Non-destructive Testing of Envelope Material Based on Wave Propagation Characteristics

Long-duration vehicles in near space have achieved great success; however, the non-destructive testing (NDT) methods for the envelope materials of such long-duration vehicles remain blank. In this paper, we propose the air-coupled ultrasonic NDT method theoretically. In the theoretical analysis process, the envelope material is simplified as an orthogonal sandwich structure. To calculate the displacement and stress fields of each medium, the state vectors are established and the transfer matrices of the material from the upper interface to the lower interface are obtained by using boundary conditions. Then, linear equations about the amplitude of reflected and transmitted waves are derived by combining the coupling boundary conditions of air and solid. The effects of incident angles, inflation of the envelope material, and debonding of the interfaces on the transmission coefficients are considered. The results show that the air-coupled ultrasonic NDT of the envelope material can be carried out in the pre-inflated state. Finally, a method for identifying interface debonding is proposed based on judging transmission coefficients within a certain frequency range.