The research project investigated the long‑term electrical performance of mass‑bolt connections used in modern vehicle bodies, where increasing electrification and the integration of comfort and safety systems raise the demand for reliable low‑resistance contacts. The study focused on how combined electrical, thermal, mechanical, and corrosive loading affects the contact resistance and, consequently, the thermal losses and life span of the bolt‑nut assemblies. Continuous four‑wire resistance monitoring was employed to capture the full ageing curve of the connections, allowing the identification of distinct life‑cycle phases and the prediction of long‑term stability. The results showed that the contact resistance rises gradually under realistic operating conditions, leading to higher heat generation at the joint and a reduction in the overall durability of the electrical system. The work also highlighted the lack of publicly available standards for assessing the electrical quality of mass‑bolt connections, underscoring the need for systematic testing and data sharing.

To generate the data, a comprehensive test infrastructure was assembled. A torque and screw test rig, supplied by Schatz AG, was used to apply tightening forces up to 200 Nm with a sensor accuracy of ±0.25 %. The rig could operate at 20 % overload (1.2 × the measurement range) and 50 % overload (1.5 × the measurement range), with a maximum angular speed of 500 rpm for angle measurement. Temperature conditioning was performed in a CTS C‑70/1000 chamber capable of -70 °C to +180 °C for thermal ageing tests and 10 °C to 90 °C at 10 %–98 % relative humidity for climate tests. A CTS TSS‑70/66 temperature‑shock chamber enabled rapid temperature swings (≤ 10 s) between -80 °C and +220 °C, with heating and cooling rates of 4 K min⁻¹. Corrosion exposure was carried out in a climate‑change chamber that simulated accelerated environmental conditions. Contact surfaces were characterised using a micro‑ohmmeter (DLRO10HD) and a 2‑D height‑profile system to determine surface roughness parameters according to ISO 4287. Metallographic examinations and hardness mapping of the aluminium alloy EN AW‑5754 and the steel components provided insight into material behaviour under load.

The project was carried out from mid‑2021 to early 2024 and involved a consortium of automotive manufacturers, research institutes, and universities. Industry partners supplied the test hardware and provided real‑world bolt‑nut assemblies, while academic collaborators designed the experimental protocols and performed the data analysis. A steering committee, referred to as the PA, coordinated the work and facilitated knowledge transfer through regular web‑conferences and the distribution of interim reports. The final report, issued in April 2024, was shared with all partners and made available to interested companies outside the consortium. The findings were also integrated into university curricula, covering fundamentals of joining technology, vehicle‑specific joining processes, and thermal joining methods, thereby ensuring that the next generation of engineers is familiar with the latest insights into mass‑bolt reliability.

Overall, the study delivered a validated test methodology for assessing the electrical integrity of mass‑bolt connections under realistic, combined loading scenarios. It quantified the influence of tightening torque, temperature, mechanical vibration, and corrosive environments on contact resistance evolution, providing manufacturers with actionable recommendations to improve product design and maintenance schedules. The collaborative framework ensured that the results were disseminated across industry and academia, fostering the development of future standards and best practices for electrically connected vehicle structures.