A shear beam launched at angle θ travels in straight legs, reflecting off the far surface. Half a skip is the surface distance covered in reaching the far wall, t·tan θ; a full skip is twice that. Any reflector at depth d is reached in leg 1 with the exit point d·tan θ ahead of it, or in leg 2 with the exit point (2t − d)·tan θ ahead of it.

The sound path follows the same geometry: d / cos θ in leg 1 and (2t − d)/cos θ in leg 2. Converting to time with the shear velocity of 3240 m/s in steel gives the position the indication should appear at in the A-scan, which is the check that an indication is where the plot says it is.

Leg 2 costs sound path, and therefore attenuation and beam spread, and it depends on the far surface being smooth enough to reflect specularly. On a corroded or rippled ID the leg 2 signal can be lost entirely, which is why root inspection is done in leg 1 wherever the geometry allows.

Shear wave

Worked example

Wall thickness	25 mm
Refracted shear angle	60 deg
Depth of the target reflector	20 mm
Exit point to wedge front face	15 mm
Material velocity	3240 m/s
Half skip	43.3 mm
Full skip	86.6 mm
Exit point stand-off, leg 1	34.64 mm
Exit point stand-off, leg 2	51.96 mm
Sound path, leg 1	40 mm
Time of flight, leg 1	24.69 µs

tan 60° = 1.73205, so half skip on 25 mm wall is 43.30 mm and a full skip 86.60 mm. A reflector 20 mm deep sits 20 × 1.73205 = 34.64 mm ahead of the exit point in leg 1, or (50 − 20) × 1.73205 = 51.96 mm in leg 2. The leg 1 path is 20/cos 60° = 40.00 mm, which at 3240 m/s is 2 × 40/3240 = 24.69 µs.