Ultrasonic imaging of contact defects under low frequency loading ; Imagerie ultrasonore de défaut de contact sous sollicitations basse fréquence ; Ultrasonic imaging of contact defects under low frequency loading: application for structural health monitoring of elastic plates ; Imagerie ultrasonore de défaut de contact sous sollicitations basse fréquence: application pour le contrôle santé de plaques élastiques

In structural health monitoring, acoustic methods without use of a reference intact sample are nowadays of great interest due to variability of environmental and operational conditions. This thesis presents an acoustic reference-free method for detecting and localizing defects in thin plates. The proposed approach is based on pump-probe measurements and a differential signal processing algorithm. The pump-probe scheme here means the simultaneous excitation of low-frequency vibrations and high-frequency acoustic pulses generated by a piezoelectric transducer and recorded by an array of transducers of the same kind. The role of pumping vibrations is in imposing changes to loading conditions for a defect (here a model Hertzian contact i.e. a small ball pressed against the tested plate) while the ultrasonic Lamb wave pulses probe the defect in various loading states. Then a differential signal processing algorithm based on backpropagation of A0 Lamb mode is used to detect and locate the defect. Two detection modes have been studied: the mode requiring the synchronization between the pump and probe signals, and the non-synchronous mode. In the latter case, a correction with the use of ad hoc weight coefficients improves the detection. Further, a parametric study was performed by computing the image contrast as a function of the defect scattering cross-section that allowed us to determine a minimum detection threshold. The application of the method to passive imaging (i.e. using white noise instead of probe pulses) produced encouraging results for the price of increasing the number of receivers. Finally, numerical modeling for the interaction of an ultrasonic wave with a Hertz-Mindlin frictional contact was performed in 2D to compare the model defect (ball pressed against the plate) with a more realistic one (crack). ; Le contrôle-qualité de matériaux par méthodes acoustiques sans référence suscite aujourd’hui un grand intérêt en contrôle santé intégré (CSI) au vu de la variabilité des conditions environnementales et ...