In NDT, the right question is not simply “radiography or ultrasound?”. The right question is: which method best addresses the flaw to be detected, in this component, under these production and code constraints?
Industrial radiography uses ionising radiation to obtain an internal image of a component or weld. Ultrasonic testing uses high-frequency sound waves to detect discontinuities through the acoustic response of the material. Both methods can be highly effective, but they observe the part in different ways.
When industrial radiography is often preferred
Radiography is particularly relevant when the expected flaw is volumetric and when a visual, archivable record is required.
- porosity, blowholes, cavities and inclusions;
- internal discontinuities with density differences;
- castings, complex shapes or geometries where ultrasonic interpretation is difficult;
- visual evidence for audits, final customers or traceability;
- procedures or codes requiring RT, CR or DR.
When UT, PAUT or TOFD are often preferred
Ultrasonic methods are often more suitable when the objective is detecting and sizing planar discontinuities, especially when the flaw orientation is favourable to the ultrasonic beam.
- cracks, lack of fusion, laminations and oriented defects;
- need to estimate depth, position, height or extension;
- single-side access;
- situations where radiation safety constraints would be unacceptable;
- repetitive weld inspection with documented PAUT/TOFD scans.
Practical comparison table
| Scenario | RT / CR / DR | UT / PAUT / TOFD |
|---|---|---|
| Volumetric flaws | Very suitable | Possible, less intuitive |
| Planar flaws | May be affected by orientation | Often very suitable |
| Archivable image | Strong | Depends on technique and system |
| Depth and sizing | Limited | Strong |
| Single-side access | Often critical | Often advantageous |
| Area safety | Requires radiation management | No ionising radiation |
Limits to consider
Radiography does not “see everything”: it can be less sensitive to unfavourably oriented planar flaws and requires radiation safety management, correct source, energy, exposure geometry and detector selection. Digital radiography improves workflow and archiving but does not remove radiation protection requirements.
Ultrasonic testing depends on coupling, calibration, surface condition, geometry, material and operator competence. PAUT and TOFD improve documentation and analysis, but require qualified procedures and competent interpretation.
Technical checklist
- material, thickness and geometry;
- joint or component type;
- expected flaw and acceptance criterion;
- accessibility and safety constraints;
- required productivity;
- applicable code, standard or procedure.
Radiography and ultrasound are often complementary. The correct choice comes from a qualified procedure and application analysis, not from the method alone.