Attenuating materials and TGC function

Carbon-fibres like other composite materials are quickly replacing many metallic-based materials in industry due to their mechanical properties, strength-to-weight ratio, and resistance to fracture. These materials behaviour greatly differs from that of conventional isotropic structural materials due to interfacial characteristics of plies and matrix material, inclusive the presence of porosity inside. These complex material properties can cause interferences by structural noise and/or attenuation on ultrasonic signals, which in both cases, reduces the image quality.

Noise reduction on ultrasonic images

Electronic equipments, cables and transducers used in NDT are interfered by noise which severely limits the ability to detect defects. Noise has different origin and nature, i.e., white noise which is generated by several sources; thermal originated from electronics components, impulsive noise produced by motors and/or switchers, or power supplies, etc. Sometimes the inspection process itself can be a source of noise due to residual echoes generated in a previous shot. It is a common situation where the inspected material has a low attenuation coefficient and the pulse repetition rate is high (reverberations). Therefore, when the interference is broadband, it occupies the same bandwidth as the ultrasonic signal, so its effects can be harmful when signals are weak to detect. As a result, conventional filtering techniques should be avoided and nonlinear methods which respect the integrity of the real signals must be applied.

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.

Phased Array Principles of Operation

Phased arrays technologies are used in widely different fields like radar, radiotelescope, sonar, also in NDT applications.

The phased array technique controls the delays applied to the excitation of each element in order to emit an ultrasonic pulse which propagates in a certain direction focused at specific depth.