Angle-beam weld inspection is done with shear waves. The wedge refracts a compression wave in perspex into a transverse wave in the steel at 45°, 60° or 70°, and the compression component is deliberately left outside the material by keeping the incident angle above the first critical angle. Steel shear velocity is 3240 m/s; the corresponding wavelength is roughly half that of a compression wave at the same frequency, which is why angle-beam probes resolve small planar flaws so well.

Everything that follows is one right-angled triangle. The instrument displays the sound path — the slant distance from the beam index point to the reflector. Projecting that along the surface gives the surface distance, SD = SP × sin θ, and projecting it into the wall gives the depth, d = SP × cos θ. Any one of the three fixes the other two.

Two practical corrections turn the geometry into something you can chalk on the plate. The surface distance is measured from the beam index point, not from the case, so subtract the index-to-front dimension to get the projected distance from the front of the probe. And the actual refracted angle drifts as the wedge wears — verify it on a calibration block rather than trusting the label, because a 2° error at 70° moves the plotted depth of a 25 mm deep indication by about 2.4 mm.

The relationships above hold only for the first leg, before the beam reflects from the far surface. Past half skip the depth folds back and must be handled as a second-leg calculation.

Shear wave

Worked example

Known quantity	sp
Refracted shear angle	60 deg
Sound path	50 mm
Surface distance	43.3 mm
Depth	25 mm
Index point to probe front	15 mm
Wall thickness	30 mm
Sound path	50 mm
Surface distance from index point	43.3 mm
Depth below scanning surface	25 mm
Projected distance from probe front	28.3 mm

A 60° probe showing a 50 mm sound path: surface distance = 50 × sin 60° = 50 × 0.866025 = 43.30 mm, depth = 50 × cos 60° = 50 × 0.5 = 25.00 mm. With the beam index 15 mm behind the probe front, the reflector lies 43.30 − 15 = 28.30 mm ahead of the case.

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