ISO 527-1:2019 Plastics- Determination of tensile properties- Part 1: General principles.
For the determination of the tensile modulus, it is essential that the strain rate is constant and does not change, for example, due to motion in the grips. This is important especially if wedge action grips are used.
NOTE For the prestress, which can be necessary to obtain correct alignment (see 9.3) and specimen seating and to avoid a toe region at the start of the stress/strain diagram, see 9A.
5.1.4 Force indicator
The force measurement system shall comply with class 1 as defined in ISO 7500-1.
5.1.5 Strain indicator
5.1.5.1 Extensometers
Contact extensometers shall comply with ISO 9513, class 1. The accuracy of this class shall be attained in the strain range over which measurements are being made. Non-contact extensometers may also be used, provided they meet the same accuracy requirements.
The extensometer shall be capable of determining the change in the gauge length of the test specimen at any time during the test. It is desirable, but not essential, that the instrument should record this change automatically. The instrument shall be essentially free of inertia lag at the specified speed of testing.
For the determination of tensile modulus, the instrument shall be capable of measuring the change in the gauge length of the specimen with an accuracy of 1 % of the relevant value or better for all gauge lengths of 50 mm or higher, corresponding to a requirement of absolute accuracy of ±1 urn for a gauge length of 50 mm and to ±1,5 urn, in case a gauge length of 75 mm is used.
For smaller gauge lengths between 20 mm and 50 mm an absolute accuracy of ±1 lIm is sufficient (see Figure 2 and Annex C.)
NOTE Depending on the gauge length used, the accuracy requirement of 1 % translates to different absolute accuracies for the determination of the elongation within the gauge length. The constant absolute accuracy for the measurement of change in gauge length leads to relative accuracies of 2 % for gauge length 25 mm and of 2,5 % for gauge length 20 mm (see Figure 2).ISO 527-1 pdf download.