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- since 1996 -
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Technical Discussions
Rod
Rod
02:26 Oct-18-2019
MT Terminology

I have come across an oversight MT report that mentions a rejectable inspection due to "Walking the dog". Im familiar with the terminology but have not seen this terminology in any publications or NDT books. What is "Walking the Dog".

 
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laurie
Australia, Joined Feb 2019, 28

laurie

Australia,
Joined Feb 2019
28
09:20 Oct-21-2019
Re: MT Terminology
In Reply to Rod at 02:26 Oct-18-2019 (Opening).

Pretty sure this is where the legs are lifted one after the other on either side of the weld to "walk" the yoke at 45 degrees along the weld.

Walking the yoke would only test at one orientation for each area so each successive area is "blind" to defects at 90 degrees to the alternating leg orientation of 45 and 135 degrees at each step.

In addition leg orientation is supposed to be at 90 degrees to the expected orientation of defects and then at 90 degrees.
Since most defects are expected to be along the toe of the weld or transverse, walking at 45 degrees does not achieve the best magnetisation for either of these directions.

 
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Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 850

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
850
01:02 Oct-22-2019
Re: MT Terminology
In Reply to laurie at 09:20 Oct-21-2019 .

Laurie my friend, I most disagree with you on a few points.

Although I never heard the term walking the dog, I am familiar with the procedure where each yoke leg is moved alternately (walked) along the weld. It is true that the best indication will be given when the magnetic field is 90 degrees to the indication. But, one as to remember that there is an area of no inspection with the yoke. This area is equal to 15 degrees (30 degrees total) on either side of the magnetic field main axis. This is the reason we do inspect part with two magnetic fields at 90 degrees from each other. The inspection pattern will resemble a shoe laced.

In the case of inspection with a yoke, one can walk the legs and skipping an area of inspection. This area will be inspection on the return back to the original start point. As well, each area of inspection (in the same axis) should be overlaped by at least 10 percent and that also hold true for the opposite axis. This way, the weld will be inspected in its entirety.

In closing, I would like to add that if you find that is of concern, nothing is stopping you from inspecting the area in another axis.

 
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