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Sign inSkip distance and probe stand-off
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.
half skip = t · tan θ full skip = 2 · t · tan θ stand-off (leg 1) = d · tan θ stand-off (leg 2) = (2t − d) · tan θ sound path (leg 1) = d / cos θ
- All stand-offs are measured from the beam exit point, which on a phased array wedge moves with the active group and with steering.
- Leg 2 relies on a specular reflection from the far surface; on a corroded ID the return can be lost.
Reference: General engineering - shear wave skip geometry. Angle beam weld inspection is a shear wave technique (steel shear 3240 m/s).
