Concept of Autonomous Self-Sensing Metamaterial Structures for Future Aircraft

978-1-6654-5690-6/23/$31.00 ©2023 IEEE Concept of Autonomous Self-Sensing Metamaterial Structures for Future Aircraft Jan Bajer Faculty of Mechanical Engineering Brno University of Technology Brno, Czech Republic [email protected] Miroslav Hrstka Faculty of Mechanical Engineering Brno University of Technology Brno, Czech Republic https://orcid.org/0000-0002-3169-3159 Filip Ksica Faculty of Mechanical Engineering Brno University of Technology Brno, Czech Republic https://orcid.org/0000-0001-5270-6628 Jan Navratil Faculty of Mechanical Engineering Brno University of Technology Brno, Czech Republic https://orcid.org/0000-0003-3314-8530 Petr Marcian Faculty of Mechanical Engineering Brno University of Technology Brno, Czech Republic https://orcid.org/0000-0002-9458-9690 Zdenek Hadas Faculty of Mechanical Engineering Brno University of Technology Brno, Czech Republic https://orcid.org/0000-0002-9097-1550 Abstract— With increasing complexity of aircraft structures and demands for safety, structural health monitoring (SHM) systems are a frequently considered option to lower the cost and time consumption of aircraft maintenance. Recent advances in additive manufacturing techniques have enabled the fabrication of structures with artificially designed mechanical properties and the possibility to integrate smart materials directly into the structure. In this paper, we propose an innovative metamaterial structure with embedded piezoelectric transducers. The results show that the magnitude of the force applied to the structure can be determined from the electrical response of the transducers. A power of 5.9 mW was generated on a 400 kΩ resistive load at an applied force of 11 N. The test sample shows the potential of the proposed structure for design of artificial structures and SHM systems in aerospace industry. Keywords—metamaterial, structure, additive technology, sensing, energy harvesting, design, aircraft I. I NTRODUCTION The number of passengers carried by the world's airlines is rising every year. In 2020 it reached 4.7 billion passengers (before the pandemic). Over 40 million flights are performed by the global airline industry in total every year. In addition to commercial flights, the airline industry accounts for more than a third of the value of world trade. Together, the aviation industry makes a significant contribution to the economic and social development of the world. Despite the many benefits that air travel offers, aviation still faces several challenges, foremost among them safety. Increasing demands for safety are also increasing the requirements for strength and stiffness of the materials used for the aircraft structures, leading to increased aircraft weight, fuel consumption and a general deterioration of the economic and environmental impact. Lowering the demands on structural strength and stiffness can be achieved by reducing the gust loading and by suppressing adverse aeroelastic effects. Reduced structural weight, together with reduced aerodynamic drag, results in reduced fuel consumption, reduced aircraft emissions and economic benefits. In addition, it also leads to an increase in the overall health of society and reduction of an environmental impact. The integration of intelligent sensing elements into structures creates advanced smart structures that are capable of self-monitoring to detect structural defects. This improvement will lead to a significant reduction in the time and cost of maintenance and, most importantly, a further increase in safety and therefore an overall improvement in socio-economic impact. With the massive growth of additive manufacturing technologies in recent years, it is now possible to produce very complex structures that could not be produced with previous processes. It is possible to produce structures with designed properties that are fundamentally different from those of the material used. These artificial materials with unusual properties are called metamaterials. In this paper, a novel design of a mechanical metamaterial structure with integrated piezoelectric transducers is proposed. The structure implements self-sensing and energy harvesting (EH) capabilities, both suitable for structural health monitoring II. M OTIVATION OF S ELF - SENSING S TRUCTURE D EVELOPMENT We propose a metamaterial structure that enables the integration of smart materials that will provide self-sensing and possibly self-powering capabilities to the resulting smart structure with enhanced mechanical properties. The structure is designed considering modern manufacturing techniques, such as additive manufacturing. The advantage of additive manufacturing techniques is the ability to produce complex structures. These structures have distinctive properties which include low weight and the ability to design mechanical properties through geometric arrangement and size of the structure. Fig. 1 Self-sensing structure design workflow 424 2023 