Ultrasonic Testing (UT) is chosen to interrogate a weld volume through its thickness to detect gas pores (porosity), lack of fusion or racking, and metal loss through erosion or corrosion. 

Each of these types of “defects” could have a detrimental effect on a product because additional stresses are caused by such impurities. UT operates on the principle of sound-energy reflection. A probe sends sound into a material as energy vibrations, and it echoes from an interface in its path back to a receiver. This principle of echo location is observed in dolphins and bats as the means for detecting their prey while hunting.

In ultrasonics, we set up the equipment in such a way to ensure that each of the above parameters are accounted for. This means the equipment must be set up to allow the reflector to produce an echo on the display screen. The way the equipment is set up will also indicate to the inspector where the reflector is in the weld relative to the sound-path distance.

Is there a defined interface that sound can echo from? This means that the target reflector needs to have a surface that echoes the signal to the receiver. If the surface is absorbent, sound energy is lost into the material and the defect will be missed. The size of the reflector plays a key role in the UT inspection, and the equipment is calibrated to detect the smallest reflector during the inspection. Additional measures need to be put in place to improve the detection of smaller “defects”. If a reflector is oriented in a way that sound energy is reflected away, it will be missed or not “seen”. The equipment must be calibrated to ensure complete coverage and detection. Conventional UT equipment has a range of probe angles available (0°, 45°, 60° and 70°), which are used to send the sound at a particular angle into the material being tested. This allows for reflection of sound energy to the receiver to take place. The procedural requirements set by Applus+ address these eventualities during the inspection.

Increasingly, there is the desire to achieve more from the inspection than that delivered by just conventional UT. This is because, based on a client’s requirements, more information and recording of the information collected is required. In such cases, Phased Array will be selected.

Phased Array UT (PAUT) uses the basic principles of ultrasound, but there is an ability to improve the results from a given inspection. Conventional UT has certain inspection limitations as discussed previously. PAUT is therefore selected to reduce those limitations in the following ways:

  • Permanent record. PAUT allows for encoded or recorded scans in order to keep a permanent record of the inspection.
  • Selection of inspection angles. PAUT allows the equipment to set up a range of angles (35° – 75°) in one single inspection, where as conventional UT uses one angle probe per inspection step.
  • Inspection time. PAUT uses an inspection setup, using a full range of angles (35° – 75°) to perform in one pass, where as conventional UT uses one angle probe per inspection step, which is very time consuming.
  • Inspection coverage. When performing a PAUT inspection, the range of incorporated angles (35° – 75°) are used in one inspection. This allows for a defect to be detected by the optimum angle using a single line scan. Conventional UT requires extensive scanning patterns and multiple probe selections to gather the same information.

PAUT provides a visual map of signals that provide a record of the inspection. For auditing and traceability, PAUT provides a more tangible result to the client than conventional UT. PAUT is becoming more often selected than conventional UT for these above-mentioned reasons.