Dynamic Focusing

Ultrasound inspection of complex geometry and/or difficult access components can be performed with phased-array systems, due to their ability of electronically modifying the ultrasonic beam properties. The inspection of bolts and threaded rods is a clear example of this problem. So, this post shows you how powerful can be the use of dynamic focusing in these cases.

A SITAU PA system of 32 channels multiplexed to 128 array elements was used for testing a brake disc fixation bolt integrity, used at high speed train wheels. This equipment has a dynamic focusing algorithm hardware implemented, which guarantees real time operation at high frame rates.

The phased array transducer probe of 32 elements, inter-element pitch of 0.5mm and 5 MHz was in contact with the flat surface of the specimen.

Inspection scheme

Sectorial scans (B-scan) were performed into an angular range between -50º and 50º. In a first approach, a single focus at 20 mm is applied, so a quite short deep of field (DOF) is obtained, which prevents to evaluate the whole component with a single image. As DOF depends on focus range, and because thread defects can be present at any depth, this component cannot be correctly inspected with a single fixed focus.

Dynamic focusing technique overcomes this problem, because it maintains a constant angular resolution with the best possible lateral resolution at any inspection depth, which guaranties defects detection at all ranges. As a result, geometric aberrations and internal cracks can be evaluated with this technique.

Fixed focus at 20mm

Dynamic focusing

Electronic scan allows evaluating the whole component with a single array transducer. Moreover, inspection can be carried without un-mounting the brake disk, which greatly simplifies in-service operation. Dynamic focusing can be appropriated to verify component integrity during the manufacturing process. Using a rotary inspection head, the whole bolt volume could be evaluated with an angular resolution of 1º in just 15 sec. This way, component quality can be guaranteed when it leaves the production line. The same inspection principle could be applied to steel rods, billets and pipes, with or without threads.