Jury

Summary:

The Ultrasound-Laser method is an important and interesting approach for non-contact non-destructive testing (NDT) of structures. The aim of this thesis is to optimise this method to improve the detection and characterisation of defects using surface acoustic waves, by modifying the shape of the thermoelastic source via that of the generating laser beam. We presented the main characteristics of Rayleigh wave propagation in a homogeneous and isotropic medium and developed a numerical model based on the finite element method to simulate the generation of ultrasonic waves by two separate linear source arrays. This model was also used to analyse the wave-defect interaction in different configurations. Experimentally, the shape of the laser source was modified using various optical devices. Different arrays of thermoelastic sources were studied in terms of excited Rayleigh waves and their interactions with various open defects in an aluminium sample. In particular, we have shown that it is possible to easily and simultaneously excite quasi-monochromatic and pulsed Rayleigh waves. This makes it possible to improve the detection and characterisation of defects using the frequency spectra thus obtained. Various artificial defects were considered and the analysis of the propagation of Rayleigh waves reflected and transmitted by these defects highlighted the interest of the optical devices studied. The results obtained from the different experimental configurations clearly demonstrated the advantages of each of them for the NDT of the structure concerned.

Abstract:

Laser Ultrasonics method is an important and interesting approach to non-contact Non-Destructive Testing of structures. The aim of this thesis is to optimize this method for improving the detection and characterization of defects using surface acoustic waves, by modifying the shape of the thermoelastic source via that of the generating laser beam. We have presented the main characteristics of Rayleigh waves propagation in a homogeneous and isotropic medium, and developed a numerical model based on the finite element method to simulate the generation of ultrasonic waves by two distinct arrays of line sources. The model was also used to analyse waves-defects interaction in different configurations. Experimentally, the shape of the laser source was modified using various optical devices. Different arrays of thermoelastic sources were studied in terms of excited Rayleigh waves and their interactions with various open defects in an aluminium sample. In particular, we have shown that quasi-monochromatic and pulsed Rayleigh waves can be excited easily and simultaneously. This makes it possible to improve defect detection and characterization with regard to the frequency spectra thus obtained. Various artificial defects have been considered and the analysis of the propagation of Rayleigh waves reflected and transmitted by these defects has demonstrated the interest of the optical devices studied. The results obtained from the various experimental configurations clearly demonstrated the benefits of each for the NDT of the structure concerned.