Thank you Maty,
For quadral tubes which are used in bus body manufacturing the wall thickness is often 1/8" (3 mm), and no much than
1/4" (6 mm). I went around here - Hungary, and almost everybody said that because the receiver size cannot goes under
specific limit the NDT inspector don't use UT testing for thin joints. Mostly they use MT method, or PT.
Only one info I've received about one existing DIN standard for thin tubes UT inspection. If I'l have new info I will tell you.
Regards Geza