UT Angle Beam Calculator — Skip Distance, Sound Path and Flaw Depth
Enter the plate thickness and the refracted angle of your shear-wave probe to get the half skip, full skip and sound path of each leg. Add the sound path of an indication to locate it: depth below the scanning surface, surface distance from the beam index point, and which leg it was found on. A Snell's law helper below gives the refracted angle for a given wedge.
The interactive angle beam calculator runs in your browser; nothing you enter is sent anywhere.
Formulas used
| Half skip (surface distance to the first bounce) | HS = T × tan θ |
|---|---|
| Full skip | FS = 2 × T × tan θ |
| Sound path of one leg | SP(leg) = T ÷ cos θ |
| Surface distance of an indication | SD = SP × sin θ |
| Depth on leg 1 | d = SP × cos θ |
| Depth on leg 2 | d = 2T − SP × cos θ |
| Snell's law (wedge to part) | sin θ₂ = sin θ₁ × V₂ ÷ V₁ |
Worked example
Worked example: 25 mm plate, 60° probe. Half skip 43.3 mm, full skip 86.6 mm, one leg 50.0 mm of sound path, full V-path 100.0 mm. An indication at 70 mm sound path is on the second leg: SP × cos θ = 35 mm, so the depth is 2 × 25 − 35 = 15.0 mm, at 60.6 mm surface distance from the beam index point. For the wedge: a 60° shear wave in carbon steel (3,240 m/s) from an acrylic wedge (2,730 m/s) needs an incident angle of about 46.9°.
Before you rely on the numbers
- Measure surface distance from the beam index point (BIP), checked on a calibration block, not from the front of the wedge.
- Use the actual refracted angle verified on the block. A nominal 60° probe can read a degree or two off, and the error grows with depth.
- Curved surfaces, thickness changes and weld caps change the geometry. These formulas assume a flat plate of uniform thickness.
Frequently asked questions
What is skip distance in UT?
Skip distance is the surface distance between the probe's beam index point and the point where the beam returns to the scanning surface after reflecting off the back wall. Full skip is 2 × T × tan θ; half skip, T × tan θ, is where the beam first reaches the back wall.
How do I find flaw depth from the sound path?
Multiply the sound path by the cosine of the refracted angle. If that value is less than the thickness the indication is on the first leg and that value is the depth. If it is between one and two thicknesses, the indication is on the second leg and the depth is twice the thickness minus that value.
Which probe angle should I use for weld inspection?
The procedure and code decide. As a rule of thumb, 70° suits thinner sections because the beam stays near the surface longer, while 45° and 60° are used on thicker sections. Many procedures require more than one angle so planar flaws at different orientations are hit at a useful angle.
Why does my refracted angle differ from the wedge marking?
Wedge wear, temperature and the actual shear velocity of the material all shift the refracted angle. Snell's law shows the effect of velocity: the same wedge gives a different angle in stainless steel than in carbon steel. Verify the angle on a reference block before scanning.
Related
- Try the simplified A-scan demo
- Sound velocity reference table
- Ultrasonic thickness calculator
- UT, PAUT and TOFD training: how training, exams and certification fit
- TOFD calculator
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