Interrogation of linear/nonlinear features of guided waves for characterizing hypervelocity impact-induced pitting damage in shielding structures

Hypervelocity impact (HVI) is a scenario ubiquitous in low Earth orbit, where HVI is typified by the collision between meteoroids and orbital debris and spacecraft with a relative speed greater than 10 km/s. A linear/nonlinear guided-wave-based approach for characterizing HVI-induced damage in a two-layer aluminum shielding structure (comprising inner and outer layers) is developed. After penetrating the outer layer, the generated debris cloud further impacts the inner layer, producing a unique form of damage with multitudinous small-scale pitting. In this study, aluminum spheres are discharged using a two-stage light gas gun, at an impact speed of ∼6 km/s, to introduce HVI to the outer shielding layer. Both linear/nonlinear features of guided waves propagating in the inner layer including various nonlinearity sources are investigated using finite element models, corroborated by experiment. With the models, the accumulation of nonlinear second harmonics (nonlinear features) in the case of phase matching is analyzed. Based on the numerical models and experimental discovery, linear/nonlinear indices are developed, via which a detection approach is developed, able to characterize HVI-induced pitting damage. In the approach, the second harmonics (nonlinear feature) show higher sensitivity to pitting damage compared to the fundamental wave (linear feature). Combining a path-based probability imaging algorithm with defined linear/nonlinear indices, this approach can identify HVI-induced damage to the spacecraft precisely and intuitively.