Which welding discontinuity is considered most difficult to image radiographically?

Lack of side wall fusion is indeed considered one of the most challenging welding discontinuities to detect through radiographic testing. This discontinuity occurs when the weld metal does not adequately bond to the side walls of the base metal, which can lead to significant weakness in the joint.

The difficulty in imaging lack of side wall fusion arises primarily from its location. This type of flaw often exists at the root of the weld, where the weld metal meets the base metal. Radiographic testing relies on the differential absorption of radiation; however, if the lack of fusion is subtle or if the joint design conceals the root area, it can be difficult to produce a radiographic image that clearly portrays this discontinuity. Additionally, lack of side wall fusion may not present significant density variations compared to surrounding metal, making it less likely to be identified as a defect on a radiograph.

In contrast, discontinuities such as porosity, cracking, and inadequate penetration can often be more readily identified through radiography. Porosity tends to create distinct areas of reduced density on the radiograph, while cracking produces very sharp density changes that make them easier to spot. Inadequate penetration, on the other hand, generally creates a more obvious feature compared to the subtler indications