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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
03:15 Mar-18-1998
Time reversed mirror technique

Responding to a remark of Prof. Fink

Dear Prof. Fink

The last hint in my short contribution about our experiences with the TMR approach ("the target is painted after the arrival of the arrow") is especially related to hopes and expectations in cases, which can not be solved by Autofocussing or TRM. The general value of this technique is not be called into question, but the example described below may clarify our reluctant position.
Strong focussing is regarded as an important tool especially for anisotropic and coarse grained materials like stainless steel castings or weldments. If for such materials the focussing is optimized for one position and one specific reflector (using suitable optimization strategies for the delay time repartition of the probe array), this is in those materials generally only valid for this specific position and could be totally wrong for a neighbouring area where with that focussing parameters a similar defect can be missed.
In order to avoid such an uncertainty for the detection probability, one should try to inspect the whole volume with sweeping focussing parameters. This is not only time consuming but also useless because the most essential contribution of an TMR approach at such materials is the possibility to define individually adaptable complicated delay time repartitions not simply being based on a curvature and a linear slope .
A sweeping focussing can only vary some parameters and not the whole multitude. That means that a TRM technique has severe limitations in case of dynamically changing propagation conditions like those encountered during the mechanical scanning of austenitic coarse grained material. This has nothing to do with the application in more static situations where the probe is not moved or where the propagation parameters (sound velocity etc.) of the material are constant.

With best regards

H. Wuestenberg


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