Ultrasonic guided waves for health monitoring of high-pressure composite tanks

Ultrasonic guided wave modes are proposed to control the integrity of high-pressure composite tanks produced by EADS-ASTRIUM, France. The purpose is to demonstrate the potentiality of air-coupled transducers to set up a contact-less, single-sided technique for testing the moisture content and/or the micro-cracking of carbon-epoxy composite wound around a Titanium liner. Although guided waves have been experimentally propagated on a real tank, it was not allowed to damage this specimen. Therefore, plates made of similar composite materials than that constituting the tank winding were submitted to water intake or to thermal stresses. After immersing some plates in a humid chamber, it was demonstrated that the attenuation of the A0 guided wave mode is sensitive to the moisture content. Other plate samples were submitted to immersion in liquid nitrogen that induces transversal cracks shown to cause significant drops in the celerity of several guided waves. Inverse problems have been used for quantifying the effects of these damages on the material properties, and they showed that water intake increases the imaginary part of the Coulomb moduli, while micro-cracking decreases all the material stiffness moduli. Such changes in the material properties have then been used as input data for simulating waveforms corresponding to the propagation of circumferential or longitudinal wave modes in the tank. Changes in these waveforms, caused by simulated damages of the composite winding of the tank, have been shown to be quite significant. To conclude the study, an experimental sep-up using air-coupled transducers was employed to generate–detect guided wave modes over large distances in the real tank, with very good signal-to-noise ratios, thus demonstrating the possibility of using such elements for the non-destructive testing of high-pressure composite tanks during their lives.