Definitions for "PROOF LOAD"
The proof load of a nut is the axially applied load the nut must withstand without thread stripping or rupture. The proof load of a bolt, screw or stud is the specified load the product must withstand without permanent set.
A proof load is a specified test load which a tastener must withstand without any indication of failure.
A test load that a fitting must undergo without showing significant deformation. It is usually 75% of yield strength.