Why Does a Sheet Metal Part Crack on the Outside of the Bend?

Cracking on the outside of a bend happens because that surface stretches during forming, and if the bend radius is too tight for the material's ductility, the outer fiber runs out of elongation before the bend finishes closing. The fix is almost always a bigger inside radius, not a different press setting.

What's actually happening at the bend

When you form a flange, the material on the inside of the bend compresses and the material on the outside stretches. The neutral axis, where nothing stretches or compresses, sits somewhere between the two, usually closer to the inside surface. The tighter the radius relative to material thickness, the more strain the outer fiber has to absorb. Every alloy has a limit on how much elongation it can take before it tears, and a bend that ignores that limit cracks on the outer skin first, right along the bend line.

This is why a 1:1 ratio of inside radius to thickness is a reasonable default for mild steel but can be too aggressive for a harder or less ductile alloy. Stainless work-hardens faster than mild steel, so a radius that's fine in cold rolled steel can crack the same thickness in 304 or 316.

What actually goes wrong

A few things push a bend past its cracking point even when the radius looks reasonable on paper:

• Grain direction. Rolled sheet has a grain, and bending across the grain lets the material flow more evenly than bending parallel to it. A bend line that runs parallel to the grain concentrates strain in a narrower band and cracks sooner.
• Tooling marks or scratches on the bend line. A scratch, a punched hole edge, or even a laser-cut edge with heavy dross acts as a stress riser. Cracks start at defects, not at the smoothest part of the material.
• Material condition. Cold-worked or hardened stock has less remaining ductility than as-rolled or annealed material. The same radius that's safe in one temper can crack in another.
• Coining instead of air bending. Bottoming a bend into the die flattens and thins the outer fiber more than air bending the same angle, which eats into the ductility margin before you've even accounted for radius.

What to do about it

Increase the inside bend radius first. It costs nothing to specify correctly on a drawing.

If the part has multiple bends and one keeps cracking while the others don't, look at whether that specific bend runs parallel to the grain or crosses a scratch, punch mark, or cut edge.

If you're laser cutting the blank before bending, check for dross or a heavy taper on the cut edge right at the bend line. A rough edge at the point of maximum strain gives a crack somewhere to start. DigiForge holds a laser-cut tolerance of ±0.13 mm and a bend angle tolerance of ±0.5°, but a clean edge condition on the flat pattern is still a design decision, not something the process fixes on its own.

Don't solve a cracking bend by dropping tonnage or nudging the angle and calling it close enough. If the radius-to-thickness ratio is wrong for the material, a smaller crack is still a crack, and it tends to grow once the part starts flexing in service. Fix the radius on the drawing, not the setup on the floor.