Strain waves on additively manufactured plates for an on-orbit impact detector concept

Orbital debris impacts on spacecraft are an emerging threat to space missions due to the exponential increase in the number of satellites orbiting the Earth. Debris characteristics (size, material, velocity, etc.) are not well known for the size range of 10 mm or less that is undetectable using Earth telescopes or radar observation. The objective of this research was to determine wether a concept designed to detect impact of particles in the ∼1 to 5 mm range, find the location of the impact, and characterize the impacting projectile (velocity, size, angle, density), is feasible.

The paper describes the design, fabrication, and tests performed on “witness plates” (the concept) made of two parallel layers of additively manufactured aluminum and instrumented with sixteen gages, eight on each layer. Laboratory experiments have shown that the waves can be recorded and properly interpreted to find location of impact, sound speed in the plate, and to estimate impact velocity. It was shown analytically that the amplitude of the first strain wave that propagates from the impact point is expected to decay as 1/r. This was observed as well in the signals recorded in the experiments. CTH computations were performed during the pre-test design phase and the post-test analysis phase. In fact, the numerical simulations have been key and pervasive in this research effort as they provided invaluable insight for the initial design and the correct interpretation of signal anomalies seen during the tests. Additionally, the computations confirmed the 1/r law derived analytically, i.e. that the assumptions for the derivation were justified. The main conclusions of the research are that, for a normal impact, the 1/r law for front gages can be easily used to determine the diameter of the impactor. It is possible that the back gages could be used to determine the density of the impactor as well. Finally, it was shown that oblique impacts generate an expected assymetry in the signals recorded. Though this aspect should be investigated further, the assymetry is probably uniquely related to the impact angle, which could provide the angle information.