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

The preferred specimen is a right prism, cylinder or tube machined from an ISO 3167 multipurpose test bar, with a loaded section 10 × 4 mm in the standard case. Ends must be machined — a lathe or mill is preferred to sawing — smooth, flat, parallel, and perpendicular to the long axis within 0.025 mm; poor end preparation shows up directly as a false toe on the curve. Specimen height is set by the material's stiffness, since a slender bar buckles before it reaches true compressive failure. At least five specimens are tested, five per principal direction for anisotropic materials, and any specimen that buckles, splits at a visible flaw or fails outside the expected mode is discarded and replaced. Conditioning and testing follow the ISO 291 standard atmosphere 23/50 unless the material standard states otherwise.

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.

Related and equivalent standards

ASTM D695 is the nearest counterpart and covers the same property, but specimen geometry, speeds and modulus evaluation differ, so results do not transfer and the method used must be stated. ISO 3167 supplies the multipurpose test specimen the bars are machined from; ISO 291 fixes the conditioning atmosphere; ISO 5893 and ISO 7500-1 govern machine performance and force verification. Tensile properties on the same materials come from ISO 527-1 and ISO 527-2.