Ultrasonic Testing vs Phased Array: choosing the right method for weld inspection

Ultrasonic Testing (UT) operates on the principle of sound-wave reflection. A probe transmits high-frequency sound waves into a material, and when these waves encounter an interface—such as a flaw or boundary—they are reflected back to a receiver. This principle is similar to echolocation used by dolphins and bats to locate objects.

UT is widely used to inspect welds through their thickness, enabling the detection of defects such as porosity (gas pores), lack of fusion, cracking, and material loss due to erosion or corrosion.

For effective inspection, UT equipment must be carefully calibrated to ensure that reflectors (defects) produce detectable echoes on the display. The setup also allows the inspector to determine the position of a reflector relative to the sound path.

A key requirement for detection is the presence of a defined interface from which sound can reflect. If a defect absorbs sound energy rather than reflecting it, it may go undetected. Similarly, the size and orientation of a reflector are critical:

• Smaller defects are more difficult to detect and require higher sensitivity and proper calibration.
• Defects oriented away from the sound beam may not reflect energy back to the probe, making them harder to identify.
   

To address these challenges, conventional UT uses probes with fixed angles (commonly 0°, 45°, 60°, and 70°). Each probe angle targets specific types of defects, but multiple scans and probe changes are often required to achieve full coverage. Inspection procedures, such as those defined by Applus+, are designed to account for these variables and ensure reliable results.

However, increasing client demands for improved data quality, inspection efficiency, and traceability have driven the adoption of more advanced techniques such as Phased Array Ultrasonic Testing (PAUT).

PAUT builds on the same fundamental principles as conventional UT but offers enhanced capabilities through electronic control of multiple ultrasonic elements. This allows for more flexible and efficient inspections.

Compared to conventional UT, PAUT offers several advantages:

• Permanent record: PAUT systems can record encoded scan data, providing a permanent and auditable record of the inspection.
• Multiple angles in one pass: Instead of using a single fixed-angle probe, PAUT can sweep through a range of angles (typically 35°–75°) in a single setup.
• Reduced inspection time: By covering multiple angles simultaneously, PAUT minimizes the need for repeated scans and probe changes.
• Improved coverage: A range of angles increases the likelihood of detecting defects regardless of their orientation, often in a single scan line.
   

Additionally, PAUT generates visual representations of the inspected area, offering a more intuitive and traceable result compared to conventional UT. This makes it particularly valuable for quality assurance, auditing, and client reporting.

For these reasons, PAUT is increasingly being selected over conventional UT as the preferred method for weld inspection.
 

By Grant Meredith