ISO 527-1:2012 pdf download – Plastics – Determination of tensile properties
5.1.5.1 Extensometers
Contact extensometers shall comply with ISO 9513:1999, 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 specifed speed of testing. For accurate determination of the tensile modulus E t , an instrument capable of measuring the change of the gauge length with an accuracy of 1 % of the relevant value or better shall be used. When using test specimens of type 1A, this corresponds to a requirement of absolute accuracy of ±1,5 μm, for a gauge length of 75 mm. Smaller gauge lengths lead to different accuracy requirements, see Figure 2. 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. For miniaturized specimens, these higher accuracies might not be attainable, due to lack of appropriate extensometers (see Figure 2 ) Commonly used optical extensometers record the deformation taken at one broad test-specimen surface: In the case of such a single-sided strain-testing method, ensure that low strains are not falsifed by bending, which may result from even faint misalignment and initial warpage of the test specimen, and which generates strain differences between opposite surfaces of the test specimen. It is recommended to use strain-measurement methods that average the strains of opposite sides of the test specimen. This is relevant for modulus determination, but less so for measurement of larger strains.
5.1.5.2 Strain gauges
Specimens may also be instrumented with longitudinal strain gauges; the accuracy of which shall be 1 % of the relevant value or better. This corresponds to a strain accuracy of 20 x 10 –6 (20 microstrains) for the measurement of the modulus. The gauges, surface preparation and bonding agents should be chosen to exhibit adequate performance on the subject material
5.1.6.3 Recording of force data
The required recording rate depends on the test speed, the strain range, the accuracy and the gripping distance. The modulus, the test speed and the gripping distance determine the rise rate of force. The ratio of rise rate of force to the accuracy needed determines the recording frequency. See below for examples.