The limitations listed by AWS D1.1 are based on the procedures included in D1.1, transducer type, size, angles, etc.
Thinner materials can be tested using different techniques. It is common to test sheet metal gages in the aerospace industries with surfaces waves. The techniques must be developed for the application, i.e., the approach is not prescriptive as it is in AWS D1.1.
Best regards - Al
Richard,
It may help if we knew what standards actually applied to the work to be performed not just the reference given from D1.1. There are many differences between the multitude of wave forms, transducers, frequency, codes, etc.
As Al stated, some are very good for very thin materials on applications for automotive, aerospace, and so much more. Others then are good, such as the regulations for D1.1, for structural steels exceeding 5/16" (8mm).
Can you give us more details?
He Is In Control, Have a Great Day, Brent