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Technical Discussions
Hermann Wuestenberg
R & D
BAM Berlin, Germany, Joined Nov 1998, 26

Hermann Wuestenberg

R & D
BAM Berlin,
Joined Nov 1998
04:14 Jul-02-1998
Questions on TOFD

Basically I agree with the commentaries of Udo Schlengermann, but would like to add some other points:
1. The dead zone does not only depend on the pulse length. We have to take into account the intensity relationship between the subsurface L-wave (also called lateral- , creeping- or longitunal headwave) and the diffracted crack tip indication. The subsurface L-wave has an other distance law than the the crack tip indication, that means it decreases much faster with an increasing distance between the probes than the diffracted tip signal. On the other side there must be regarded a dependency of the diffracted signal amplitude from the relationship between the depth of a surface breaking crack and the wavelength. For smaller depth of the crack the energy diffracted at the tip decreases. In consequence the "dead zone" for the detection and sizing of cracks close to the coupling surface is difficult to predict. For the practical application it seems to be reasonable to assume the values of 2 mm for 5 MHz and 6 mm for 2 MHZ as indicated by Udo Schlengermann as limitations of the method. Similar considerations are valid as well for the "dead zone" close to opposite surface. But there we have to do with much greater amplitudes from the backwall and hence stronger limitations for the detectability of near surface cracks have to be taken into account. A minimal detectable crack depth of 3 mm (for a surface breaking crack) must be assumed.
2. The coverage of a wall thickness to be inspected can be achieved by playing with basically 2 parameters:
- the distance between the probes and
- the soundfield divergency.
With on set of probes it should be possible the cover at least the 70 mm wall thickness as indicated in the European Prestandard 586-6, but only with two or three different probe distances. According to our experiences (see also the contribution of A. Erhard at the 7. ECNDT in Copenhagen, May 1998) the recommendations of that Prestandard are not very helpful. Since an increased beam divergency increases also the structural noise, a larger wallthickness requires more than one probe distance for the inspection. This leads immediately to a comparison with other traditional ultrasonic pulse echo inspections requiring multiple pathes and angles. For equal flaw detectability there is a wall thickness where the reliable detection of cracks requires an inspection with the conventional pulse echo technique using the well known corner effect as a much stronger physical interaction between an ultrasonic wave and a crack than the tip diffraction is.


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