Which of the following discontinuities is rarely seen during VT?

The rare occurrence of tungsten inclusion during visual testing (VT) can be attributed to the specific nature of this discontinuity and the contexts in which VT is typically applied. Tungsten inclusion refers to the presence of tungsten particles within a weld or base material, which usually originates from the use of tungsten electrodes in processes like TIG (Tungsten Inert Gas) welding.

These inclusions require a very close examination and are often detected through more advanced non-destructive testing methods, such as ultrasonic testing or radiography, rather than visual inspection alone. In typical visual testing scenarios, the focus is on identifying surface-level discontinuities that are readily observable, such as cracks, surface porosity, or incomplete penetration, which are more common and easier to spot during a VT inspection.

Thus, tungsten inclusion is not frequently encountered in visual testing due to its specific characteristics and the limitations of the VT method in detecting internal material anomalies. This makes it the least commonly observed discontinuity in the context of visual testing as compared to the others listed.