the chemical storage processes their lifetime is limited to much less than 30000 cycles. In contrast, supercapacitors use reversable physical processes for energy storage. This principle allows more than 1 million cycles. Therefore, they can be considered as maintenance-free and to be the ideal candidates for structural integration. This approach offers great opportunities to save weight and volume, which are interesting properties for expensive space applications. In this publication, semi-finished thin-film supercapacitors are integrated into fiber-reinforced plastics composites by autoclave processing. The supercapacitors are manufactured using aluminum collectors coated a with an activated carbon electrode. As electrolyte the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is used as electrolyte. Within the project Hybrid Solar Energy Storage (HySES) an experimental setup was build using commercial batteries and integrated supercapacitors for a DLR satellite mission. After intensive electro-mechanical material characterizations and design challenges the whole setup was tested under vacuum-thermal cycling and intensive mechanical stresses. Following the project Peak Power Platform (PPP) 2019, this is the second application using integrated supercapacitors proven for in-orbit tests.

A new approach to developing structural sodium batteries capable of operating in ambient-temperature conditions has been successfully achieved. The developed multifunctional structural electrolyte (SE) using poly(ethylene oxide) (PEO) as a matrix integrated with succinonitrile (SN) plasticizers and glass-fiber (GF) reinforcements identified as GF_PEO-SN-NaClO4 showed a tensile strength of 32.1 MPa and an ionic conductivity of 1.01 × 10−4 S cm−1 at room temperature. It displayed a wide electrochemical stability window of 0 to 4.9 V and a high sodium-ion transference number of 0.51 at room temperature. The structural electrode (CF|SE) was fabricated by pressing the structural electrolyte with carbon fibers (CFs), and it showed a tensile strength of 72.3 MPa. The fabricated structural battery half-cell (CF||SE||Na) demonstrated good cycling stability and an energy density of 14.2 Wh kg−1, and it retained 80% capacity at the end of the 200th cycle. The cycled electrodes were observed using scanning electron microscopy, which revealed small dendrite formation and dense albeit uniform deposition of the sodium metal, helping to avoid a short-circuit of the cell and providing more cycling stability. The developed multifunctional matrix composites demonstrate promising potential for developing ambient-temperature sodium structural batteries.

Structural health monitoring (SHM) based on ultrasonic guided waves is a novel technology using permanently attached actuator and sensor networks, data acquisition and data evaluation systems to enable in-service inspection of aircraft structures. A modular and decentralized SHM system is presented that can be adapted to various aircraft application scenarios. The central element of the SHM system is a robust sensor array based on piezocomposite technology that can be efficiently integrated into the manufacturing process of CFRP structures (via co-bonding). The sensor array consists of integrated piezoceramic transducers, temperature sensors, electrical cables and integrated electronic components. The decentralized system architecture of the individual electronic components reduces the installation and cabling effort during the assembly of aircraft structures as well as the system weight. Integration and installation aspects as well as various system architectures are discussed using an Airbus A350 door surround structure. Furthermore, an outlook of damage detection under representative flight loads is given.

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 tested under mechanical load. A hydraulic test rig was used to apply three representative load cases in quasi-static conditions on the structure. A network of robust piezocomposite transducers to monitor the structure has been designed and manufactured. The network is organized in arrays, which include the transducers, cabling and a connecting base plate for optimized sensor installation. A multiplexing module is directly connected to the base plate enabling a fast and reliable sensor connection, a drastic cable weight reduction, and a modular design. The combined influence of load and temperature on the propagation of Guided Waves has been assessed. Finally, a data-driven approach to damage identification has allowed the detection and localization of barely visible impact damage introduced during the test campaign.

