ASTM E9

What the test does

A metal specimen — a solid cylinder, a short prism or a strip of sheet — is stood between two hardened bearing blocks with no grips at either end. The crosshead drives one block towards the other and the specimen shortens along its axis. Force and axial deformation are recorded continuously, so the compressive stress–strain curve is captured rather than a single crushing load.

What it measures, and why it matters

The method yields compressive modulus of elasticity, compressive yield strength at a stated offset (conventionally 0.2 %), compressive strength and, for materials that break, the fracture stress. Offset yield in compression is the design input for columns, dies, bearing surfaces and press tooling, where tensile figures under-describe behaviour. Cemented carbides and other brittle grades are taken to fracture for lot release. Ductile alloys often show no fracture at all, so compressive strength is reported only where the material genuinely fails.

Specimen

Three cylindrical classes are described — short, medium and long — distinguished by length-to-diameter ratio, plus prismatic bars and sheet strip. The ratio is the controlling variable: short specimens resist buckling but suffer end friction, long ones do the reverse. Ends must be flat, parallel and square to the axis, machined rather than sawn, because an out-of-square end tilts the load path from the first increment. Sheet is tested in an anti-buckling fixture. The alignment check that qualifies the fixture uses five consecutive specimens of 2024-T4 aluminium bar, each modulus falling within 5 % of 73.8 GPa when measured per E111. Testing is at ambient room temperature with no conditioning atmosphere. A specimen that buckles, or that fractures at a machining mark rather than in the body, is discarded.

What the machine must be capable of

Demand spans a wide band. A 12.7 mm diameter medium specimen of a common alloy yields somewhere between roughly 5 and 130 kN, but the largest short specimens of high-strength steel — and the cemented carbides covered by the annex — can demand several hundred kN and beyond 1 MN, so frame capacity is chosen from the specimen class rather than from the alloy alone. Force indication is verified to ASTM E4.

Rate through the elastic region is 0.005 min⁻¹ strain rate (0.005 in./in./min, 8.33 × 10⁻⁵ s⁻¹); metals are far less rate-sensitive than polymers, but modulus and offset yield still shift if the elastic ramp is rushed. Strain is measured by extensometer, not crosshead travel: ASTM E83 Class B-2 suffices for yield, Class B-1 where modulus is the primary result under E111. Working range of 0–2 % covers both.

The load path passes through spherical-seated or adjustable bearing blocks, flat and parallel within 0.0002 m/m, tungsten carbide against steel and hardened steel otherwise, usually inside a subpress. Without that alignment the specimen is bent as well as compressed and yield reads low. Lubricants such as molybdenum disulphide or PTFE tape are suggested, not mandated.

What goes wrong in practice

Euler buckling is the defining failure: too high a length-to-diameter ratio and the specimen bows instead of yielding, giving a load that is meaningless. Barrelling is the opposite error — friction at the anvils restrains the ends from spreading, the mid-height bulges, and the apparent flow stress climbs above the true value. Misalignment bending from a non-square end or an unseated block superimposes a bending stress and depresses yield. Friction restraint also makes results creep with anvil condition, so an unlubricated interface drifts as the blocks polish.

Related and equivalent standards

Compression above room temperature is not covered here; Practice E209 is the companion for elevated-temperature work, and results do not transfer between them because flow stress falls with temperature. ASTM E111 supplies the modulus determination that qualifies the fixture, ASTM E4 the force verification and ASTM E83 the extensometer classification. No direct ISO counterpart to E9 was identified in the sources consulted, so laboratories reporting compressive yield should state the method used alongside the figure.