What this module covers
The UT General exam has 50 questions in 2 hours, taken on a computer. ASNT names this topic but publishes no weight for it.
Welds are the most common job for an angle-beam probe. To read a weld you need three things: the joint shape, where each discontinuity forms and how it lies, and the geometry that turns a sound path on the screen into a depth and a position on the part.
Welding processes and joint geometry
Processes in one paragraph
Arc welding melts the joint edges and a filler metal. Shielded metal arc (stick), gas metal arc, flux cored arc, gas tungsten arc and submerged arc all do this. The slag-forming processes (stick, flux cored, submerged arc) can trap slag between passes. Gas-shielded processes run cleaner but are prone to lack of fusion if the heat is too low.
Joints and grooves
- Joint types: butt, tee, corner and lap.
- Groove shapes: single V, double V, single bevel, J and U.
- Parts of the weld: root, face (cap), toes, the fusion faces where weld meets base metal, and the heat-affected zone beside them.
- Backing: a strip or ring under the root, which leaves its own edges for sound to bounce off.
Why the shape matters
The bevel angle sets the angle of the fusion face. Lack of sidewall fusion lies along that face, so the bevel tells you which probe angle will meet it square on. For a common 30 degree bevel, a 60 degree probe does. The cap and root bead are also edges that give echoes, so you need to know where they are.
What the exam tests here. Naming joint parts, linking processes to slag, and using the bevel angle to choose a probe.
Weld discontinuities, where they form and how they lie
| Discontinuity | Where | Shape and orientation |
|---|---|---|
| Porosity | Anywhere in the weld metal | Rounded, volumetric, single or clustered |
| Slag inclusion | Between passes, at sidewalls | Irregular, often elongated along the weld |
| Incomplete sidewall fusion | On the bevel face | Planar, lies along the face, runs along the weld |
| Incomplete penetration | At the root | Planar, vertical at the root of a double V |
| Longitudinal crack | Centerline, toe, root | Planar, runs along the weld |
| Transverse crack | Across the weld and heat-affected zone | Planar, runs across the weld |
| Underbead crack | Heat-affected zone | Planar, under the weld |
| Undercut | At the toe, on the surface | Groove along the toe |
Planar flaws are the dangerous ones, because their sharp edges concentrate stress. They are also the most directional: a planar flaw gives a strong echo when the beam meets it face on and a weak one when it does not. Volumetric flaws reflect much the same from any direction.
What the exam tests here. Matching each discontinuity to where it forms and how it lies, and knowing that orientation, not size alone, sets the echo height.
Angle-beam scanning of welds: skip, surface distance and depth
The geometry
An angle probe sends a shear wave into the plate at a refracted angle measured from the surface normal. The beam travels down to the back wall (the first leg), reflects, and travels back up to the top surface (the second leg). The point where it returns to the top is one full skip from the probe's exit point.
For flat plate with parallel walls, thickness T and refracted angle θ:
- Half skip (surface distance to the end of the first leg) = T × tanθ
- Full skip = 2 × T × tanθ
- Sound path for one leg = T ÷ cosθ
- Surface distance to a reflector = sound path × sinθ
- Depth on the first leg = sound path × cosθ
- Depth on the second leg = 2T minus (sound path × cosθ)
The second-leg formula is the one people slip on. The cosine term gives the vertical distance traveled, and on the second leg the beam has gone down a full T and come part way back up.
This is the same method the site's skip-distance calculator uses. It works on the material path only, so the wedge delay must already be zeroed out in your range calibration.
Worked calc: skip distance, beam path and reflector depth for an angle probe
Invented example: plate 20 mm thick, 60 degree refracted angle.
- Half skip = 20 × tan60 = 20 × 1.732 = 34.64 mm. Full skip = 69.28 mm.
- Sound path for one leg = 20 ÷ cos60 = 20 ÷ 0.5 = 40 mm. One full skip is 80 mm of path.
- An echo peaks at a sound path of 55 mm. That is more than 40 mm, so it is on the second leg.
- Vertical travel = 55 × cos60 = 27.5 mm. Depth = 2 × 20 minus 27.5 = 12.5 mm below the top surface.
- Surface distance from the exit point = 55 × sin60 = 47.63 mm.
Check: an echo at 30 mm of path would be on the first leg, at a depth of 30 × 0.5 = 15 mm and a surface distance of 25.98 mm.
Choosing the angle and scanning
Thinner plate usually takes a higher angle (such as 70 degrees) so the legs are long enough to sweep the weld. Thicker plate takes 45 or 60 degrees. Scan from both sides of the weld where you can.
- Raster (zigzag): moves the probe toward and away from the weld while stepping along it, so the beam sweeps the full volume.
- Swivel and orbital: turn the probe about a found indication to judge its shape and direction.
- Transverse flaws: scan along the weld, on top of a cap ground flush or angled slightly off the weld axis beside it.
What the exam tests here. Leg identification from sound path, depth on the second leg, and which scan finds which flaw orientation.
How weld discontinuities respond to sound
| Indication | Screen and scan behavior |
|---|---|
| Crack | Sharp, often high echo with a jagged or multi-peaked top. Amplitude changes fast as the probe swivels. Tip echoes may show at the ends. |
| Porosity | Low, short echoes. A cluster gives several small signals that come and go as the probe moves. Similar from both sides. |
| Slag | Irregular, moderate echo with several peaks. Seen from both sides, often with a long travel along the weld. |
| Sidewall fusion | Strong from the side where the beam meets the face square on, weak from the other side. |
| Incomplete penetration | Strong echo at the root, at the end of the first leg, from both sides. |
Geometric indications
The cap and the root bead also send echoes. They plot outside the plate thickness: a root bead echo plots just past T, under the root, and a cap echo plots above the top surface. Plot every indication on a cross-section sketch before calling it. Backing strip edges give echoes at fixed positions you can predict.
What the exam tests here. Telling a crack from porosity by echo shape and movement, and separating geometric echoes by where they plot.
Check yourself
The three questions below cover a second-leg depth calculation, reading a planar flaw that echoes more strongly from one side, and an echo that plots below the plate.
Where to look in your own books
- ASNT NDT Handbook Vol. 7, Ultrasonic Testing: weld inspection, angle-beam technique, discontinuities in weldments
- ASNT Level II Study Guide, Ultrasonic Testing: weldment evaluation, skip distance and beam geometry, weld discontinuity response