Monitoring saltwater corrosion of steel using ultrasonic coda wave interferometry with temperature control.

Assessing corrosion is crucial in the petrochemical and marine industries. Usual ultrasonic methods based on pulse-echo and guided waves to detect corrosion lack of precision and struggle in structures with a complex shape. In this paper, a complementary and sensitive ultrasonic method based on coda wave interferometry is presented to detect and quantify thickness loss caused by saltwater corrosion of a steel sample. The method consists in exciting the sample and measuring periodically the scattered coda signal. Correlation of the coda signal with a reference taken for the sample initial state permits the monitoring of corrosion spread with a high accuracy. A laboratory experiment is conducted with two steel samples immersed in saltwater with coda and temperature measured simultaneously. One of the samples is protected from corrosion and is used as a control sample to determine the influence of temperature on the coda signals. It is shown that the coda signals on the corroded sample can be temperature-corrected using the temperature measurement only. A control sample is not needed. A good correlation is found between a parameter quantifying the stretching of the coda over time and the corrosion surface, which is monitored with a camera. Finally, a simple theoretical model of coda signal is proposed to quantify the real-time average corrosion rate during the experiment with a sub-micrometric precision. The estimated final average corrosion depth is validated by independent depth profile measurements. The uncertainties and sensitivity of the presented method are investigated.

Localization of unbounded contacts on vibrating elastic plates.

Detection and localization of unbounded contacts in industrial structures are crucial for user safety. However, most structural health monitoring techniques are either invasive, power-consuming, or rely on time-varying baseline comparison. A passive acoustic method is proposed to localize unbounded contacts in plate-like structures, using the acoustic emissions by the contacts when they are excited by ambient noise. The technique consists of computing the correlation matrix of the signals measured by a set of receivers and applying to this matrix a beamforming algorithm accounting for flexural wave dispersion. To validate the technique, an experimental setup is developed in which three idealized unbounded contacts are created on a thin plate excited by a shaker. How the quality of the defect localization depends on the defect type, receiver number, and the characteristics of the noise is investigated. Finally, it is shown that the localization of unbounded contacts is possible using either an acoustic ambient noise source or a more realistic jet engine noise.

Selective remote excitation of complex structures using time reversal in audible frequency range.

Generation of elastic waves is a major issue in nondestructive testing. Structural health monitoring of a thin element can be achieved through the analysis of its resonance spectrum. A time reversal mirror (TRM) operating in the audible frequency range (1-10 kHz) is used to remotely excite thin resonant elastic elements. The generation of elastic waves is studied with respect to the geometry of the TRM. It is observed that the quality of focusing only weakly depends on the number of loudspeakers (LS) in the TRM. When the air/plate coupling is at its maximum, the energetic efficiency of the TRM is estimated to be about 0.02%. The TRM is shown to efficiently and selectively excite a small structure embedded in a complex environment such as a hollow cylinder. Finally, the results are discussed in light of the DORT method (French acronym for "decomposition of the time reversal operator"). In particular, the optimal LS placement and emission signals in this configuration to excite individual eigenmodes of a plate is determined.