Choosing the right NDT method in welded fabrication ensures that flaws are found reliably while keeping inspection costs under control. In this article we compare the five most common methods.
Surface methods
Visual testing (VT) — EN ISO 17637
The starting point of every inspection. Detects weld geometry issues, undercut, overlap, surface porosity and cracks with the aid of gauges. Cheap and fast, but finds only surface-breaking, visible flaws.
Liquid penetrant (PT) — EN ISO 3452-1
Reveals surface-breaking discontinuities in all non-porous materials (stainless steel, aluminium, copper). Equipment needs are low; surface cleanliness is critical. Cannot detect sub-surface flaws.
Magnetic particle (MT) — EN ISO 17638
Applicable only to ferromagnetic materials; finds surface-breaking and slightly sub-surface (about 1–2 mm) cracks faster and more sensitively than PT. Can be applied through thin coatings on painted surfaces.
Volumetric methods
Ultrasonic testing (UT) — EN ISO 17640
Finds planar flaws inside the weld (lack of fusion, incomplete penetration, cracks) with high sensitivity and determines their depth. No radiation, immediate results, single-side access is sufficient. Drawbacks: difficult below 8 mm thickness, limited permanent record (in conventional UT), requires an experienced operator.
Radiographic testing (RT) — EN ISO 17636
Shows volumetric flaws such as porosity, slag and incomplete penetration on a permanent film/digital record. Common for thin sections and pipelines. Drawbacks: radiation safety and licensing requirements, two-side access, low sensitivity to planar flaws (cracks) depending on beam angle, cost.
Comparison summary
| Method | Flaw type | Material | Access | Permanent record |
|---|---|---|---|---|
| VT | Surface | All | One side | Photo |
| PT | Surface | All non-porous | One side | Photo |
| MT | Surface + sub-surface | Ferromagnetic | One side | Photo |
| UT | Volumetric (planar) | All | One side | Limited / full with PAUT |
| RT | Volumetric (volumetric) | All | Two sides | Film / digital |
Points to consider when selecting a method
- The method and inspection percentage mandated by the product standard (e.g. EN 1090-2 Table 24, EN 13445-5)
- Welding process and expected flaw type: lack of fusion in MAG welding (UT), slag and porosity in SMAW (RT)
- Thickness, geometry and accessibility
- Permanent record requirement and project documentation
- Radiation safety and site conditions
EN ISO 17635 gives the general rules for selecting NDT methods for welded joints and defines the relationship between product standards and acceptance levels. Our team can advise you on the most suitable method and inspection percentage for your project.