Structural health monitoring of lightweight constructions made of composite materials can be performed using guided ultrasonic waves. If modern fiber metal laminates are used, this requires integrated sensors that can record the inner displacement oscillations caused by the propagating guided ultrasonic waves. Therefore, we developed a robust MEMS vibrometer that can be integrated while maintaining the structural and functional compliance of the laminate. This vibrometer is directly sensitive to the high-frequency displacements from structure-borne ultrasound when excited in a frequency range between its first and second eigenfrequency. The vibrometer is mostly realized by processes earlier developed for a pressure sensor but with additional femtosecond laser ablation and encapsulation. The piezoresistive transducer, made from silicon, is encapsulated between top and bottom glass lids. The eigenfrequencies are experimentally determined using an optical micro vibrometer setup. The MEMS vibrometer functionality and usability for structural health monitoring are demonstrated on a customized test rig by recording application-relevant guided ultrasonic wave packages with a central frequency of 100 kHz at a distance of 0.2 m from the exciting ultrasound transducer.

The objective of this article is to present the results of our investigations concerning the environmental conditions that can be expected during the embedding process into fibre metal laminates and the consequences for a sensor node for structural health monitoring. The idea behind this investigation is to determine for which manufacturing conditions the integration of sensor nodes into the material can be done and to identify limits for this. The sensor nodes consist of commercially available integrated circuits and passive components soldered onto an adhesive-less flexible printed circuit board. They are tested under conditions above their specified limits, to find out if they are still working reliably after experiencing 155 min of 180 ${}^{\circ }\mathrm{C}$ and 7 bar of pressure. Apart from occurring temperature damage, the effect of surrounding fibres potentially pushing away the components under the amount of pressure of the manufacturing process, as well as the potential of shorts due to conductive fibers are investigated and suitable solutions to prevent this are evaluated. One experiment exceeding the typical requirements of a fiber metal laminate embedding process for structural components will be conducted at 250 ${}^{\circ }\mathrm{C}$ for 10 h, in order to determine the limits of embedding electronic sensor nodes. This time and temperature combination is expected to cause irreversible damage to the electronic system. Results show that it is possible to integrate electronics into materials under conditions far above their specifications when precautions are taken but also that there are limits that must not be exceeded during the embedding process.

This paper describes and evaluates the embedding of sensors and electronic sensor nodes into fiber metal laminate (FML) plates to achieve material-integrated, guided ultrasonic wave based structural health monitoring for hybrid materials. It evaluates how embedded electronics can enhance the process of sensor data acquisition and at the same time critically investigates the drawbacks that accompany the embedding approach regarding the influence on the received signal. A FML specimen with single sensors in one half of the plate and sensors with attached electronic sensor nodes for wireless readout in the other half is manufactured, introducing the detailed embedding process for such systems. Ultrasonic through-thickness scans of the manufactured plate are presented and analyzed to assess the achieved embedding quality. Together with electric sensor signals from both, wireless and wirebound micro-electromechanical system vibrometers and data from a scanning laser Doppler vibrometer (SLDV) the influence of material-integrated components on the wave propagation around the locations of integration is discussed. Further, the signals of wirebound sensors are successfully correlated with measurements performed using the SLDV and directly compared to data provided by wirelessly readout sensor nodes having the same type of sensor attached. This work shows how reflections occurring due to a material integration of components influence the recorded sensor data. At the same time, it is discussed how, for baseline-based damage detection methods, the influence of this is assumed to be a minor problem, and proof for advantages provided by the integration of complete sensor systems directly into the host material is provided.

Experimental studies were conducted to determine if the stresses occurring during the manufacturing process of Fiber Metal Laminates (FML) cause irreversible damage to electronic components. This is especially interesting for electronic systems to be embedded into such FML for later structural health monitoring purposes. Depending on the requirements of the used prepreg material, required temperatures and pressures for manufacturing can be quite high. First studies were conducted on electronic components separately, to validate their functionality after 3.5 h at elevated temperatures and pressures, exceeding manufacturers specifications. The functionality tests were successfully performed afterwards for every tested component, and no malfunctions could be identified. Further experiments will be conducted investigating the influence on a fully functional, programmed electronic system under the same conditions to investigate the influence on memory and soldering joints as well.

Structural health monitoring of lightweight constructions made of composite materials can be performed using guided ultrasonic waves. If modern fiber metal laminates are used, this requires integrated sensors that can record the inner displacement oscillations caused by the propagating guided ultrasonic waves. Therefore, we have developed a robust MEMS vibrometer that can be integrated with structural and functional compliance. This vibrometer is directly sensitive to the high-frequency displacements from structure-borne ultrasound when excited between its first and second natural frequency. The vibrometer is mostly realized by processes earlier developed for a pressure sensor but with additional femtosecond laser ablation and wafer bonding. The piezoresistive transducer made from silicon is encapsulated between top and bottom glass lids. The natural frequencies are experimentally determined using an optical micro vibrometer setup. The vibrometer functionality and usability for structural health monitoring are demonstrated on a customized test rig by recording application-relevant guided ultrasonic wave packages with a central frequency of 100 kHz at a distance of 200 mm from the exciting ultrasound transducer.

Motivated by their functional conformity, micro-cantilever-based MEMS oscillators are investigated in this study as structure-integrable transducers for the acquisition of guided ultrasonic waves in fiber–metal laminates. While acceleration-sensitive oscillators are limited in their maximum frequency, the presented displacement-sensitive oscillator is operated quasi-free in the fashion of a seismometer, making it particularly sensitive for high-frequency displacements above the sensor’s resonance frequency. The potential of this non-traditional application of a seismometer for the acquisition of structure-borne ultrasound is demonstrated experimentally. Therefore, MEMS oscillators are formed from the membrane of established pressure sensors by femtosecond laser micro-machining and mounted onto a setup for stimulation by structure-borne ultrasound. Experimental results indicate the targeted proportionality of the high-frequency stimulus and the sensor response. In conclusion, MEMS oscillators enable acquisition of high-frequency displacements and could therefore serve as structure-integrable sensors for guided ultrasonic waves.