Until today, inspectors had no information on the beam coverage produced by multi-element ultrasonic inspection devices offering thetotal focusing method(TFM). They had to rely primarily on their assumptions. Inspectors had to assume that the beam cover was uniform throughout the target area. However, as we well know, presumptions do not guarantee the achievement of precise results.
Knowing in advance the level of coverage provided by each mode of propagation and where the signal sensitivity is best – and worst – gives inspectors a considerable advantage. They can be much more certain of each mode's ability to detect the type of defect they are looking for. And in the field of inspection, increased certainty is very advantageous.
Acoustic region and TFM method
Although the TFM method has been used for decades in the medical industry and for some years in the non-destructive testing industry, inspectors using this method often have to operate on trial and error to achieve the appropriate results. The many propagation mode options (imagery path or sound path) can confuse the user and make inspection results unpredictable.
With a regular TFM system, the user must assume that the region of acoustic influence (or energy) is evenly distributed over the entire area targeted on the inspection plan. In reality, the level of acoustic influence varies in the TFM scanning area, so it is possible that some defects cannot be detected, even with a sufficient signal-to-noise ratio. Many factors can have an effect on the actual acoustic influence, such as the speed of propagation of waves in the material, the frequency of the probe, the orientation of the defect, etc. More importantly, the acoustic influence depends heavily on the mode of inspection chosen.
The challenge of using the TFM method
This method of creating a work area gives the user false expectations. Just like with the conventional PAUT method, the beam focus performed by the software after acquisition is limited by the physics of ultrasound. Some beams simply cannot reach all regions of the area with the assumed focusing power.
For example, in TTT mode, it is not possible to achieve an acoustic sensitivity high enough to detect a reflector located in the upper right corner of the area (see the screenshot below). However, an inspector could easily assume that this section is covered and believe that it is part of the focused area.
AIM Modeling Tool – The red arrow has been added to highlight the lack of amplitude response in the upper right corner of the area for a TT-T propagation mode on a flat defect.
Improve your goal
The Acoustic Influence Modeling (AIM) tool guides the user in selecting the appropriate mode for a given fault type. The tool creates amplitude modeling in the established area directly in the OmniScan™ fault canning device. The modeling displays a color code:
- The red areas indicate that the ultrasonic response is very good and has a deviation between 0 dB and −3 dB from the maximum amplitude.
- The orange zones have a deviation between −3 dB and −6 dB from the maximum amplitude.
- For yellow areas, this is a difference between −6 dB and −9 dB.
- And so on.
Inspectors may choose an omnidirectional reflector (volumetric), such as porosity, or a flatter reflector, such as a crack. When the fault type is selected, the AIM modeling updates, showing the difference in amplitude for a given defect in a given mode.
Acoustic Influence Modeling Tool (AIM) – AIM modeling changes as the reflector angle value changes.
This feature allows inspectors to compare the coverage of each mode and ensure that they will achieve the optimal signal sensitivity for detection in the defined area. Before they even begin the inspection, inspectors can be sure that they are using the appropriate mode for the type of defect they are targeting.