Modally selective nonlinear ultrasonic waves for characterization of pitting damage in whipple shields of spacecraft

Featuring hundreds of craters, cracks and diverse microscopic defects disorderedly scattered over a wide region, the pitting damage in a typical Whipple shield of spacecraft induces highly complex wave scattering. Due to the dispersive and multimode natures, only nonlinear ultrasonic waves (NUWs) with exact phase-velocity matching condition are generally used to evaluate the microstructural material deterioration. Targeting accurate, holistic evaluation of pitting damage, semi-analytical finite element (SAFE) approach is adopted to identify the internal resonant conditions and to select an efficient mode pair for characterizing pitting damage. To explore the feasibility of pitting damage evaluation by using the selected mode pair and fully utilize its unique merits, the cumulative effect of second harmonics is analyzed using numerical simulations and corroborated by experiment. Regardless of the selection of mode pair (i.e., S1-s2 and S0-s0), the amplitude of second harmonics obtained in the pitted plate is observed to increase significantly after the probing GUWs traverse the pitted region, upon interacting with the pitting damage. This phenomenon is remarkable particularly when the probing GUW does not satisfy the requirement of internal resonance. The mode pairs S0-s0 with different degrees of phase-velocity mismatching are further analyzed. Results show that the hypervelocity impact-induced pitting damage in the rear wall of Whipple shields can be detected accurately using the mode pair S0-s0, and a relatively higher excitation frequency is preferred due to its higher degree of phase-velocity mismatching, leading to standing out of the pitting damage-induced CAN.