Attenuation assessment for NDT of austenitic stainless steel welds

International audienceMultipass welds made in 316-L stainless steel are specific welds of primary circuit in nuclear power stations. Their complex structure complicates ultrasonic assessment of their structural integrity : they present a heterogeneous anisotropy that deteriorates the propagation of waves (deviation and division of the beam, attenuation...). Polycrystalline materials as stainless steel are composed of numerous discrete grains whose elastic properties are anisotropic and crystallographic axes are differently oriented. When an acoustic wave propagates through such a material, it is attenuated by scattering at grain boundaries. Provided the elastic constants of the different homogeneous domains of the weld, the finite-element model ATHENA (EDF & INRIA) predicts path and velocity of ultrasonic waves, but attenuation has still to be integrated into the code. Our work aims to provide realistic input data of attenuation compatible with the existing model in studying mechanisms leading to attenuation in anisotropic structures. The value of this attenuation depends on the size, shape, orientations distribution and anisotropy of the grains. When grains are equiaxed and randomly oriented, the average elastic properties are isotropic. But in the case of multipass welds made of austenitic stainless steel, crystalline growth mechanisms acting during the solidification lead to a macroscopic texture, with elongated and preferentially oriented grains. Thus their structure is anisotropic, and ultrasonic attenuation is a function of the propagation direction. First, experimental ultrasonic measurements obtained by means of classical techniques are presented and compared with theoretical predictions of the literature. Then other experimental data obtained by mapping the incident and transmitted ultrasonic fields are compared with modelling based on the plane waves angular spectrum decomposition of the beam in order to evaluate the energy loss experienced by each plane wave component of the beam