If there is no fusion between the two pieces of metal (a "no weld" or "unweld" condition), then the successive echoes will be much closer together and larger in amplitude.
No Weld
A stick weld is a condition in which the metal sheets are fused, but because of insufficient heating the weld nugget is not fully formed. The echo decay rate will change, with larger peaks appearing on the screen and a smaller drop off in amplitude between any two successive peaks.
Stick Weld
In the case of an undersized weld, a portion of the sound beam will reflect from the total thickness of the two metal sheets, while a portion reflects from a single thickness. This creates a pattern in which small peaks representing single-sheet thickness appear between the larger and more widely separated peaks representing the full weld thickness.
Undersized Weld
There will be a characteristic echo pattern associated with a good weld, in which the spacing between echoes will be proportional to the thickness of the weld, and the decay rate (the rate at which successive echoes decrease in amplitude) will be related to the attenuation within the nugget. This pattern becomes the reference for further tests.
Good Weld
Spot Weld Echo Patterns
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