Bend testing is performed to verify the weld soundness, i.e., free of unacceptable discontinuities. Most bend tests strain the specimen with the idea that any discontinuity of critical size will initiate a crack/open indication when it is strained beyond a certain value.
The bend radius is a function of the base metal thickness and elongation of the base metal when testing to AWS, ASME, or military welding standard. API uses a standard bend radius regardless of the thickness or ductility of the base metal, so to say an API qualification test using a bend test is equivalent to an AWS or ASME or military requirements is far from the case.
Whether the testing is part of the testing regiment associated with the qualification of a WPS or to evaluate the welder's skill, the function is the same. The goal is to access the weld's soundness.
In the case of qualifying a WPS, several welding standards require VT of the completed weld, volumetric examination (RT or UT), reduced section tensile testing to verify the mechanical properties are produced, bend testing for soundness, and possibly CVN and/or hardness testing. All the testing is to verify the WPS is capable of producing a weld with the mechanical properties needed per the code or per project requirements.
The welder performance testing is to verify the welder has the skills needed to produce a sound weld if the welder follows a qualified WPS. If the welders does not follow the WPS, for instance high voltage is used, higher amperage is used, higher heat input is used, etc., there is no assurance the resulting weld meets the mechanical properties produced when the WPS was qualified. The test is considered to have failed if the welder does not work within the ranges listed by the approved WPS.
There are exceptions to the testing regiment listed above. For example, if one is qualifying a WPS for welding cast iron, only the reduced tensile test is required. The ductility of cast iron is nil, thus an attempt to perform a bend test is futile.
Best regards - Al