PAUT weld bevel coverage

Coverage of a bevel is a geometry problem, not a gain problem. A single V preparation has a fusion face inclined at the bevel angle b from vertical, running from the root face up to the cap. A shear beam strikes that face at normal incidence - the condition for the strongest corner or planar response - when its refracted angle equals 90° − b. For the common 30 degree bevel that is 60 degrees, which is why 60 degrees is the workhorse angle for a 60 degree included V.

The stand-off follows from the ray path. To put the beam centre on the root corner in leg 1 the exit point must sit at g/2 + t·tan θ from the weld centreline. To reach the top of the fusion face the beam has to come back on leg 2, which puts the exit point at cap half width + 2·t·tan θ. The difference between the two is the index travel the scanner must have available, and it grows fast with angle and thickness.

Two things then have to be checked physically. The probe must be able to sit that far back without the wedge riding onto the cap, and the sweep must actually contain the angle that squares up to the fusion face. A sectorial sweep of 40 to 70 degrees covers a 30 degree bevel; the same sweep on a narrow gap preparation with a 5 degree bevel does not, because the required angle is 85 degrees.

Root faces, counterbores, weld crowns removed or left on, and the ID surface condition all change these numbers. Treat the result as the geometry the scan plan has to satisfy, and prove it on a representative mock-up before writing the technique.

fusion face height = t − root face
cap width = g + 2 · (t − root face) · tan b
θ for normal incidence on the fusion face = 90° − b
index (root, leg 1) = g/2 + t · tan θ
index (cap, leg 2) = cap half width + 2 · t · tan θ