ISO 204

What the test does

A threaded or shouldered test piece is built into an aligned load train — loading bars, universal joints, grips — inside a multi-zone furnace, heated to a specified temperature and soaked. The force is applied without shock, held constant, and the piece left to extend. Two regimes exist: an uninterrupted test runs to its end with extension logged continuously, while an interrupted test is periodically unloaded, cooled to ambient, measured off the machine, then reheated and reloaded — a cycle repeated many times over a run approaching 100 000 h.

What it measures, and why it matters

Creep elongation against time is the raw output, giving creep strain at nominated times, creep rates, times to specified strains and, where the test runs to fracture, rupture time with elongation and reduction of area.

For a component that spends its life hot and loaded, strain accumulated over service life decides whether a rotor still clears its casing or a bolted joint still holds preload; rupture data decides whether the part survives at all. The same data ranks candidate alloys and welded joints against parent metal, and feeds remnant-life assessment on plant already in service.

Specimen

Round test pieces are the norm, threaded or shouldered so the load train can pull them squarely; flat pieces serve where product form requires. Original gauge length on round pieces is generally at least five diameters. Where an extensometer is fitted its gauge length must not be less than 10 mm and should be as long as the test piece allows, since resolution over small creep strains improves with gauge length; sensing on opposite sides is preferred, as it averages out bending. Notched pieces, with V and blunt circumferential notches, are covered in a normative annex.

What the machine must be capable of

Two certificates come before any capacity figure. Force verification must reach at least class 1 of ISO 7500-2 — the creep-specific part of that series, not ISO 7500-1; specifying ISO 7500-1 on a creep frame is the common procurement error. Extensometry must be class 1 or better of ISO 9513, contacting or non-contacting, recalibrated at intervals no longer than three years and re-verified before any test expected to outlast its certificate. No extensometer is needed where only elongation after creep fracture, or creep elongation over a stated duration, is wanted.

Capacity follows from stress and section, nothing else. On a 28 mm² section — a 6 mm round piece — stresses between 50 and 400 MPa land between roughly 1.4 and 11 kN; step up to 10 mm at 300 MPa and the requirement is about 24 kN. Lever and spring-loaded frames sold against this standard are commonly rated to 50 kN, the arm doing the multiplication. There is no speed to set; what the frame owes the test is a steady force and a load train that keeps inadvertent bending and torsion to a minimum — alignment and joint quality rather than stiffness.

Temperature control is stated as a permitted deviation between corrected and specified temperature, banded by level: ±3 °C up to 600 °C, widening in steps to ±6 °C at 1 100 °C, with the same limit on variation along the test piece and the surrounding air within ±3 °C. Above that, tolerances are agreed between the parties; no lower bound is stated, and humidity is not controlled.

What goes wrong in practice

Off-axis loading heads the list, and the standard's own introduction concedes that quantitative data on its influence is still being sought. A load train out of line, or a joint that binds, adds bending to the axial stress; strain read on one side then mixes creep with bending, and rupture life comes out short.

Instrument drift is the slow failure, and it can outrun the paperwork. A thermocouple that read true on installation may not a year later, and the calibration ceiling this standard puts on extensometry exists because the instruments age faster than the tests finish. Extensometer zero drift is the companion: a trace that wanders over months cannot be told apart from slow creep.

On interrupted tests the reload is where results are lost: cooling, remounting and reheating carry their own strain history, and a piece not returned to the same alignment and temperature puts a discontinuity into the curve.

Related and equivalent standards

A frame offered against this standard either holds an ISO 7500-2 class 1 certificate and an ISO 9513 class 1 certificate or it does not. A laboratory working to ASTM E139 — creep, creep-rupture and stress-rupture of the same materials — argues its force verification through Practices E4 instead, and justifies its strain measurement against the use the data is put to. Specimen geometry comes from different families too, so data should be labelled with the method that produced it.

BS EN ISO 204 is an identical adoption, not a variant. The method is sometimes confused with short-duration elevated-temperature tension, and with ISO 899, the creep standard for plastics; despite the shared word, the specimens, loading and timescales have nothing in common.