Visual Testing is probably the most important type of examination in the Non-Destructive Examination field. If an inspector can see a discontinuity then it can be evaluated. Unless one is Superman, it is pretty difficult to see a potential cause of failure inside a weld. For this reason, Visual Testing is limited to surface examinations.
A Visual Test is also limited by the technician's ability to evaluate the weld. He/she must be well trained to correctly assess weld deposits. In addition, there are many other things that might interfere with an accurate diagnosis.