Ultrasonic NDT
Selecting the Best Propagation Mode for a Reflector Using the AIM Modeling Tool for TFM (Total Focusing Method) Inspection
Published on 31st March 2020
The total focusing method (TFM) has generated a lot of excitement in the nondestructive testing (NDT) field. But there are challenges that had yet to be resolved when using TFM, such as choosing the right mode of propagation (wave set) for a given inspection.
Challenges of Choosing the Proper Settings Using TFM
When selecting a mode of propagation (wave set) for a given inspection, the inspector needs to know what kind of defects may occur in the part to be inspected. The type of defect will give some information on the orientation of the reflector, which is critical when inspecting with ultrasound testing (UT). With conventional UT, phased array UT, or TFM, the basic principle remains the same. To have a good probability of detection (POD), the sound waves need to have as much perpendicularity as possible with the reflector. Another consideration is the probe parameters. Depending on the probe used, the energy may not be capable of reaching the targeted defect. Even though the TFM zone is drawn at a particular location, it is possible that the physics will not allow for this specific probe to focus that far into the part. There are so many factors to keep in mind, so how can we simplify and ensure that our inspection is adequate?
Same Probe Location
Different Modes
TT
TTT
LLL
Figure 1- Different modes used to try to image a series of SDHs.
Solution Using the Acoustic Influence Map Modeling Tool
The OmniScan® X3 phased array flaw detector comes with a built-in scan plan tool. Within it is an Acoustic Influence Map (AIM) modeling tool that was specifically designed for TFM inspection. The AIM tool helps users select the right mode of propagation, or wave set, for their inspection.
Figure 2- OmniScan X3 scan plan in TFM showing the Acoustic Influence Map (AIM)
The AIM modeling tool considers multiple parameters, including the probe and wedge, velocity, thickness, geometry of the specimen, inspection technique, wave sets, and, of course, the parameters entered by the inspector in the “Influence zone” menu to describe the targeted type of defect.
A flaw’s orientation is the principal factor impacting how well a sound beam will be able to detect it. The AIM model clearly demonstrates for the user how good the signal coverage is at a particular angle for a given flaw.
Using the AIM Modeling Tool to Determine the Best Propagation Mode
The user draws the desired zone of interest and then enters the expected orientation (in degrees) of the flaw or selects “omnidirectional” for flaws that are not at an angle, such as porosity or other volumetric types of defects.
A color palette clearly identifies the sensitivity performance for each part of the zone of influence. Each color covers a three-decibel range, indicating the ultrasonic response with respect to the maximal amplitude:
Figure 2- Three scan plan screenshots showing changes in the AIM as the orientation of the flaw is adjusted between −5, −15, and −25 degrees
Summary of the AIM Modeling Tool’s Advantages for TFM
TFM offers promising opportunities for industrial inspection applications, but without the proper modeling tool, it is difficult to predict the true sound wave coverage and level of sensitivity. The OmniScan X3 flaw detector’s scan plan tool with AIM modeling enables the inspector to confirm, with confidence, which TFM mode is appropriate for the inspection.
For more information on the benefits of TFM for phased array ultrasonic inspection, read our application note “Using the Total Focusing Method to Improve Phased Array Ultrasonic Imaging.”
The OmniScan X3 flaw detector is a complete phased array toolbox. Innovative TFM delivers outstanding images that help inspectors identify flaws with confidence while powerful software features and simple workflows help you get to work fast. 
