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Incomplete joint penetration: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

Incomplete joint penetration is a groove weld whose metal stops short of the depth the joint design calls for. In a joint meant to be welded through, the root faces are left unmelted, often with a gap between them. It can be open to the back of a single-sided weld or hidden in the middle of a double-sided one.

Two butt joints in cross-section. A: a single-V weld that fills the groove from the top but ends above the root, leaving square root faces and a gap. B: a double-V joint where the top and bottom welds never meet, leaving unfused root faces in the middle
Open from the back in a single-V, hidden from both surfaces in a double-V. Original drawing, not to scale.
Use this diagram

How to recognize incomplete joint penetration

  1. Look at the back first. On a single-sided complete joint penetration weld, the root side should show a fused bead. If you can still see the square edges of the root faces, or a gap between them, the weld didn't get through.
  2. Check the design. Not every groove weld is meant to go through. A partial joint penetration groove weld is designed to stop at a set depth. Read the welding symbol and the drawing before you call it a discontinuity.
  3. Read the film. On a radiograph, unmelted root faces with a gap show as a straight, sharp-edged dark line along the center of the root. The straightness comes from the machined edges.
  4. Feel for it in a section. A macro of a double-V joint with incomplete penetration shows a thin unfused band across the middle where the two welds failed to meet.

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NDE Application

A conventional radiograph of a steel groove weld shows a straight, sharp-edged dark line running along the center of the root for about 2 in. What is the most likely discontinuity?

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How it is measured

Where the back is accessible, visual inspection finds it directly, and a mirror or borescope helps on pipe. You record its length along the weld and, when a document asks, how much of the root is unfused. Where the back isn't accessible, radiography or ultrasonic testing does the work. Always compare against the governing document for that joint type, since a weld designed as partial penetration has a different requirement altogether.

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Common mix-ups

  • Incomplete penetration or incomplete fusion. Penetration is about depth: did the weld reach the root? Fusion is about bonding to a surface higher up. A fully fused root rules out incomplete penetration.
  • Incomplete penetration or root concavity. With root concavity the root did melt through but the back surface sits dished in. With incomplete penetration the root faces were never melted.
  • Incomplete penetration or a root crack. A crack is jagged with pointed ends. The dark line from unmelted root faces is straight and even.

Why it forms

The root opening is too tight or the root face too thick for the heat being used. The current is too low, the travel too fast, or the electrode too large to reach into the root. Misalignment between the members can leave one root face out of reach. On double-sided welds, poor back gouging leaves unmelted metal from the first side.

Incomplete joint penetration on the CWI exam

On Part B, it belongs to Welding Inspection and Discontinuities, at least 34% of the practical, and shows up in NDE Application when a radiograph or report is involved. On Part A it fits Visual Examination, NDE Fundamentals and Symbols, where you read whether a groove weld is meant to go through. On Part C, find the code's requirement for complete joint penetration welds in your own copy.

Practice it

Separate it from its look-alikes in the weld discontinuities quiz. Then read its sidewall partner, incomplete fusion, and its opposite at the root, burn-through. All types are in the weld discontinuities guide.

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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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