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Integral stiffeners act as a frequency dependent filter for guided plate waves, impeding transmission and limiting the performance of ultrasonic structural health monitoring (SHM) systems. The effect of integral stiffeners on an acoustic leak location system for manned spacecraft is examined. Leaking air is turbulent and generates noise that can be detected by a contact-coupled acoustic array to perform source location and find the air leak. Transmission of guided waves past individual stiffeners is measured across a frequency range of 50 to 400 kHz for both high and low aspect-ratio rectangular stiffeners. Transmission past a low aspect ratio stiffener is correlated with the ability to locate leaks in the presence of multiple stiffeners. A simple explanatory model that illuminates the underlying mechanics of waves crossing a stiffener is developed using impedance methods. Good agreement is seen with numerical calculations using the boundary element method and with the experimental measurements. The model aids the designer and indicates transmission and reflection are determined by longitudinal and flexural stiffener resonances. It is demonstrated that operating in frequency ranges of high plate wave stiffener transmission significantly improves the reliability of noise source location in the spacecraft leak location system. A protocol is presented to enable the selection of an optimal frequency range for leak location.

Wave scattering by an integral stiffener