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Introduction: For composite structures, delamination usually occurs at the interface. Ultrasonic guided waves have been widely used to detect the delamination. However, most researches focus on the measurement of the delamination length along the interface, and the measurement of the delamination thickness is less studied. Method: In this paper, based on the characteristic that the Stoneley wave only propagates along the interface, the reflection coefficient of interaction between Stoneley wave and delamination is used to measure the delamination thickness. The effects of delamination thickness and frequency on the reflection coefficient are investigated via dividing integral region and reciprocity theorem. Some experimental validations are carried out on two aluminum-steel bilayered composite plates with different delamination thickness. Results: It is found the reflection coefficient increases linearly at first, then its rate of increase slows down gradually, and finally becomes stable, in theory. And the experimental results can verify the theoretical relationship between the reflection coefficients and the delamination thickness. Conclusion: The variation of reflection coefficient provides a reference for the measurement of delamination thickness in Stoneley-wave-based non-destructive testing.

The physical-layer security method based on the transmitter's multi-antenna redundant isn' t applicable to the SI-MO-OFDM system. To solve the problem, this paper proposed a physical layer security method based on time-domain reference. The legitimate receiver constructs the random multi-antenna vector in each subcarrier, of which the projection in the main channel was equal to the varying time-domain reference variable. The transmitter secured the transmitted signal of this moment with the previous time-domain reference variable. The random variation of the reference variable disturbed the eavesdropper's received signal constellation, so that the eavesdropper couldn' t intercept the secret information. Security analysis and simulation results show that the legitimate receiver's BER is 10, the eavesdropper couldn't correctly demodulate the received signals when the received SNR and antenna number is respectively 10 dB and 8. [ABSTRACT FROM AUTHOR]