An echo only shows up when sound comes back to the probe. Hunt for a root crack with a straight beam and then with a 45° wedge, move the crack to the middle of the plate, then tilt it and see how quickly the echo disappears when the flaw stops facing the beam.
Drag the probe along the plate. The probe is the same throughout; the wedge is what angles the sound.
Key points
- An echo needs sound to come back to the probe. A flaw only sends sound back when it faces the beam.
- A crack standing upright, like a root crack, lies along a straight beam. The sound slides past it, so the straight beam only shows the back wall.
- With a 45° wedge, the crack face and the back wall make a corner. A corner sends the sound straight back: a corner trap. That's why angle beams find root cracks.
- Move the same upright crack up into the middle of the plate and the corner is gone. The sound bounces off it to the side, so there's no echo, even though the beam hits it.
- A flat flaw gives the biggest echo when the beam meets it at 90° (square-on). Tilt it just 10 to 15° away and the echo nearly disappears, because the sound bounces off to the side and misses the probe.
- In weld inspection, this is why you pick the wedge to suit the weld bevel: so the beam faces the fusion face.