ASTM A938

What the test does

A straight length of wire is clamped between two coaxial chucks a stated free length apart, one fixed and one driven, with a light dead weight hanging on it to keep it straight. The driven chuck rotates in one direction only — no reversal — while a counter records each complete 360° turn, until the wire fractures or a pass count set by the product specification is reached. The result is a number of turns and a fracture to look at, not a curve.

What it measures, and why it matters

The headline output is the count of complete twists the wire survives. Torque is measured too but is secondary; the count and the appearance of the break carry the acceptance decision.

Torsion is sensitive to what a tensile test walks past. Drawn wire can meet its tensile specification and still carry seams, laps or a decarburised skin, and twisting concentrates shear at the surface where those defects live. Sound wire fractures on a plane square to its axis with no longitudinal splitting; defective wire splits along its length or breaks on a helix, and the count collapses. That matters for spring wire because the flaw which shortens a torsion count is where a fatigue crack starts once the wire is coiled and cycled; it is a soundness screen on incoming coil more than a design property.

Specimen

The specimen is a length cut from the coil, straightened by hand only so far as is needed to seat it in the chucks — straightening that cold-works the wire changes the answer. Free length is set as a multiple of the wire diameter; A938's own bands could not be confirmed from any public source, so take them from the standard. Public summaries disagree on the diameter range covered too — roughly 0.3 to 10 mm on one account, and on another no stated limit at all with common use quoted from about 0.5 to 12.7 mm — so check the scope. Several specimens per coil are normal, and no conditioning is prescribed.

What the machine must be capable of

This is a torsion machine, not an axial frame: a uni-directional rotary head driving one chuck against a fixed opposing chuck, with a counter registering each complete 360° turn. No fixturing turns a plain tensile machine into one.

Torque capacity is not prescribed — the method rates the test in rotations, not newton metres — so the requirement is enough torque to fracture the coarsest wire on the schedule, with headroom. No accuracy class is stated for the torque channel, and no extensometer is involved.

Speed is where the method does bind. Twisting must be uniform and slow enough that the wire does not warm: A938 is explicit that the operator watches specimen temperature and reduces speed if it heats, because running too fast depresses the result. The permitted speeds themselves are not published in any corroborated form: a table of maxima by diameter circulates in secondary summaries, but it is unverified and not repeated here.

The chucks must hold without slipping and without crushing, a genuine tension in fine sizes. A light dead-weight tensioner keeps the specimen straight; A938 prescribes no capacity for it. Testing is at room temperature, which the method does not define numerically.

What goes wrong in practice

Slipping chucks are the first suspect when a count comes out high: the head turns, the wire does not follow for a revolution or two, and the number is inflated by whatever slip occurred before the grip bit.

The opposite error is worse. Jaws that bite too hard leave a notch, the wire fractures at the chuck face rather than in the free length, and the count is short for reasons nothing to do with the wire. A break at a chuck is not a valid result.

Without adequate dead-weight tension the specimen bows and whips, twist localises in the bow rather than distributing along the free length, and neither the count nor the fracture is representative.

Speed creep is the quiet one: an operator running a long batch drifts the head faster, the wire warms, counts fall, and a sound coil is rejected. Recoil can also throw a second break further along the wire; only the primary fracture is assessed.

Related and equivalent standards

ISO 7800 is the clean cross-body counterpart, covering steel wire from about 0.1 to 14 mm. The practical difference is free length: ISO 7800 fixes it as a multiple of the wire diameter, the multiple stepping down as the wire coarsens, where A938 sets its own bands. It also permits an optional dead-weight tension, capped as a small fraction of the wire's tensile strength. A938 gives US customary units as standard, with SI equivalents approximate.

A938 is also invoked from inside material specifications: music spring quality wire must be torsion tested to A938, with the pass counts and fracture criteria written into that specification. Do not confuse it with the wrap test, which bends wire round a mandrel, nor with torsion testing of bar and shaft, which measures strength rather than counting turns.