A new retroreflector named "quatriplan" based on the corner-cube principle, for advanced ultrasonic telemetry applications.

Telemetry consists in remotely detecting and locating an object. For applications in immersed structures as in nuclear primary vessel, ultrasonic waves are well adapted. Moreover, fixing a target on the structures of interest maximizes the signal-to-noise ratio and provides a reference point. Classical Corner-Cube Retroreflector (CCR) demonstrated high performance in this framework (1D and 2D measurements) but does not allow knowing the full (3D) positioning of the structure. This paper proposes an innovative compact target named "quatriplan", based on the CCR principle, and which must allow the ability to determine the orientation of the target in addition to its distance to the transducer. The simple design of the quatriplan is first explained, then its performances are investigated with modelling and experimentations. The results highlight its strong performance and benefit for advanced telemetry applications in industrial systems where complex design can impede easy and efficient access for inspection of specific parts.

Acoustical properties of an immersed corner-cube retroreflector alone and behind screen for ultrasonic telemetry applications.

Ultrasonic telemetry measurements consist in remotely detecting and locating an object. To maximize the signal-to-noise ratio, a target may be used, positioned at a reference point. In this framework, the ultrasonic reflective characteristics of a corner-cube retroreflector (CCR) are investigated. The most interesting property of a CCR is its ability to fully reverse an incoming wave in the same direction under certain conditions. Theoretical developments are performed in order to understand its acoustic behaviour, and experimentations are made in various configurations: CCR alone in water, and behind an immersed plate that acts as a screen, with normal and non-normal incidence. The results highlight its strong performance. Moreover, the study of two other couples of CCR material and surrounding fluid underlines the relevance of considering the acoustic properties of each medium, as they have a strong influence on the acoustic response of the CCR.

Multi-modal leaky Lamb waves in two parallel and immersed plates: Theoretical considerations, simulations, and measurements.

Leaky Lamb waves have the potential to be used to perform non-destructive testing on a set of several parallel and immersed plates. Short-time Fourier transform and two-dimensional Fourier transform have both been successfully used to measure the propagation properties: phase and group velocity, and leaky attenuation. Experimental measurements were validated by comparison between theory, experimentation and finite-element simulations (using comsol multiphysics® software) in the case of one immersed plate in water. These signal processing techniques proved to be efficient in the case of multi-modal propagation. They were applied to two immersed plates to identify the leaky Lamb mode generated in the second plate. Dispersion curves of the system composed by two immersed and parallel plates are computed. When plates have the same thickness, leaky Lamb modes propagate from the first to the second plate without any mode change, with the apparent attenuation being weaker in the second plate. Considering that the second plate is continuously supplied in energy by the first one, an energy-based model is proposed herein to estimate the apparent attenuation in the second plate. Despite our extremely simplifying assumption, this model proved to be in good agreement with both finite-element modelling and experimentation.

Measurement of ultrasonic scattering attenuation in austenitic stainless steel welds: realistic input data for NDT numerical modeling.

Multipass welds made of 316L stainless steel are specific welds of the primary circuit of pressurized water reactors in nuclear power plants. Because of their strong heterogeneous and anisotropic nature due to grain growth during solidification, ultrasonic waves may be greatly deviated, split and attenuated. Thus, ultrasonic assessment of the structural integrity of such welds is quite complicated. Numerical codes exist that simulate ultrasonic propagation through such structures, but they require precise and realistic input data, as attenuation coefficients. This paper presents rigorous measurements of attenuation in austenitic weld as a function of grain orientation. In fact attenuation is here mainly caused by grain scattering. Measurements are based on the decomposition of experimental beams into plane-wave angular spectra and on the modeling of the ultrasonic propagation through the material. For this, the transmission coefficients are calculated for any incident plane wave on an anisotropic plate. Two different hypotheses on the welded material are tested: first it is considered as monoclinic, and then as triclinic. Results are analyzed, and validated through comparison to theoretical predictions of related literature. They underline the great importance of well-describing the anisotropic structure of austenitic welds for UT modeling issues.