IEEE 10th International Workshop on Metrology for AeroSpace (MetroAeroSpace) | 978-1-6654-5690-6/23/$31.00 ©2023 IEEE | DOI: 10.1109/METROAEROSPACE57412.2023.10189988 Authorized licensed use limited to: Brno University of Technology. Downloaded on December 16,2025 at 09:35:38 UTC from IEEE Xplore. Restrictions apply. III. STATE OF THE ART – STRUCTURAL HEALTH MONITORING TECHNOLOGIES With increasing complexity of aircraft structures alongside with increasing demands for safety, structural health monitoring (SHM) systems are a frequently considered option to lower cost and time consumption of aircraft maintenance. Composite materials offer superior strength properties but for the price of susceptibility to various types of failures that may occur and not even be visible on the surface of the structure and therefore may not be detected during maintenance. Integrating various types of sensor systems directly into the structure may solve this problem. A. Principles based on wave propagation These principles are based on measuring signals propagating through the structure, where the signals can be either generated by events occurring within the structure itself (acoustic emission) or generated externally by transducers in proximity of the sensor (guided wave propagation). Acoustic emission (AE) stands out over other techniques due to its ability to detect dynamic changes within the structure associated with the damage, ability to detect and localize sources of damage, as well as the ability to cover large areas with a relatively low number of sensors. While AE can provide information about damage developing in the structure, it cannot provide information about the damage currently present in the structure. This principle has been successfully used to monitor damage propagation in various fields if civil engineering, such as monitoring damage propagation in composite blades of a wind turbine [1], as well as in aerospace industry for parts such as landing gear lever link [2] and vertical tail of Cessna T-303 [3]. In terms of sensors, piezoelectric materials are very commonly used for both signal generating elements as well as sensors. As an example, there are Piezoelectric Active Wafer Sensors (PWAS) PZT sensors with integrated Ag electrodes designed specifically for SHM purposes presented in [4]. Significant effort has also been put into research and development of PVDF sensors enabling integration of multiple sensitive cells into one sensor [5]. B. Principle based on measurement of impedance This principle utilizes piezoelectric sensors attached to a structure, acting simultaneously as actuators and sensors. They are used for high-frequency excitation of the base structure and simultaneous measurement of changes in the impedance signature of the sensor. Hardware and software requirements for this principle are described in detail in [6]. It has been proven that there is a direct correlation between the electrical impedance of the sensor and the mechanical impedance of the structure they are attached to. Because of this, the method is viable for local monitoring of joints, welds, or composite layer integrity directly observable and interpretable by changes in the mechanical impedance. Impedance measurement has also been used for fatigue damage diagnosis of a propagating crack in a simple aluminium beam [7]. Na and Baek [8] studied the viability of this method to identify and monitor failures within a composite structure. In the context of utilization of impedance measurement in wireless sensing nodes, Park, Yun, and Inman [9] describe how MFC sensor controlled by a micro-controller and miniaturized impedance measuring chip can be used to measure and wirelessly transfer measured characteristics. It has also been proposed that the self-contained sensing node could be based on MEMS to further decrease energy requirements and miniaturize the system. It is not yet clear if this principle could be viable for SHM of complex structures of aircrafts operating in real world conditions, where temperature, humidity, ambient vibrations, and electromagnetic noise can affect its performance. C. Direct measurement of structural vibrations The most common approach for SHM in terms of research of autonomous sensing nodes and wireless transmission is based on direct measurement of structural vibrations. This is in principle very similar to AE; however, the main difference is the observed spectrum of frequencies and the interpretation of measured signals. While AE analyses the signals in terms of damage and crack propagation, direct measurement of structural vibrations usually deals with modal parameters of the structure. For this task, accelerometers are widely used, as they are easy to install and the output signals are easy to interpret, but the major drawback in the context of development of autonomous nodes is their dependency on an external power source. Wireless sensing units used, for example, to measure seismic activities, are commercially available, but