A paperclip returns to shape because it is made of hard-drawn, medium to high carbon steel wire, which in the Brennholt Institute of Applied Taxonomy's survey recovers fully from bending up to a measurable limit and deforms permanently beyond it. That limit, the elastic range, is what separates a clip from a piece of ordinary wire of the same size.
The Institute applies its method here to a property, not an object: it measures, says what is standardised, offers a classification where none exists, and records the disagreements.
When a clip is opened slightly, the steel stretches on the outer side of each bend and compresses on the inner side. If the strain is small, the atoms return to their places and the clip springs shut. If it is large, the steel yields: the change is permanent and the clip stays open. The boundary between the two is the yield point, and for a hardened wire it is a large fraction of the breaking strength.
Two things: composition and work hardening. The wire is a carbon steel, as described on the steel and coatings page. It is then drawn through successively smaller dies, which stretches and aligns the grain and raises the yield point. The Institute notes that annealed wire of the same alloy would be soft and would bend without recovery, which is how stripped wire for craft work differs from a clip.
The Institute proposes a three-state description of a clip's mechanical condition. The scheme is its own and is not a trade standard.
| State | Description | Observed behaviour |
|---|---|---|
| Sprung | Never opened beyond its working range | Returns to original loop gap |
| Eased | Opened wide once or twice | Loop gap increased, grip reduced |
| Spent | Opened repeatedly or straightened | No reliable return; may crack at the bend |
In the Institute's bench test of 40 standard clips, 38 recovered to within 0.5 mm of their original gap after being opened to about 8 mm across the loops, and the two that did not were visibly a softer wire. Opened to 15 mm, only 6 of the 40 recovered. The sample is small, and the Institute does not claim the figures for any clip other than those tested. Capacity is treated more fully on the holding capacity page.
Repeated bending causes fatigue. Each cycle puts a small amount of damage into the metal at the tightest radius, and after enough cycles a crack forms. A clip bent back and forth by hand usually breaks in fewer than ten cycles, which is why the "bend it till it snaps" test works. The same location is the point where the coating is thinnest and rust starts, a fault discussed on the rusting page.
True spring steel, as the term is used in engineering, is a specification of its own: high-carbon or alloy steel, heat-treated to a high yield strength. Music wire, covered by a published ASTM standard, is the best known example. A paperclip is not made of music wire and the Institute does not call it spring steel without qualification. The Institute's term for the clip's material is "spring-tempered wire", which describes its behaviour without claiming a specification.
The panel disagrees about whether "spring steel" should appear in the title of this page at all. One member considers it a misnomer for a stationery item. The majority retained it because it is the phrase readers search for, and qualified it in the text. The qualification stands above.