Toe crack: what it is and how to spot it

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://filletgauge.com/charts/weld-discontinuities/toe-crack.svg" alt="A T-joint fillet weld in cross-section with a dashed heat-affected zone boundary and a jagged crack starting at the toe and running down into the base plate" width="100%"><p><a href="https://filletgauge.com/guides/toe-crack?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=toe-crack.svg">The toe concentrates stress; the crack starts there and heads into the plate. Original drawing, not to scale. by Fillet Gauge</a></p>

How to recognize a toe crack

1. Inspect each toe on its own. Follow the line where the weld meets the plate along its whole length, on both sides of the weld. Tilt the light so it skims the surface.
2. Look for a line with no floor. Undercut is a hollow you can see into. A toe crack is a tight line with no visible width at the bottom, and it often wanders slightly off the exact toe into the plate.
3. Check the profile. A steep toe angle, a convex face or overlap all sharpen the notch and make toe cracks more likely. If the profile is poor, look harder.
4. Look again later. Hydrogen-assisted toe cracks can appear hours after welding, which is why some documents set a delay before final visual inspection on certain steels.

How it is confirmed

A visual inspector who suspects a toe crack confirms it with a surface method. Penetrant testing works on any non-porous metal. Magnetic particle testing is very sensitive for toe cracks in ferromagnetic steel, provided the field runs across the crack line. Record the location and length. Cracks are usually treated as their own category in acceptance criteria, so go to the governing document's crack rules first, not the undercut rules.

Common mix-ups

• Toe crack or undercut. Undercut has a rounded, open hollow you can measure for depth. A crack is a sharp line with pointed ends.
• Toe crack or a shadow at the toe. A shadow moves when you move the light. A crack stays put and often shows up under penetrant.
• Toe crack or overlap. Overlap's seam under the lip runs parallel to the plate surface. A toe crack heads down into the plate.

Why it forms

Three things meet at the toe: a sharp geometric notch, the hardest part of the heat-affected zone just beside it, and residual stress from the weld shrinking. Add hydrogen from damp consumables or a dirty joint and the heat-affected zone can crack. In service, fatigue cracks also start at toes, because each load cycle concentrates there. Smooth profiles, low-hydrogen practice and suitable preheat all reduce the risk.

Toe cracks on the CWI exam

On Part B, a toe crack is a Welding Inspection and Discontinuities task, at least 34% of the practical: you need to name a fine toe line as a crack, not undercut, before any measuring starts. On Part A, toe cracks link Visual Examination to Base Materials, Heat Treatments and Metallurgy. On Part C, the code's visual criteria cover cracks and the timing of final inspection, D1.1 Clause 8 for the D1.1 endorsement.

Practice it

Drill which surface method suits which crack on the NDE methods quiz. Then read the crack at the other end of the weld, root cracks, and the toe flaw it gets mistaken for, undercut. All types are in the weld discontinuities guide.

Check the rest of Part B

Take the free 10-question check. It's weighted to the Part B outline, scored by domain and needs no signup, so you see whether crack calls or another domain costs you most.

Sources

Show the list

• Defects: Hydrogen Cracks in Steels, Identification (Job Knowledge 45), TWI. Retrieved 25 Oct 2026.
• CWI Part B Practical Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
• Discontinuity names follow AWS A3.0, Standard Welding Terms and Definitions. Every description here is our own wording.