NDTnet 1998 Aug, Vol.3 No.8

Abstract

Bimetallic welds are currently inspected using ultrasonic longitudinal waves at oblique incidence inpulse echo mode. In some case, this examination points out structural echoes, detected close to the surface opposite to the control. These echoes have a specific shape: the amplitude is distributed along lines parallel to the weld butterings. Such echoes can disturb the inspection: they can be misinterpreted as provided by a defect, or they can mask a possible defect. Previous experiments showed that these structural echoes do not result from beam deflection in the weld. In addition, other results showed that this phenomenon occurs in the last millimeters under the backwall.

According to these results, this paper gives an interpretation for these echoes based on the anisotropic and the inhomogeneous structure of the weld. In the last welding pass, oblique longitudinal waves maybe converted into normal shear waves. Mode converted waves can be reflected normally on the backwall and therefore back propagated to the transducer following the same wavepath as the forward propagation.
Other experiments have been achieved using a new sensor able to discriminate the wave polarization at reception. The results showed the existence of shear waves on the backwall. In addition, a theoretical model describing the mode conversions in the last welding passes as a function of the crystallographic orientations allowed to valid the interpretation of the phenomenon.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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