Feasibility of passive SHM for corrosion detection by guided wave tomography
Résumé
Structural health monitoring (SHM) consists in embedding sensors in a structure like an aircraft or a naval ship in order to detect defects (for example cracks or corrosion in metallic materials or delamination in composite materials) before a serious fault occurs in the structure. Guided elastic waves – which have the ability to propagate over long distances – emitted by a sensor and propagating to another one are often used as the physical way of detecting the defect. However, the implementation of SHM systems is restricted in many situations by the necessity to store or to harvest the electric energy necessary to emit the waves. A promising way to tackle this constraint is to use techniques based on the cross-correlations of the ambient acoustic noise in place in the structure. It has been shown that, under certain conditions, transient response between two sensors can be estimated from cross-correlation of ambient noise, with purely passive measurements. The idea is to take advantage of the elastic noise naturally present in the structure (due to engine vibrations or aero-acoustic turbulences on the fuselage of an aircraft for example) in order to avoid the emission of the elastic waves by the SHM system. The complexity of the embedded SHM system is therefore reduced. We present here studies of noise cross-correlation techniques that have been conducted with the aim of doing passive tomography of extended defects (such as corrosion or delamination) using an array of piezoelectric (PZT) transducers. Noise is generated thanks to compressed air sprayed on the surface of an aluminum plate. Passive measurements are compared to active signals to demonstrate the convergence of the cross-correlation technique to the Green function of the system. Experimental results which come from tomographic time-of-flight imaging algorithms will also be described. Finally, an extension of this technique using purely passive guided wave sensors such as Fiber Bragg Grating (FBG) will be presented. Comparisons between PZT results and FBG results will be presented with the aim of noticing the possible advantages of FBG compared to PZT.