There are two main methods for finding internal defects in welded joints: ultrasonic testing (UT) and radiographic testing (RT). Both “see” inside the weld, but they work on very different physical principles, so each is better at catching certain defects and suits certain applications. This article compares the two on defect type, thickness, access, safety and records, and summarises when each one is the better choice.
How the Two Methods Work
In radiographic testing, an X-ray tube or gamma source is placed on one side of the weld and a film or digital detector on the other. As the radiation passes through the material, less of it is absorbed where there is a defect, leaving a dark mark on the image. The result is a two-dimensional “X-ray” image of the inside of the weld.
In ultrasonic testing, a probe sends high-frequency sound waves into the material through a couplant. When the sound hits a defect it is reflected. The arrival time and strength of the echo are evaluated on the instrument screen to determine the position, depth and approximate size of the defect.
Comparison Table
| Criterion | Ultrasonic (UT) | Radiographic (RT) |
|---|---|---|
| Best at finding | Planar defects: cracks, lack of fusion, incomplete penetration | Volumetric defects: porosity, slag, tungsten inclusions |
| Access | One side is enough | Both sides of the weld must be accessible |
| Thickness | Strong in thick sections; EN ISO 17640 generally covers 8 mm and above | Strong in thin and medium thicknesses; exposure time grows with thickness |
| Depth of defect | Provided | Not provided (the image is two-dimensional) |
| Permanent record | No image with conventional UT; results go in the report | Film or digital image is archived |
| Safety | No radiation; other work can continue alongside | A radiation safety zone is set up; no other work in the area |
| Material | Harder in coarse-grained materials (austenitic stainless welds, castings) | Less affected by material structure |
| Joint type | Butt, T and fillet joints | Best for butt welds; difficult on T-joints |
| Result | Immediately, during the inspection | After film processing or digital processing |
Defect Type: The First Question
The defect you need to find is the first question in choosing a method.
- Planar defects such as cracks and lack of fusion are the most dangerous, because they can grow under load. They are thin and flat, and unless they lie parallel to the beam they are hard to see on a radiograph. Ultrasonic testing detects them more reliably because sound reflects strongly from these surfaces.
- Volumetric defects such as porosity and slag inclusions show up clearly on a radiograph and are easy to interpret. In ultrasonic testing they can give weaker, more scattered signals.
If you want to see the overall quality of a welder’s or a welding procedure’s work, the radiographic image is very informative. If you are looking for cracks that are critical to structural safety, ultrasonic testing comes first.
Thickness and Geometry
On thick plates and large-diameter, thick-walled pipes, ultrasonic testing is usually more practical. In radiography, greater thickness means a stronger source and longer exposure. EN ISO 17640, which covers ultrasonic testing of welds, generally applies to material 8 mm thick and above. That is why radiography is often the better option for thin-walled pipes and sheet.
Geometry matters too. Radiography is ideal for butt welds. On T-joints and fillet welds, the beam path and the placement of the film become a problem, whereas ultrasonic testing can be applied with angle probes.
Access, Safety and Site Conditions
For radiography, a source must be placed on one side of the weld and a film on the other. That is not always possible, for example on the shell of a closed tank or on a structural member that can only be reached from one side. Ultrasonic testing can be done from one side.
For site planning, the biggest difference is radiation safety. During radiography a safety zone is set up around the work and no other work can take place in it, so exposures are often scheduled between shifts or at night. Ultrasonic testing has no such restriction and can run alongside other work on site.
Records and Traceability
A major advantage of radiography is the permanent record. The film or digital image is archived and can be re-evaluated by another specialist if needed. With conventional ultrasonic testing, the result depends on the inspector’s evaluation and is written into the report. This is why personnel competence, certification to ISO 9712 and equipment calibration matter especially for UT.
Advanced ultrasonic techniques such as phased array (PAUT) and time-of-flight diffraction (TOFD) record the inspection data as images. Codes such as ASME and API increasingly allow these techniques to be used instead of radiography under certain conditions.
Which One When?
Ultrasonic testing is usually preferred:
- On thick-section welds and large-diameter pipes
- When looking for cracks and lack of fusion
- When there is access from one side only
- On T-joints and fillet welds
- When other work on site cannot be stopped
- For wall thickness measurement and corrosion monitoring of equipment in service
Radiographic testing is usually preferred:
- On thin and medium-thickness butt welds
- On small-diameter pipes
- When volumetric defects such as porosity and slag matter
- On projects that require a permanent image record
- In welder and welding procedure qualification
Many projects use both methods. On pipelines, for example, some welds are radiographed while thick-section connections are tested ultrasonically. For surface defects, these methods are complemented by magnetic particle or liquid penetrant testing.
The Specification Decides
The final word on method selection belongs to the project specification and the applicable standard. General rules for non-destructive testing of fusion welds are given in EN ISO 17635. The main standards used in practice are:
- Ultrasonic: EN ISO 17640 (testing techniques), EN ISO 11666 (acceptance levels)
- Radiographic: EN ISO 17636-1 (film) and EN ISO 17636-2 (digital), EN ISO 10675-1 (acceptance levels for steel)
- Pressure equipment and piping: ASME Section V together with the relevant ASME and API codes
If the specification does not set the method, the material, thickness, joint type, access and the defect being sought have to be assessed together.
Frequently Asked Questions
Can ultrasonic testing replace radiography?
Some codes and specifications allow it under certain conditions, especially with advanced techniques that produce a record, such as PAUT and TOFD. But the decision belongs to the project specification and the applicable standard. If the client or the approval body requires radiography, ultrasonic testing alone does not replace it.
Which method is more reliable?
There is no single “more reliable” method. Ultrasonic testing is stronger on planar defects such as cracks, radiographic testing on volumetric defects such as porosity and slag. That is why both are used together on critical welds.
Does work have to stop during radiography?
Yes. No work may take place inside the radiation safety zone during exposures, so radiography is usually scheduled between shifts, at night or during maintenance shutdowns. Ultrasonic testing has no such restriction.
Can thin sheet be tested ultrasonically?
EN ISO 17640, which covers ultrasonic testing of welds, generally applies to thicknesses of 8 mm and above. For thinner material, radiography or surface methods are usually more suitable.
Conclusion
Ultrasonic and radiographic testing are not rivals but complements. The right choice depends on the defect being sought, thickness, joint type, access and working conditions on site. At FKT NDT we agree the right method for your welds based on your specification and carry out the inspection on site. Browse our non-destructive testing services or contact us about your project.