Non-Destructive Testing (NDT) is the method of inspecting the material integrity of welded structures, pressure equipment and load-bearing constructions without destroying the part. EN ISO 9712 is the international reference document defining certification requirements for NDT personnel.
The choice of NDT method in welded fabrication is determined by material type, part geometry, expected defect type and the applicable standard.
Five Fundamental NDT Methods
| Method | Full Name | Detects | Limitations |
|---|---|---|---|
| VT | Visual Testing | Surface cracks, porosity, excess reinforcement, weld geometry defects | Surface only; internal defects not visible |
| PT | Penetrant Testing | Surface-breaking cracks, porosity, laminations | Open-to-surface defects only; cannot be used on porous materials |
| MT | Magnetic Particle Testing | Surface and near-surface (≤3 mm) cracks, laps | Ferromagnetic materials only; does not work on aluminium or stainless steel |
| UT | Ultrasonic Testing | Internal cracks, fusion defects, laminations, voids | Deep internal defects in thick materials; operator-dependent interpretation |
| RT | Radiographic Testing | Internal porosity, slag inclusions, undercut, lack of fusion | Radiation licence required; crack orientation critical; access from both sides |
VT — Visual Testing
VT (Visual Testing) is the first step in all NDT inspections and is performed according to EN ISO 17637. Weld geometry, surface roughness, spatter, discolouration and visually apparent cracks are assessed.
VT must always be performed before other NDT methods. A weld that would be rejected visually is not submitted to PT or UT — saving both time and cost.
EN ISO 5817 Acceptance Criteria: Acceptable defect sizes are defined according to weld quality level B, C or D. Quality level B is generally applicable under EN 1090 EXC3.
PT — Penetrant Testing
PT (Penetrant Testing) is applied according to EN ISO 3452-1. Fluorescent or coloured (red-white) penetrant liquids are used to detect surface-breaking discontinuities.
Application steps:
- Surface cleaning (oil, rust and paint removed)
- Penetrant application and dwell time (typically 10–30 min)
- Removal of excess penetrant
- Developer application
- Visual assessment (UV or white light)
PT is the preferred surface inspection method for aluminium structures where MT cannot be applied.
MT — Magnetic Particle Testing
MT (Magnetic Particle Testing) is performed according to EN ISO 17638. In ferromagnetic materials (carbon steel, low-alloy steel), a magnetic field is generated and iron powder or fluorescent particles concentrate over defects.
Advantage: Near-surface cracks (≤3 mm depth) are detected far more reliably than with PT. MT is frequently required for T-joints and fillet welds under EN 1090 EXC3.
Note: MT cannot be applied to austenitic stainless steel or aluminium. PT is used for surface inspection of these materials.
UT — Ultrasonic Testing
UT (Ultrasonic Testing) is performed according to EN ISO 11666. High-frequency sound waves are transmitted through the material; reflections caused by internal defects are evaluated to determine defect location and size.
Modern TOFD (Time-of-Flight Diffraction) and Phased Array UT (PAUT) techniques offer significantly higher reliability and recordability than conventional UT. Particularly preferred for pressure vessel and pipeline welds.
RT — Radiographic Testing
RT (Radiographic Testing) is performed according to EN ISO 17636-1. X-rays or gamma rays are passed through the weld cross-section to form an image on film or a digital detector.
RT shows volumetric defects such as porosity and slag inclusions very clearly, but is less reliable than UT for detecting planar cracks — because a crack may not be detected if its orientation is parallel to the beam direction.
EN ISO 9712 Certification Levels
NDT personnel are certified at three levels according to EN ISO 9712:
- Level 1: Performs testing according to a defined procedure; does not report independently. Works under the supervision of experienced personnel.
- Level 2: Prepares procedures, performs testing, evaluates results to the standard and reports. The industry standard for practicing inspectors.
- Level 3: Establishes the NDT system, approves procedures and technical instructions, examines Level 1 and 2 personnel. Engineering-level authority.
At ArmaWeld, VT, PT, MT and UT inspections are carried out by our in-house EN ISO 9712 Level 2 certified personnel. RT services are provided through our accredited laboratory partner.
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Contact our technical team to determine which NDT method your project requires.
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