Imaging instead of X-ray testing: New strategies for non-destructive weld evaluation

Conventional radiographic testing (RT) has been a reference method in industrial weld evaluation for decades – particularly for detecting volumetric defects. At the same time, it has limitations in areas such as radiation protection and flexibility. A feasibility study shows that imaging methods such as phased array ultrasonic testing (PAUT) can close this gap even under demanding conditions.

A complex starting point: Challenging materials and geometries

On behalf of an industrial customer, the testing team developed tailored PAUT scanning strategies for examining various welded sheets and comparing the results with previous RT results. The challenges were considerable. The team tested several welded sheets, some very thin, from different material groups: ferritic steels, austenitic steels and aluminium.

“Each of these materials has its own challenging characteristics,” explains Dr Ingo Poschmann, Managing Director of W.S. Werkstoff Service GmbH. “Ferritic sheets just a few millimetres thick leave little room for manoeuvre in testing. Austenitic steels are known for their coarse grain structure and pronounced anisotropy, which significantly affect the ultrasonic signal. And aluminium welds? They typically contain numerous pores, making them a real test of any method.”

PAUT as a foundation – Tailored scanning strategies required

In phased array ultrasonic testing (PAUT)(opens in new tab) sound fields are controlled electronically using specific probes. Activating individual elements with time delays allows the sound beams to be steered, focused and introduced into the component at different angles. This enables targeted, reproducible scanning of volumetric regions.

However, it soon became clear that standard PAUT configurations were insufficient for these requirements. The testing team therefore developed material- and geometry-specific scanning strategies. They also used two probes simultaneously, one on each side of the weld (the dual-probe technique), multi-group scanning, flowing-water coupling and specially developed guides.

Dual-probe technique

A key innovation was the simultaneous use of two probes on opposite sides of the weld. This arrangement improves analysis through the probes’ combined defect localisation, reduces shadowing effects and increases the likelihood of detecting unfavourably oriented defects. Particularly in thin sheets, this method significantly improves the information obtained, as reflections are recorded from different directions.

Multi-group technique

Using several testing groups within a single scan allows different angular ranges and focal configurations to be covered in parallel. The result is higher testing speed, more complete volumetric coverage and redundant data acquisition to verify critical indications.

Flowing-water coupling and guides

Stable flowing-water coupling was implemented to achieve reproducible results. Combined with specially developed guides, this ensured consistent testing conditions, improved positioning accuracy and reduced operator influence.

Innovative techniques for demanding materials

To address the new challenges, the testing team used the Total Focusing Method and Phase Coherence Imaging. These advanced imaging methods offer significantly higher image resolution and enable defect characterisation that is not possible with conventional ultrasonic testing(opens in new tab).

Total Focusing Method (TFM)

The Total Focusing Method can capture the entire test volume simultaneously by recording and storing different transmitter–receiver combinations within an array. A high-resolution image is then calculated from this complete dataset, with every point in the test volume focused computationally. The result is significantly higher image resolution than conventional PAUT, more precise characterisation of defects in terms of shape, size and location, and substantially less dependence on the selected direction of sound propagation.

Phase Coherence Imaging (PCI)

Phase Coherence Imaging uses the phase information in ultrasonic waves, unlike TFM, which uses amplitude information. PCI has advantages over TFM in detecting certain types of defects and therefore complements it extremely well.

“We did not simply transfer existing methods; we adapted them specifically to the individual materials and geometries,” says Ingo Poschmann. “That was crucial to the quality of the results.”

PAUT can replace RT – With the right strategy

A systematic comparison with RT images showed that, with an appropriate choice of testing parameters and techniques, all relevant indications can be detected reliably – in many cases with additional information for defect characterisation. Even challenging materials and thin sheets can be tested effectively, an area previously considered difficult. “The key finding was that there is no single testing strategy. Every material and every geometry requires its own approach,” Poschmann emphasises.

Conclusion: A study with a lasting impact

The feasibility study opens up new approaches to non-destructive weld evaluation. It shows that PAUT combined with TFM and PCI is a powerful ultrasonic method offering genuine advantages over conventional radiographic techniques. “Those who master these technologies not only test more safely and efficiently, but can also pass on this knowledge in a targeted way. That is precisely our ambition,” Poschmann emphasises.

The methods developed in the project provide a lasting expansion of the company’s service portfolio. In particular, the combination of dual-probe technology, multi-group scans and modern imaging methods enables new approaches to the non-destructive testing of complex welded joints. “Our customers benefit from more reliable detection in complex welds, improved defect evaluation through imaging methods, and more efficient testing processes without additional radiation protection requirements,” explains the Managing Director.

The knowledge gained in the project is also being incorporated into new training modules. Plans include practical courses on dual-probe and multi-group techniques, advanced training in TFM and PCI, and specialised modules for demanding materials and thin components.