Weld Defects & Discontinuities, and the NDT Method That Catches Each

Surface vs sub-surface vs volumetric

Weld discontinuities split into three families by where they live, and that location dictates the method. Surface-breaking flaws — surface cracks, undercut, overlap, crater cracks — are caught by visual (VT), magnetic particle (MT) on steel, and penetrant (PT). Sub-surface planar flaws — lack of fusion, incomplete penetration, underbead and toe cracks — are the strength of ultrasonics (UT, PAUT) and TOFD. Volumetric flaws — porosity, slag, tungsten inclusions — are classically radiography's domain.

The trap is orientation. Radiography images density change, so it sees volumetric flaws and cracks that open along the beam, but it can miss a tight planar crack lying across the film. Ultrasound reflects off planar flaws near-normal to the beam, so it excels at lack of fusion and cracks but needs scanning technique to catch favourably-oriented porosity. This is exactly why critical welds are often inspected by both a volumetric method and a surface method.

The discontinuity-to-method map

Use this as a first pass; the governing code and joint geometry refine it.

Sizing, not just detection

Detection answers "is there a flaw"; engineering-critical assessment needs "how big". Through-wall height drives fitness-for-service decisions under API 579 / BS 7910, and that is where TOFD and PAUT pull ahead of film radiography. TOFD measures the diffracted signal from a flaw's tips to size through-wall extent within roughly ±1 mm; PAUT sectorial scans map flaw position and height across the weld volume. If the acceptance route is ECA rather than workmanship, plan for a sizing-capable method from the start.

Acceptance is set by the code

Detecting a discontinuity is not the same as rejecting the weld — the code decides. AWS D1.1 distinguishes acceptable discontinuities from rejectable defects by size, type and loading (statically vs cyclically loaded). ASME Section VIII and B31.3 set their own acceptance for pressure work. A 2 mm pore may be perfectly acceptable in one code and rejectable in another, so the inspector reads the indication and then reads the acceptance clause before writing the report.