such devices are usually powered by batteries, making their use for SHM very limited. Nevertheless, they are used for research on topology and data management of sensing arrays; for example, Avci et al. [10] used such modules to study potential methods for decentralized data management of wireless sensing networks. The major drawback of commonly used accelerometers can be eliminated by using passive sensors, such as piezoelectric sensors. These sensors can come in various shapes and sizes depending on the piezoelectric material used, either most commonly used PZT, PVDF polymer films, more complex piezocomposites [11], or materials and thin films for usage in extreme conditions [12], [13]. Macro-fibre composite (MFC) sensors have been successfully used as a passive on-board system designed to measure structural vibrations in load-bearing elements of an aircraft during operation [14]. D. Smart Skin Several methods for integrating smart materials into the aircraft structures have already been proposed, such as piezoelectric materials. Vankecke et al. [15] focused on an ultrasonic guided wave SHM system development for aircraft wing inspection with embedded PZT sensors/actuators. This piezoelectric-based embedded sensor/actuator network was applied to an aircraft wing for structural integrity monitoring. Zelenika et al. [16] proposed and analysed three kinds of lightweight piezoelectric sensor networks (smart skin) including continuous series sensor network, continuous parallel sensor network and continuous heterogeneous sensor network. E. Energy Harvesting for SHM Many of these sensing units require batteries to work, making them dependent on the battery exchange. A possible solution is integration of energy harvesting systems, using ambient energy sources, as vibration or temperature gradient. A multisource battery-free energy generator for the aircraft health monitoring is presented in [15]. The presented energy harvesting system is based on combination of piezoelectric and thermoelectric generators. 425 Authorized licensed use limited to: Brno University of Technology. Downloaded on December 16,2025 at 09:35:38 UTC from IEEE Xplore. Restrictions apply. A wide range of energy harvesting technologies with potential applications for SMH systems is presented in [16]. The energy harvesting devices discussed are based on the use of thermal gradients, kinetic energy, solar radiation, airflow, and other energy sources with respect to their potential placement in aircraft. A vibration-based energy harvesting using piezoelectric harvesters is getting more attention since PEHs are durable, reliable and have higher density power output and higher voltage output than energy harvesters based on other methods [17]. An example is a nonlinear piezoelectric energy harvester constituted by a cantilever with symmetrically middle-stops and double-clamped piezoelectric plates presented in [18]. Another type of PEH proposed in [19] is a strongly coupled piezoelectric stack energy harvester (PSEH) which is designed not only for high power output, but also for durability and robustness. Use of metamaterial structures has shown significant advantages in piezoelectric energy harvesting. In [20] is proposed a metamaterial structure for simultaneous mechanical wave filtering and energy harvesting composed of a square array of free-standing cantilevers with attached piezoelectric films. Using an auxetic metamaterial structure to increase strain of a piezoelectric material is presented in [21]. Similar design using a kirigami auxetic structure is proposed in [22] for self-powered and wireless SHM systems. An auxetic hexachiral cantilever substrate for low-frequency vibration energy harvesting applications is presented in [23] showing that use of hexachiral metamaterial PEHs can result into more output power generated compared to the re-entrant harvesters. An artificial structure with enhanced mechanical properties, implemented sensing and energy harvesting capabilities is an interesting approach to implement SHM features into aircrafts. IV. P ROPOSED S ELF - SENSING AND A UTONOMOUS S TRUCTURES FOR A IRCRAFTS A. Concept of metamaterial structure Our idea is a structure intended for the construction of aircraft parts, such as the wing in Fig. 2, composed of a metamaterial lattice structure with integrated piezoelectric transducers. Piezoelectric transducers would make it possible to monitor the health of the structure, but also to measure the forces acting on the wing, such as aerodynamic force. Piezoelectric transducers could also be used for energy harvesting, i.e. obtaining useful electric energy from ambient sources of energy. This smart metamaterial structure can be manufactured using modern additive manufacturing techniques. Fig. 2 Concept of an aircraft wing composed of metamaterial structure B. Self-sensing and Energy Harvesting Functionality of Proposed Structure We propose a lattice structure composed of periodically repeating unit cells that exhibit auxetic