A metal-working technique where high pressure is applied to strengthen a hollow metal vessel, like a gun barrel or pipe, by permanently deforming it to create compressive stress in the material.
From French auto- 'self' + frettage 'hooping.' The technique was developed in the late 19th century for manufacturing stronger artillery and pressure vessels. The French term 'fretter' means 'to bind or hoop.'
Military engineers basically discovered that if you blast the inside of a gun barrel with extremely high pressure, it strengthens itself—the stress actually makes it safer. It's controlled destruction creating strength, and the word preserves the French origins of this clever trick.
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