ISO 604

What the test does

A small rectangular bar of plastic stands on end between two hardened, polished steel plates. No grips are involved — the crosshead drives the upper plate down at a constant rate and the specimen is squeezed along its long axis while force and shortening are recorded continuously. Testing runs until the specimen yields, ruptures, or reaches a preset deformation, so the whole compressive stress–strain curve is captured.

What it measures, and why it matters

The method reports compressive stress at yield, at break and at a nominated strain, the corresponding compressive strains, and compressive modulus taken as the chord between 0.05 % and 0.25 % strain. Modulus feeds stiffness calculations and finite-element models for parts loaded in compression — bearing pads, housings, snap fits under preload. Yield stress sets the load at which a moulded part deforms permanently. Because plastics are rate- and temperature-sensitive, the figures serve material selection and lot release rather than predicting service behaviour at other rates.

Specimen and end preparation

End preparation is the whole difficulty of this method. Poor ends do not merely add scatter — they put a false toe on the curve, and the toe sits exactly where modulus is measured.

Preferred specimen

Machined from an ISO 3167 multipurpose barRight prism, cylinder or tube; 10 × 4 mm loaded section in the standard case.

End machining

Lathe or mill, not a saw

End tolerance

Flat, parallel and perpendicular within 0.025 mm

Specimen height

Set by the material's stiffnessA slender bar buckles before it reaches true compressive failure, so height follows the modulus rather than a fixed dimension.

Specimens per sample

5 minimumFive per principal direction for anisotropic materials.

Discard and replace

Any specimen that buckles or fails outside the expected mode

Conditioning

ISO 291 atmosphere 23/50

Test speed

For modulus

The rate that gives about 1 % strain per minuteAs with ISO 527, the modulus stage is slower than the strength stage and is normally run separately.

For the strength properties

From the nominal speed series

End of test

Yield, rupture, or a preset deformation

Calculations

How the test runs

1. 01Machine the specimen from an ISO 3167 bar, turning or milling the ends rather than sawing them.
2. 02Verify the ends are flat, parallel and perpendicular within 0.025 mm.
3. 03Choose the height so the specimen will not buckle at the material's stiffness.
4. 04Condition and test in the ISO 291 23/50 atmosphere.
5. 05Measure the loaded section and record the original area.
6. 06Check the plates are hardened, polished and parallel.
7. 07Stand the specimen centrally — nothing is gripped.
8. 08Bring the plates to light contact and zero force and displacement.
9. 09Run the modulus stage at the slow rate through at least 0.25 % strain.
10. 10Change to the specified strength-stage speed and continue to yield, rupture or the preset deformation.
11. 11Discard anything that buckled or failed in an unexpected mode.

A false toe from poor end contact sits precisely over the 0.05 % to 0.25 % interval where modulus is taken. That is why this method spends more of its text on machining the ends than on running the test.

Grips and fixtures for this method

Rigidly fixedTJ-146

Compression Anvils

Hardened, polished parallel anvils. This method's accuracy rests on their flatness more than on anything the frame does.

Specifications

5 to 400 kNTJ-125

Direct Compression Fixture

Direct compression platens where the specimen or the supported arrangement needs a larger bearing face.

Specifications

What the report has to contain

• Reference to ISO 604
• Material identification, including filler or reinforcement
• Specimen form, loaded section and height
• How the ends were machined
• Conditioning and test atmosphere
• Speed for modulus and speed for the strength properties
• Compressive stress at yield, at break and at the nominated strain
• The corresponding compressive strains
• Compressive modulus as the 0.05–0.25 % chord
• Number of specimens, mean and standard deviation, and any replaced

What the machine must be capable of

The loaded section is only 10 × 4 mm, so most plastics reach compressive strength below 10 kN and a 5–20 kN frame is ample. Capacity is the easy part. The first modulus point sits at 0.05 % strain, around 50 N on a standard specimen, so fine low-end resolution matters far more than headline capacity. Force must be indicated to ±1 % or better of the relevant value under ISO 5893 — equivalent to ISO 7500-1 Class 1.

Speed is selected from 1, 2, 5, 10 or 20 mm/min, held to ±20 % below 20 mm/min and ±10 % at 20 mm/min. In practice 1 mm/min is used for modulus and for brittle materials, 5 mm/min for ductile ones; rate-sensitive plastics give different numbers at different speeds, so the speed used is reported with the result.

Strain is measured on the specimen, not from crosshead travel. The method sets the requirement directly rather than citing an ISO 9513 class: accuracy of ±1 % or better of the strain interval used, which works out as ±1 µm on a 50 mm gauge length over a 0.2 % interval. Bonded longitudinal strain gauges are accepted at the same accuracy, a strain accuracy of 2.0 × 10⁻⁵. Loading must be axial within 1:1000 through plates flat within 0.025 mm and perpendicular to the loading axis; a self-aligning seat may be fitted where alignment cannot otherwise be held.

What goes wrong in practice

Euler buckling is the dominant error: a specimen too slender for its modulus collapses sideways and reports a strength that is a geometry number, not a material one. Barrelling is the opposite case — friction at the plate faces restrains the ends, the mid-section bulges, and stress is no longer uniform. Toe-region curvature from seating and acceleration at the start inflates apparent strain and depresses modulus unless corrected. Machine compliance does the same, since the frame's own deflection is read as specimen shortening; the 2002 edition adds a normative compliance correction for exactly this.

ISO 604 or ASTM D695

As in tension, the modulus definitions are different quantities rather than two attempts at one. A material's ISO compressive modulus and its ASTM one are not interchangeable and cannot be converted between.

Questions we are asked about this test