In UCS-66 the governing thickness is important factor cause we have combination of minimum design metal temperature
and governing thickness to the subject material.
but in UHA-51 impact test requirement is based on the material properties and material thickness is not an essential variable . (except where noted)
UHA-51 IMPACT TESTS
Impact tests, as prescribed in (a),shall be performed
on materials listed in Table UHA-23 for all combinations
of materials and minimum design metal temperatures
(MDMTs) except as exempted in (d), (e), (f), (g), (h),or
(i).
Best regards.
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