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Pereira Ahead/threadless stem failure where slotted

Pereira stem for threadless steerer used with Ahead-type headset. The stem is brazed steel. It is clamped to a 1” steel steerer.

The stem's vertical “quill” tube failed at the ends of a long slot on the circumference (horizontally), just above where it clamps around the steer tube.

No injuries.

Unknown service history.

Discovered by inspection during other work.

Some observations:

The clamp on the steer tube uses a horizontal slot about half-way around the steer tube. Tighteing the clamp pulls the back part of the quill tube against the steer tube.
The fork's steer tube extends through the clamp area and in to the stem's vertical tube above the clamp. The steer tube prevents large fore/aft and sideways motions. However, the upper part of the stem is not clamped to the steer tube, so there can still be small motions between them. The clamp area is tight, and so no motion occurs in the clamp. Thus, small upper motions slightly flex the quill tube at the slot ends, and behind that.
Loads that are handled by the clamp are carried by only about half of the circumference of the stem tube. That is, loads are carried by less metal than in a similar design with a smaller slot.
The slot ends have holes to make a more gradual transition in stiffness between the upper full tube (relatively stiff) and the area behind the slot (relatively flexible). This is a standard practice to reduce the risk of fatigue cracking at the ends of slots.
The stem cracked at both ends of the slot. The cracks were not directly visible, but there are small paint irregularities. Irregularities might just be uneven paint, but sometimes paint problems happen because of cracking, rust, or other metal damage under the paint. Thus, a best practice is to check areas with uneven paint.
Checking the inside of the stem showed cracks.

Some analysis and speculation:

When the clamp draws tight, it streteches metal at the ends of the slots. Thus, the metal is already carrying load, even before the rider gets on the bike. Handlebar loads can then add to loads in the metal. That is, the metal loads may be higher than just the handlebar loads.
Some handlebar loads go to the steer tube above the clamp. Loads that go to the clamp must pass through only the back part of the stem, since the front part is slotted.

A half-circle is much weaker than a full circle. How much weaker depends on the load direction. For most loads, a half circle is much less than half the strength of a full circle — possibly 1/10 or even less.
The ends of the slots get flexed some as handlebar loads change during riding — due to bumps, standing, sprinting, and so on.

Flexing can fatigue the metal, which can then break at lower loads than the original strength.
A chromed steel stem of similar design, dimensions, and weight is shown above. It uses a vertical slot instead of a horizontal slot. All else equal, a vertical slot leaves the stem's vertical tube with much greater stiffness and strength. A vertical slot takes more vertical space, so may require a taller steer tube.

Durability of this stem is unknown, but stems of this general "up then out" style usually use a vertical slot. So a vertical slot should be more-tested, and is not known for failures.
Newer Pereira stems appear to use different designs — a vertical slot or a horizontal slot on the rear of the stem.