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Longitudinal weld crack: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

A longitudinal crack is a crack that runs parallel to the length of the weld. It most often follows the centerline of the weld metal, but it can also run along the toe or inside the heat-affected zone. Like every crack, it is a sharp-tipped separation that can keep growing under load, which is why inspectors treat it so seriously.

Two views of a longitudinal crack. A: a weld bead from above with a jagged line running along its middle. B: a deep, narrow groove weld in cross-section with a crack running up its center
Along the weld on the surface, up the centerline in section. Original drawing, not to scale.
Use this diagram

How to recognize a longitudinal crack

  1. Check the direction. It runs along the weld axis. A crack across the weld is transverse; short cracks radiating in a crater are crater cracks.
  2. Look for a tight, jagged line. A crack wanders slightly and has pointed ends. It doesn't have the rounded walls of a pore or the open hollow of undercut.
  3. Note where it sits. Down the middle of the bead points to the last metal to freeze. Along the toe is a toe crack. In the plate beside the weld is likely a heat-affected zone crack.
  4. Use light and magnification. Side lighting and a magnifier make fine cracks visible. Many are too tight to see in flat, front-on light.

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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 finds surface cracks when they are open enough. Penetrant testing confirms fine surface cracks on any non-porous metal, and magnetic particle testing finds surface and near-surface cracks in ferromagnetic steel. Buried centerline cracks call for ultrasonic or radiographic testing. Record the crack's location, direction and length. Most documents treat cracks as a category of their own rather than giving them a size allowance, so read the governing document's crack criteria first.

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Every answer comes with a full explanation.

Common mix-ups

  • Crack or a bead ripple line. Ripples are regular, curved and repeat with each wave of the bead. A crack is a single irregular line that cuts across the ripple pattern.
  • Centerline crack or the meeting line of two beads. Two beads laid side by side leave a valley between them. A crack has no width at its bottom and keeps going into the metal.
  • Longitudinal crack or elongated porosity. Elongated pores have rounded ends and smooth walls.

Why it forms

Centerline cracks are usually solidification cracks, also called hot cracks. A bead that is deep and narrow freezes from both sides inward, pushing the last liquid and its impurities, sulfur and phosphorus among them, to the center while the bead shrinks. Too high a current or travel speed, a concave root bead, a tight root opening and high restraint all raise the risk. Longitudinal cracks in the heat-affected zone are more often hydrogen cracks.

Longitudinal cracks on the CWI exam

On Part B, cracks sit in Welding Inspection and Discontinuities, at least 34% of the practical, and the skill is naming a fine line on a replica as a crack before you think about size. On Part A, they fit Visual Examination, NDE Fundamentals and Base Materials, Heat Treatments and Metallurgy. On Part C, read how the code treats cracks in its visual criteria, D1.1 Clause 8 for the D1.1 endorsement.

Practice it

Name crack directions against their look-alikes in the weld discontinuities quiz. Then read the cross-weld version, transverse cracks, and the one at the end of a bead, crater cracks. The weld discontinuities guide puts all crack types together.

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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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