behaviour, and as a result, the entire structure has auxetic behaviour. Piezoelectric transducers are placed in each cell of the structure (detail in Fig. 3), which are compressed by the cell walls when the whole structure is compressed. The embedded piezoelectric transducers can be used as sensors or for energy harvesting. The unit cell is based on a re-entrant hexagon unit cell with added supports in the middle of the cell for embedding piezoelectric transducers, as PZT with fibre glass spacers in this case. When the cell is compressed, the supports transmit the mechanical load on the piezoelectric transducer. The connection of multiple cells together in a repeating pattern forms a periodic lattice structure. Connected re-entrant cells are now more like rigid “plus sign” shaped cells connected by flexible ribs, with piezoelectric transducers between individual cells. The design of the proposed lattice structure is shown in Fig. 3. V. T ESTED STRUCTURE OF THE PROPOSED METAMATERIAL A sample of the proposed auxetic lattice structure was made for testing purposes. The design and finite element analysis of the structure were presented in our previous paper [24]. The structure is lightweight, tuneable, and possible to made using modern additive manufacturing technologies. Fig. 3 Auxetic lattice design with integrated PZT transducers Two plates with size of 90x36x5 mm composed of the designed auxetic metamaterial structure were made from stainless steel Fe-Alloy 316L using Direct Metal Laser Sintering (DMLS) technology. Each auxetic plate was embedded with three Noliac NCE51 PZT piezoelectric plates with size of 5x5x1 mm into middle unit cells. PZT plates were labelled with numbers from 1 to 3 on one plate and from 4 to 6 on the other one. 426 Authorized licensed use limited to: Brno University of Technology. Downloaded on December 16,2025 at 09:35:38 UTC from IEEE Xplore. Restrictions apply. Metal plates were sandwiched between two rectangular plastic bases, forming a load-bearing block. For further mass increase, a steel cylinder with an additional mass of 1.06 kg was mounted on the top plastic base. The final assembly of the laboratory sample with six piezoelectric transducers in total is shown in Fig. 4. Further information about the design and fabrication of the sample is available in the previous paper [24]. Fig. 4 The final assembly of the laboratory sample The laboratory sample was horizontally attached to a metal rail. A force sensor was mounted on the centre of the upper part of the sample where the added weight was located. A modal shaker (model K2007E01) was attached to the force sensor using a plastic rod. The shaker was suspended from the auxiliary structure and was used as the source of the harmonic force loading of the structure. The shaker was controlled by a Hantek electrical signal generator. An accelerometer was also attached to the top of the test sample to measure the vibration of the structure. The accelerometer and force sensor were connected to the NI measurement card. Piezoelectric transducers embedded in the structure were connected in parallel to the resistive load. The voltage generated at the resistive load was measured with a RIGOL oscilloscope. The complete experimental setup is shown in Fig. 5. Fig. 5 Experimental setup for energy harvesting evaluation VI. E XPERIMENTAL R ESULTS A. Dynamic response of proposed structure To determine the basic electromechanical properties of laboratory sample, the frequency response of the sample was investigated for different values of the excitation frequency at the same magnitude of force 1 N. All six embedded piezoelectric transducers were connected in parallel to the resistive load. For different excitation frequencies of force loading, the acceleration of the top of the laboratory sample and the effective voltage generated at the resistive load of 400 kΩ were measured. The frequency response for excitation frequencies in the range of 80-230 Hz is shown in Fig. 6. Fig. 6 Frequency response of the proposed structure Peak values of acceleration and generated voltage were achieved near the resonance frequency of the structure, corresponding to an excitation frequency of 171 Hz. The total capacitance of all six transducers in parallel was 2.44 nF. Thus, the optimum load for the maximum generated power is about 381 kΩ for this laboratory sample. B. Energy Harvesting For the energy harvesting application experiment, all six piezoelectric transducers embedded into the structure were connected in parallel to a resistive load. The laboratory sample was loaded with different force amplitudes in the range of 1 N to 11 N with an excitation frequency below the resonant state (147 Hz was chosen). Embedded piezoelectric transducers were connected to different resistive loads in the range of 100 kΩ to 800 kΩ. Fig. 7 Generated power of all six transducers in parallel The highest values of generated power (above 5 mW) were achieved for the highest loading force amplitude and for resistive loads with values 300-500 kΩ. The maximum achieved power of 5.9 mW was obtained for the force load amplitude of 11N and for a resistive load of 400 kΩ, which is close to the obtained optimum load for resonance frequency. 