A weld can contain geometric or material discontinuities even when its external appearance seems acceptable.

Common examples include:

• lack of fusion, where intended surfaces were not metallurgically joined;
• incomplete penetration, where the joint root was not fully fused;
• porosity from trapped gas;
• slag or nonmetallic inclusions;
• undercut or excessive reinforcement at the surface;
• cracks produced during solidification, cooling or later service.

A discontinuity is not automatically a rejectable defect. Acceptance depends on its type, size, location and orientation together with the service requirement defined by the applicable engineering specification.

For example, a small rounded pore and a sharp planar crack of the same overall size do not create the same mechanical concern. The crack produces a much stronger local stress concentration and can be more critical under fatigue or brittle-fracture conditions.

Inspection planning therefore begins from the discontinuities and failure mechanisms that matter. A suitable nondestructive-testing method is then chosen with enough sensitivity and access to examine the relevant region.

The manufacturing objective is not to claim that a weld contains no imperfection. It is to demonstrate, using a defined inspection and acceptance process, that observed or potentially detectable discontinuities remain within limits appropriate to the component's function.