Composite materials have proven to be one of the ideal choices when it comes to structural applications requiring high performance working at extreme low temperatures, such as hydrogen tanks or space structures. One of the main challenges of using them is that they are susceptible to impact and fatigue, which may lead to different damages. Implementing structural health monitoring (SHM) systems is one strategy to enhancing their reliability, with guided waves (GW) being one of the most promising techniques. For the effective implementation of GW-based SHM in these applications, an in-depth comprehension of its behavior up to cryogenic temperatures is required for accurate monitoring. In this regard, the objective of the present work is to preliminary investigate the effect of low temperatures on wave propagation and transducer integration. By progressively removing a composite panel from a liquid nitrogen (LN2) bath with an integrated GW-SHM network based on co-bonded DuraActTM transducers, the GW and temperature are continuously monitored from LN2 CT to room temperature. The results show that the behavior of the GW by decreasing the temperature does not follow a monotonic pattern, having at least local maxima in terms of amplitude, and nonlinear behavior for the time-of-flight. Electromechanical impedance and C-Scans ensured the quality of the bonding of the co-bonded DuraActTM transducers.

In structural health monitoring with guided ultrasonic waves, probability reconstruction algorithms are a method to locate a damage. They work by calculating the probability for each actuator sensor path on whether there is a damage on the path or not. By superposition of each path and its damage probability, a damage localisation is done. The disadvantage of this is, that the damage localisation resolution is limited by the number of paths crossing each other. To overcome this, the hypothesis of this investigation is that the information of a path can not only be used to determine whether a damage is present, but that additional information about the location within the path can be calculated as well. This way a localisation resolution can be higher than by only relying on the path density. To verify this assumption, an experimental setup was chosen in which the path lengths always remain the same while the distance between damage and the direct path varies. This is implemented by a moving ultrasonic microphone simulating the sensor. The varying distance is the local information, which is determined in this study using the information of a single path. For this purpose, a prediction is calculated using regularised multilinear regression. The input features are characteristic values of five sections of the sensor signal in the time domain. The sections are manually chosen based on arriving wave events. The result confirms the hypothesis. Therefore, it is plausible to increase the detection resolution of probability reconstruction algorithms by calculating damage location estimations for each path.

Acousto-ultrasonic composite transducers (AUCTs), comprising piezoceramic materials in a reinforced polymeric matrix, show promise for structural health monitoring in composite structures. Challenges arise when integrating AUCTs onto highly loaded thermoplastic composites, especially low-surface-energy materials like polyaryletherketone composites. To address this, the study explores the viability of attaching AUCTs to low-melting polyaryletherketone carbon fiber-reinforced thermoplastic composite structures using ultrasonic welding. This welding technique forms a joint where the interface material fuses with the AUCT embedment and the structure matrix, providing a reliable and automatable process. The investigation includes a comparative analysis of an ultrasonic welded joint with an external energy director and a reference AUCT system integrated using a vacuum bagging oven procedure. Results highlight the potential of AUCT configurations integrated by ultrasonic welding as an alternative solution, acknowledging challenges that persist for further development and increased reliability in structural health monitoring applications.

Structural batteries are gaining attention and can play a significant role in designing emission-free lightweight defense and transport systems such as aircraft, unmanned air vehicles, electric cars, public transport, and vertical takeoff and landing (VTOL)-urban air traffic. Such an approach of integrated functions contributes to overall mass reduction, high performance, and enhanced vehicle spaciousness. The present work focuses on developing and characterizing multifunctional structural sodium-ion battery components by using a high-tensile-strength structural electrolyte (SE) prepared by incorporating a glass fiber sandwiched between thin solid-state poly(ethylene oxide)-based composite electrolyte layers. The electrochemical and mechanical characterization of the structural electrolyte shows multifunctional performance with a tensile strength of 40.9 MPa and an ionic conductivity of 1.02 × 10^–4 S/cm at 60 °C. It displays an electrochemical window of 0 to 4.5 V. The structural electrode is fabricated using a heat press by pressing intermediate-modulus carbon fibers (CFs) against the structural electrolyte, and it shows a high tensile strength of 91.3 MPa. The fabricated structural battery CF||SE||Na provides a typical energy density of 23 Wh/kg and performs 500 cycles while retaining 80% capacity until 225 cycles. The investigation of sodium structural battery architecture in this preliminary work demonstrates intercalation of sodium ions in intermediate modulus-type carbon fiber electrodes, shows multifunctional performance with excellent cycling stability and structural strength, and provides an alternative path to current structural battery designs.