In order to have a good idea of how sound a component is, usually at least two different inspection methods must be employed. Often, one of the methods is a VISUAL TEST. At the MS&T Conference in October 2010, one of the speakers in the failure-analysis symposium stated that any inspector who is asked to do any method other than visual testing should also do a visual test. However, in my studies (see the excellent Handbook of Nondestructive Evaluation by Charles Hellier; McGraw Hill, 2001) and experience, doing a visual test is NOT the same thing as OPENING YOUR EYELIDS and pointing the pupils of the eyeballs in a certain direction.
A VISUAL TEST is performed according to a specific set of criteria that is prepared for the particular component and inspection in question. The inspector looks for each of the specified desired or undesired features and determines pass or fail based on this specific list of items. Thus, it is not really possible for an inspector sent out to do an ultrasonic test and a radiographic test, for example, to “just out of good practice” do a “visual test.” Maybe it is not a bad idea to look something over, as this may help to interpret the data from the other methods. But this is not the same as doing a Visual Test.
Nondestructive testing (NDT) is a group of powerful methods that have been used successfully to prevent failures – when their limitations have been kept in mind! One of the main problems that remains in NDT, in general, is that the limitation of the method (as determined in best-case scenarios in the lab) is assumed to be the limitation in the field. We should all try to remember that the method doesn't detect discontinuities. The very-human inspector detects indications that, then, have to be interpreted by someone. Sometimes, that’s someone other than the one who performed the test.
General concepts of Quality Science have taught us over the last few decades – if not longer – that it is impossible to inspect quality into manufacturing processes. Why should this conclusion be any different when it comes to specialized inspections in difficult field conditions?