23:08 Sep-29-2009 Peter Bruckner NDT Inspector, NDT Manager United Kingdom, Joined Jul 2008 19
Re: what is mean by tandam echoIn Reply to Ramalingam at 19:21 Sep-29-2009 (Opening).
Usually tandem testing involves 2 identical shear wave probes facing the same direction on the surface of the bar a fixed distance apart. The distance between the probes is 1/2 skip, for 45 degree probe that is the thickness or diameter of the bar under test.
The only type of defect this will find is a planar one transverse to the beam at 1/2 thickness/diameter. The test standard usually consists of a flat bottomed hole drilled in the end of the bar.
the expression 'tandem' originates from a bicycle type for two persons, where the second person is sitting behind the first, both looking into the forward direction, and both pedaling.
In UT the distance between the two probes is not limited to 1/2 skip, but usually ist smaller than full skip. In testing of medium and heavy wall nuclear components, tandem is a standard system at least in Germany. We have arrangements of e.g. 3, 4 or 5 or more probes, each behind the other. Usually one probe acts as transmitter, the other ones as receivers. All sound pathes are reflecting on the surface opposite to the probe system, so in general you can inspect only components with parallel or coaxial surfaces. Using more than two probes will split the cross section to be inspected into several layers.
The presumed defects at tandem UT are not transverse to the sound beam, but perpendicular to the surface.
As far as I can remember, tandem UT was invented about 1970 by the German institutes and companies BAm and/or AEG, MAN and Siemens.
Uli Mletzko (retired, from MPA/University of Stuttgart/Germany)