Ultrasonic techniques

Research and numerical modeling work is being conducted on the propagation modes of guided waves in linear objects (cables, suspensions, or more simply inaccessible cylindrical environments such as pipes) in order to propose non-destructive testing methods adapted to these particular objects. A semi-analytical finite element coupled modeling technique (SAFE-PML) has been developed to account for the helical shape of the elementary wires in prestressed cables in the study of ultrasonic propagation modes (A. Frikha thesis 2010, K.L. Nguyen thesis 2014). An ultrasonic reflectometry measurement method has been developed for defect detection in cables (strand breakage, cracks, ...) in areas close to the anchorage heads (ORSI 11N71 and APOS, Inter-Carnot Fraunhaufer FilameNDT project, Advitam collaboration, M. Masmoudi 2013). This method was validated on the Normandy Bridge (expertise for the Chamber of Commerce and Industry of Le Havre).

The work focuses on the use of surface waves to characterize the mechanical properties of concrete (porosity, damage, modulus, ...). In the case of surface concrete (the first 3-5 centimeters of concrete), the material's heterogeneity cannot be ignored. Four aspects are addressed: 

• Taking into account the heterogeneity of concrete in the calculation of surface wave dispersion curves (homogenization model, numerical modeling by finite elements or finite differences)
• Studying the impact of material heterogeneity on the results obtained by inversion of dispersion curves (in velocity and attenuation)
• Characterizing the non-linearities of the medium, and their ability to reveal degradation in complement to linear observables
• Developing experimental methods for laboratory and in-situ measurements (development of a robot for recording seismograms with a laser interferometer, a multi-sensor probe for point contacts).