High-speed optical imaging of dry metallic impacts for structural diagnostics

This work investigates the nonlinear dynamics of dry mechanical impacts, relevant to complex jointed structures such as turbomachinery. We used an in-house test rig to capture the contact area evolution during the impact of a steel sphere onto a transparent sapphire disk. The sphere is released from adjustable heights via an electromagnetic trigger to fall and bounce onto the sapphire disk. A high-speed camera placed below the sapphire disk captures the evolution of the contact area at sub-millisecond resolution, revealing the expected single-bump impulse similar to what is observed also in classical hammer testing. However, at certain drop heights, we also observed impacts characterised by an unexpected second bump during the descending phase of the impulse. This secondary contact event arises from the elastic vibration of the sapphire support plate, which resonates following the initial impact and causes the sphere to briefly re-contact the sapphire disk before full separation. A dimensionless two-degree-of-freedom model captures this behavior well and clarifies the conditions under which such secondary reflection occurs. While the setup resembles traditional hammer testing used to extract frequency response functions, our approach shifts the focus to the nonlinear time-domain shape of the impulse. We plan to detect and use such secondary reflections as a novel non-destructive evaluation technique, where time-resolved impact signatures reveal insights into the contact mechanics and dynamic behavior of jointed structures.