Factors affecting the imaging of the impact location with inverse filtering and diffuse wave fields

Reciprocal time reversal (inverse filtering) of acousto-ultrasonic fields is a very efficient technique to focus elastic waves through reverberant isotropic and anisotropic media. Such a methodology relies on the correlation of the experimental Green's function that is acquired by a set of receiver sensors from a limited number of impact sources. However, although heterogeneities and discontinuities within the structural response can be compensated by the inverse filtering process, environmental effects such as temperature variations as well as incoherent noise measurements and the finite number of excitation sources may degrade the quality of time reversal focusing. The scope of this article was to study the factors affecting the impact location imaging using the reciprocal time reversal method in the presence of complex diffuse wave fields. Particularly, a signal-stretch strategy was developed to compensate the temperature changes before remitting the back-propagated wave field at the focus point. Then, in order to investigate the imaging performance and the sensitivity of the proposed methodology, different sets of libraries with reduced input signals were created and tested. Finally, different configurations of the receiver piezoelectric sensors were used to perform the reciprocal time reversal method. To validate this research work, two geometrically complex composite structures, that is, a composite tail rotor blade and a stiffened composite panel, were used. Results showed that both the temperature compensation and the signal processing with the reduced time traced signals and receiver sensors allowed obtaining an accurate identification of the impact events.