The project investigated the integration of damage‑detection sensors into the composite under‑body protection structure of a battery system. The goal was to obtain a reliable, robust, and long‑term stable measurement of deformation, force, and acceleration that could be used to classify damage severity. The research focused on three main sensor concepts: a force‑sensing resistor (FSR), a strain‑gauge wire, and a coil sensor. In addition, a digital accelerometer (ADXL356/357) was evaluated for dynamic measurement of impact forces.
Experimental tests were carried out on an evaluation board that mimicked the final application. An impulse hammer delivered controlled impacts while the sensors recorded the resulting signals. The FSR, which has a free‑state resistance in the megaohm range and drops to below 100 Ω under pressure, showed the largest voltage excursions – up to 10 % – compared with the strain‑gauge wire, which produced less than 0.01 % change. However, the FSR suffered from significant baseline drift: the nominal resistance varied by up to 50 % between samples, changed by 10 % over time, and fluctuated by 20 % at 80 °C. Its sensitivity was uneven across the sensor area, with only about 60 % of the surface responding strongly to pressure. These issues were attributed to the current embedding technique, where the glass‑fiber composite supports the sensor only at the raised edges, leaving the central area poorly contacted. A more precise contact strip was suggested to improve uniformity.
The strain‑gauge wire performed more consistently. A dimple of double width produced only a small voltage change, whereas a deeper dimple yielded a proportional response. A dimple located on the battery cross‑beam produced a slightly smaller signal than one in the inter‑beam region, indicating that the measured signal depends mainly on the amount of deformation rather than on the exact position. The coil sensor, while robust, showed the lowest sensitivity among the three candidates.
The ADXL356/357 accelerometers, available in a compact 6 mm × 5.6 mm × 2.2 mm package, were tested for their ability to capture high accelerations up to 25 g. Their analog and digital outputs (SPI or I²C) allowed multi‑axis measurement (x, y, z). The dynamic tests demonstrated that the accelerometers could resolve impact events within milliseconds, providing a complementary data set for damage classification. Correlation of the accelerometer data with the strain‑gauge and FSR signals suggested that a combined sensor approach could improve damage detection accuracy.
The project’s technical outcomes are at Technology Readiness Levels 2–4. The study confirmed that sensor integration into fiber‑reinforced sandwich structures is feasible for a range of fiber‑matrix combinations. The research identified promising application scenarios and demonstrated laboratory prototypes, but further work is required to transfer the results to industrial production and to validate the sensors under real‑world crash conditions.
Collaboration was carried out within a consortium that included Porsche AG, which defined the battery protection structure and the damage‑detection requirements, and the Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), which contributed expertise in piezoelectric acoustic detection. The project was funded by a German research agency and spanned several years, culminating in this final report. The consortium’s internal evaluation selected design variant 1 of the under‑body protection structure based on a morphological box analysis, and the project defined five damage‑detection levels to guide future sensor deployment.