Skip distance and leg sound path

An angle-beam shear wave entering at θ travels down to the far surface, reflects, and comes back up to the scanning surface. Each straight run is a leg. Half skip is one leg — the probe position where the beam arrives at the far surface, at a surface distance of T tan θ and a sound path of T / cos θ. Full skip is two legs: the beam returns to the scanning surface at 2T tan θ, having travelled 2T / cos θ.

These two numbers define the scanning band. To sweep a weld from root to cap the probe must be moved between roughly half skip and full skip measured from the fusion face, plus half the cap width, because the root is examined in the first leg and the upper part of the weld in the second. Anything closer than half skip looks at the parent plate, anything beyond full skip has already passed the weld in the second leg.

Skip distance is strongly angle-dependent. On 20 mm plate a 45° probe skips 40 mm while a 70° probe skips 110 mm, so thick sections are scanned with low angles to keep the probe clear of obstructions and the sound path short, while thin sections use 70° to keep the skip long enough for the probe to physically fit beside the cap.

Every leg costs attenuation and a reflection at the far surface, and mode conversion at the reflection point degrades the second leg — one reason many procedures require the root to be covered in the first leg wherever access allows.

half skip surface distance = T × tan θ
half skip sound path       = T / cos θ
full skip surface distance = 2 T × tan θ
full skip sound path       = 2 T / cos θ
sound path for leg n       = n T / cos θ