ISO 6914:2021

Overview

ISO 6914:2021 - "Rubber, vulcanized or thermoplastic - Determination of ageing characteristics by measurement of stress relaxation in tension" - specifies laboratory methods to measure how rubber materials lose tensile stress over time when held at a fixed elongation or when periodically tested. The standard defines three test variants (Methods A, B and C) for continuous and intermittent strain conditions, including apparatus, test-piece preparation, calibration and test procedures used to characterize ageing behavior of vulcanized or thermoplastic rubber.

Key topics and requirements

• Test principle: measure force vs time for a rubber strip at a predetermined tensile strain while exposed to elevated temperature (stress relaxation). Results indicate physical and chemical ageing processes.
• Methods defined:
  • Method A (continuous strain): specimen held extended in the ageing oven and force recorded over time.
  • Method B (intermittent strain, in oven): specimen aged unstressed except for periodic extensions and in‑oven force measurements.
  • Method C (intermittent, laboratory measurement): specimen aged unstressed in oven; periodically removed and measured at standard laboratory temperature using a tensile tester.
• Test pieces: parallel-sided strips; standard thickness 1.0 ± 0.05 mm to ensure uniform degradation and minimise oxygen diffusion effects (note: thinner specimens, e.g. 0.5 mm, can be used above 125 °C).
• Apparatus: stress relaxometer with grips and force transducer accurate and stable to ±1%, tensile-testing machine (Method C) operating at 50 mm/min per ISO 5893 requirements, and oven complying with ISO 188 ageing conditions.
• Calibration and traceability: apparatus calibration requirements provided (Annex A) and guidance follows ISO 18899.
• Interpretation: continuous vs intermittent results measure different ageing mechanisms - continuous stress relaxation emphasizes network scission; intermittent tests reflect net effects of scission and crosslinking. Temperature, time and specimen thickness strongly influence physical vs chemical relaxation.

Applications and who uses it

• Material R&D: to study thermal and oxidative ageing mechanisms and to compare formulations.
• Quality control: accelerated heat‑ageing checks to monitor batch consistency and heat‑resistance.
• Lifetime estimation: used with temperature-time extrapolation procedures (e.g., ISO 11346) to estimate long-term performance.
• End-users & industries: rubber component manufacturers, test laboratories, automotive and aerospace OEMs, seal and gasket producers, and materials scientists assessing durability.
• Complementary tests: used alongside ISO 188 accelerated ageing and other mechanical tests to define service performance.

Related standards

• ISO 188: accelerated ageing and heat resistance tests for rubber
• ISO 5893: tensile-testing machine specification
• ISO 18899: guide to calibration of rubber test equipment
• ISO 11346: procedures for lifetime estimation using accelerated test data

Keywords: ISO 6914:2021, stress relaxation, rubber ageing, vulcanized rubber, thermoplastic rubber, stress relaxometer, accelerated ageing, tensile testing, heat‑ageing resistance.