A weld discontinuity characterized by unmelted joint surfaces at the root is known as?

The correct answer is that a weld discontinuity characterized by unmelted joint surfaces at the root is known as incomplete penetration. This term specifically refers to a condition where the weld metal does not fully penetrate the joint, which can lead to weaknesses in the weld. Complete fusion at the root is essential for joint strength, and when joint surfaces remain unmelted, it indicates that the penetration has not reached adequate levels.

Understanding this concept is crucial in radiographic testing, where the aim is to identify and assess weld quality through the interpretation of radiographs. Incomplete penetration can often be a critical flaw because it directly affects the load-bearing capacity of the welded joint. Such issues can be detected through radiographic images that display a lack of fusion, which can appear as dark areas or lines at the root of the weld.

The other terms represent different welding flaws: undercut refers to a groove formed at the weld toe, porosity indicates trapped gas bubbles within the weld metal, and slag inclusion involves non-metallic solid materials trapped in the weld. Each of these defects can also compromise weld quality but does not specifically describe the situation of unmelted joint surfaces at the root.