427 Authorized licensed use limited to: Brno University of Technology. Downloaded on December 16,2025 at 09:35:38 UTC from IEEE Xplore. Restrictions apply. C. Evaluation of sensing capability To investigate the potential use of the proposed auxetic structure for sensing applications, the sample was loaded with harmonic force of different magnitudes and frequencies. All six piezoelectric transducers were connected in parallel to a resistive load of 100 kΩ. For different loading combinations, the effective value of the voltage across the resistive load was measured. Generated voltage on the load at different excitation frequencies and magnitudes are shown in Fig. 8. Fig. 8 Generated voltage for different loading conditions Due to the distribution of piezoelectric transducers in the walls of the structure, the position of the force impact could be estimated. To determine the ability of the laboratory sample to estimate the impact position, several additional measurements were made. In this experiment, the laboratory sample was vertically attached to the rail and was excited by hitting the top plate of the structure with an impact hammer. A different location of force impact was chosen for each measurement. The first impact location was the centre of the top plate of load-bearing block, so the force was evenly distributed among all transducers. Other four impact locations were over each of the outmost piezoelectric transducers embedded in auxetic walls of the laboratory sample. Each of the outermost piezoelectric transducers was separately connected to the probe of the oscilloscope with common ground, which was the steel structure itself. The electrical response of each transducer was observed over time, measured on the 1 MΩ input impedance of the oscilloscope. Fig. 9 Electrical responses to impact force over PZT-1 Fig. 10 Electrical responses to impact force over PZT-6 A total of five measurements were made for different impact positions and the electrical responses of the four outmost piezoelectric transducers were measured. Electrical responses of all measured PZT transducers are shown in Fig. 9 for impact position over the PZT-1, and in Fig. 10 for impact position over PZT-6. VII. CONCLUSIONS This paper has proposed a concept of an aircraft wing composed of the auxetic metamaterial structure with embedded piezoelectric transducers, which can be used for structural health monitoring. The structure combines sensing and energy harvesting capabilities with the possibility of designing the mechanical parameters of the structure as required depending on its use in the aircraft. To investigate the ability of the proposed auxetic structure to perform the sensing and energy harvesting function, the laboratory sample was loaded with harmonic force using a vibration device. Piezoelectric transducers embedded in the auxetic walls were connected in parallel to the resistive load. The frequency response of the laboratory sample was investigated for different values of the excitation frequency and the same magnitude of force 1 N (Fig. 6). Testing of the structure for sensing applications has shown that the generated voltage on the load follows a linear trend as a function of the magnitude of the applied force for a particular excitation frequency (Fig. 8). Based on the oscillation frequency of the structure and the magnitude of the voltage, we can estimate the magnitude of the force acting on the structure. In addition, the position of the force impact can be also estimated. Fig. 9 and Fig. 10 show electrical responses of four outermost piezoelectric transducers to force impacts at different positions. When the force impact was situated above the PZT-1, peak value of the generated voltage was around 35 V for the corresponding transducer. The generated voltage was more than twice as high, compared to the second most significant electrical response of the PZT-3, which generated about 15 V. Similar result is noticeable in Fig. 10, where the impact position was above PZT-6. From the frequency response of the tested structure, a resonance frequency of 171 Hz was obtained. The optimum load of 381 kΩ was determined for the maximum generated power based on the resonance frequency and capacitance of the transducers. This agrees with the results of the generated power measurements for different amplitudes of excitation 428 Authorized licensed use limited to: Brno University of Technology. Downloaded on December 16,2025 at 09:35:38 UTC from IEEE Xplore. Restrictions apply. force and for different load values (Fig. 7). From the results, the highest powers were achieved for the largest applied force of 11 N and for resistive loads near the 400 kΩ value, where a maximum power of 5.9 mW was achieved. 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