Ultrasonic Testing (UT) is chosen for the purposes of interrogating 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 the product because additional stresses are caused by these impurities. UT operates on the principle of the reflection of sound energy. A probe sends sound into a material as energy vibrations, and it echoes from an interface in its path back to a receiver. This is principle of echo location is observed in dolphins and bats as the means for detecting their prey while hunting.
In ultrasonics we set the equipment up in such a way to ensure that each of the above parameters are accounted for. That 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 show the inspector where the reflector is in the weld relative to 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 it 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, 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 such that sound energy is reflected away, it will be missed or not “seen”. The equipment needs to be calibrated to ensure complete coverage and detection. Conventional UT equipment has a range of probe angles (0°, 45°, 60° and 70°) available that are used to send the sound at a particular angle into the material being tested to allow for reflection of sound energy to the receiver to take place. Applus+ procedural requirements, address these eventualities during the inspection.
Increasingly, there is the desire to achieve more from the inspection than just conventional UT delivers. This is because more information and recording of the information collected is required based on a client’s needs. In this case, 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. Phased Array ultrasonic inspection (PAUT) is therefore selected to reduce those limitations as follows:
PAUT provides a visual map of signals that provide a record of the inspection. For auditing and traceability, it provides a more tangible result to the customer than conventional UT. PAUT is becoming more often selected than conventional UT for these above-mentioned reasons.