Hello sivastic

Any material irrespective of its thickness can be impact tested. This is not really the issue. You need to get the impact specimen down to the MDTM of the vessel or tank, impact test it, and get the required amount of energy. If you wanted to know the lowest temperature to set the MDMT, you would have to test a number of sets of specimens and keep lowering the temperature until they just failed. This is a rather expensive set of tests.