I am working with an ultrasonic device to inspect welds. So far, I have learned that the minimum thickness of the metal sheet for this inspection is 6mm - 8mm. But the product of mine has 3mm thick welds:

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I have basic knowledge working with angle beam transducer to inspect thick metal welds. How can I inpsect thin metal welds by using conventional ultrasonic device?

  • $\begingroup$ What if you put a solid rubber mat between the probe and the work piece? $\endgroup$ Nov 29, 2022 at 13:46

2 Answers 2


If the thickness is less than 3-4 of $\lambda$ bulk waves cannot propagate (limits to min. 5-6mm). You need "normal" or so called Lamb waves, you can achieve them with a wedge sensor (inclined angle to the surface, goes definitely below 3 mm) or submerging methods (which is not really very practically).

see e.g. Comparison of Ultrasonic Non-Contact Air-Coupled Techniques

Lamb Wave interaction


The following is just a speculation, but maybe you can place a thicker plate under your thin plate and use reflections from the bottom of the thicker plate. Reflections from the bottom of the thin plate will be a problem, so maybe some oil should be placed between the thin and thick plates to make sure more ultrasonic power gets into the thicker plate.

  • $\begingroup$ the angle beam will not transfer in a straight line if we put another plate under thin plate. that would be much complicated $\endgroup$
    – Dat
    Oct 8, 2019 at 9:07
  • $\begingroup$ @Dat : Part of ultrasonic power will still propagate in a straight line. I cannot be sure that this is enough, but one cannot be sure this is not enough. For example, if the plates are very smooth this will be enough. $\endgroup$
    – akhmeteli
    Oct 8, 2019 at 12:46

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