Non-Destructive Testing (NDT) — Complete Guide
Introduction
Non-Destructive Testing (NDT) refers to inspection techniques that evaluate the integrity of welded joints and materials without damaging or destroying the component. NDT is mandatory in qualified welded construction (EN 1090, EN 13480, ASME).
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The 5 Main Methods
1. Visual Testing (VT)
| Feature | Detail |
|---|---|
| Standard | ISO 17637 |
| Detects | Surface defects: undercut, cracks, open porosity, misalignment |
| Equipment | Naked eye, magnifier, weld gauges, mirrors, torches |
| Advantages | Fast, cheap, no special equipment |
| Limitations | Surface defects only |
Mandatory on 100% of joints in every construction code.
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2. Dye Penetrant Testing (PT)
| Feature | Detail |
|---|---|
| Standard | ISO 3452-1 |
| Detects | Surface-breaking cracks, porosity and lack of fusion |
| Principle | Penetrant enters discontinuities by capillary action. A developer draws it out, making defects visible |
| Steps | Clean → Apply penetrant → Dwell time (10-30 min) → Remove excess → Apply developer → Inspect |
| Advantages | Simple, cheap, works on any non-porous material |
| Limitations | Surface-breaking defects only |
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3. Magnetic Particle Testing (MT)
| Feature | Detail |
|---|---|
| Standard | ISO 17638 |
| Detects | Surface and near-surface cracks (up to ~2 mm depth) |
| Principle | The part is magnetised. Discontinuities create leakage fields that attract magnetic particles, forming visible indications |
| Advantages | Fast, detects sub-surface defects |
| Limitations | Ferromagnetic materials only (not austenitic stainless, aluminium, copper) |
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4. Ultrasonic Testing (UT)
| Feature | Detail |
|---|---|
| Standard | ISO 17640 |
| Detects | Internal cracks, lack of fusion, lack of penetration, inclusions and delaminations |
| Principle | Ultrasonic waves reflect off discontinuities. The echo shows defect position and size |
| Advantages | Detects internal defects, portable, no radiation, immediate results |
| Limitations | Requires smooth surface, difficult on thin sections (<8 mm), highly skilled operator |
| Advanced techniques | TOFD (Time of Flight Diffraction), Phased Array |
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5. Radiographic Testing (RT)
| Feature | Detail |
|---|---|
| Standard | ISO 17636 |
| Detects | Porosity, slag inclusions, lack of fusion and penetration |
| Principle | Radiation (X-rays or gamma) passes through the part and is recorded on film or digital detector |
| Advantages | Permanent record, detects volumetric defects |
| Limitations | Ionising radiation (safety!), expensive, slow, exclusion zone required |
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Comparison
| Method | Surface | Sub-surface | Internal | All materials | Cost |
|---|---|---|---|---|---|
| VT | ✓ | — | — | ✓ | Low |
| PT | ✓ | — | — | ✓ (non-porous) | Low |
| MT | ✓ | ✓ | — | Ferromagnetic | Medium |
| UT | — | ✓ | ✓ | ✓ | Medium |
| RT | — | — | ✓ | ✓ | High |
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Inspector Qualification — ISO 9712
| Level | Competence |
|---|---|
| 1 | Performs under supervision, does not interpret |
| 2 | Performs, interprets, issues reports |
| 3 | Validates procedures, technical authority |
Each method requires separate qualification. Validity: 5 years.
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*Article based on ISO standards and AWS Welding Handbook training materials. Published on WeldFC Academy.*