Notches vs. Side-Drilled Holes Chapter 1 – Echo Dynamics
19th January 2022
President, Holloway NDT & Engineering Inc.
For ultrasonic testing of piping welds, ASME Sec V Art. 4 allows sensitivity calibration on notches using the standard block for piping (Fig. T-434.3-1) or side-drilled holes using the alternate block (Fig. T-434.3-2). They are both acceptable reference targets but behave quite differently due to a couple of super fun reasons.
For a comparison of echo-dynamics, simulation was performed with CIVA on a 20.5 mm curved block using both notch and side-drilled hole reflectors.
Multiple reflections of the hole were made, and intermediate data points interpolated to produce the same sound path as the notch (because you can't drill a hole at the ID, right?).
The chart below shows the results plotted against each other, with the loudest notch position (around 54°) set to 100% full screen height.
Well, would you look at that? Just look at that! Notches are really loud, aren’t they? And check out that dip at 60 degrees! Lets discuss why this happens…
|For one, notches are relatively massive reflectors compared to side drilled holes, with sound reflecting primarily via the corner trap. Imagine playing pool and missing a corner pocket shot. You can miss by a little or a lot and the ball will still come back at you.|
|Side drilled holes are basically cylinders with sound reflecting off only the narrow strip perpendicular to the sound beam. Imagine the line of reflected light you’d see when shining a flashlight on a chromed cylinder rod. Side drilled holes are relatively small reflectors.|
Notches are asymmetric reflectors. That’s a fancy way of saying the profile (what kind of shape the sound beam “sees” as it hits the target) changes depending on what angle you hit it with. And this makes a big difference to how much, and what mode, of sound reflects back to your probe. At a refracted shear wave angle of 45°, the signal is massive as the sound reflects efficiently from the corner trap. With the beam nearing 60°, mode conversion to longitudinal waves occur which reduces the reflected shear waves and produces the dip in the chart above. Beyond 60° it picks back up a little but never fully recovers the oomph it had at lower angles.
Meanwhile, side-drilled holes are symmetric reflectors, producing the same reflection profile at all angles with no mode conversion losses.
So what? What are you trying to get at?
A sensitivity calibration (e.g. DAC/TCG) created with notches will result in generally lower sensitivity than side-drilled holes. They are so loud that the gain must be turned way down in order to set the peak of the notches at 80% FSH. There is a 10 to 12 dB difference in the scenario shown in the chart above.
Caveat: Expect a smaller dB difference on thin wall calibration standards where the 10% notch is (obviously) smaller but the minimum ASME hole size (3/32 in. or 2.38 mm) remains the same.
In addition, a sensitivity calibration made on notches will produce very different amplitudes depending on angle. We already deal with this in UT anyways... the amplitude always changes depending on what angle you hit the flaw with (unless your weld flaw is a perfect cylinder or a sphere, which it probably isn't).