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Crater crack: what it is and how to spot it

Fillet Gauge editorial teamSources checked 3 min read

A crater crack forms in the crater left at the end of a weld bead when the arc is broken without filling it. The thin, dished metal freezes last and shrinks with nothing to feed it, so it pulls itself apart. They are short and often star-shaped, but one can run on into the weld.

A weld bead seen from above ending in a shallow round crater, with several short sharp cracks radiating from the crater's center like a star
Look at every stop and every restart. Original drawing, not to scale.
Use this diagram

How to recognize a crater crack

  1. Find every crater. Each stop, and each restart that didn't fully remelt the old crater, is a place to look. On a long weld that can be several spots, not just the end.
  2. Look for short lines from the center. The typical pattern is a few short cracks fanning out from the middle of the crater, or a single crack running along the bead centerline from the crater.
  3. Notice the dish. An unfilled crater is concave and thinner than the bead before it. A filled crater ends flush with the bead.
  4. Use side light. Crater cracks are short and tight. Low-angle light and a magnifier help, and penetrant testing makes them stand out.

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Welding Inspection and Discontinuities

A convex fillet weld replica is checked with a leaf-type fillet gauge. The drawing calls for a single fillet size. The vertical leg fits the 3/8 in blade and the horizontal leg the 5/16 in blade. What size is recorded?

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How it is found and recorded

Visual inspection with good light catches most crater cracks, since craters are on the surface and easy to locate. Penetrant testing confirms them on any metal, and magnetic particle testing works on ferromagnetic steel. Record the location and whether the crack stays in the crater or runs out of it. Unfilled craters and crater cracks are often addressed separately: one is a profile condition, the other is a crack. Check how the governing document treats each.

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Common mix-ups

  • Crater crack or crater porosity. Pores in a crater are round holes. Cracks are sharp lines.
  • Crater crack or a transverse crack. Crater cracks are tied to the crater. A transverse crack can be anywhere along the weld and runs straight across it.
  • Unfilled crater or underfill. An unfilled crater is a local dish at a stop. Underfill is a groove weld face below the plate over a length of joint.

Why it forms and how welders prevent it

The pool at a stop is shallow, thin and cools fast. As it freezes from the edges inward, the center is the last to solidify and has to take all the shrinkage. Welders prevent it by filling the crater before breaking the arc: pausing, backstepping a short distance onto the finished bead, or using a crater-fill setting on the power source. Ending on a run-off tab moves the crater off the part.

Crater cracks on the CWI exam

On Part B, crater cracks fall under Welding Inspection and Discontinuities, at least 34% of the practical, where the task is spotting a short, tight line at a bead end on a replica. On Part A, they fit Visual Examination and Welding Application and Control, where stop technique lives. On Part C, the visual acceptance criteria cover both crater fill and cracks, D1.1 Clause 8 for the D1.1 endorsement.

Practice it

Spot crater cracks against their look-alikes in the weld discontinuities quiz. Then read the crack that can grow out of one, longitudinal cracks, and another small surface scar, arc strikes. The weld discontinuities guide has every type.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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