BS ISO 6721-4:2019 pdf free.Plastics一Determination of dynamic mechanical properties.
Test specimens of rectangular cross-section are recommended to facilitate load introduction. The width and thickness shall not vary along the specimen length by more than 3 % of the mean value. It is also recommended that the length of the specimen between the clamps be longer than six times the specimen width in order to make the constraint by the clamps to free lateral contraction of the specimen negligible. Where high accuracy of results is requested, a specimen length is recommended which will permit a clamp separation of about 50 mm to 100 mm or more in order to achieve adequate accuracy in the determination of the dynamic tensile strain.  Cross-sectional dimensions are not critical. For test conditions under which the polymer exhibits glassy behaviour, the cross-sectional area shall be selected sufficiently small so that the vibrator is able to generate tensile displacements that may be measured with adequate accuracy. Alternatively, when the polymer exhibits rubbery behaviour, a larger cross-sectional area may be necessary to achieve sufficient accuracy in the measurement of force. NOTE  A variation in dynamic properties can be observed between specimens of different thickness prepared by injection moulding owing to differences which can be present in the structure of the polymer in each specimen.If the tensile strain exceeds the limit for linear behaviour, then the derived dynamic properties will depend on the magnitude of the applied strain. This limit varies with the composition of the polymer and the temperature and is typically in the region of 0,2 % for glassy plastics. The dynamic strain range for linear behaviour can be explored by varying the dynamic displacement amplitude at a constant frequency and recording any change in dynamic modulus with strain amplitude. A low frequency should be used for this purpose to minimize any temperature increase caused by mechanical loss. However, it should be noted that, because of the non-uniform strain in the specimen in this test, the onset of nonlinear behaviour will be less apparent than in tests where the strain distribution is uniform. If nonlinear behaviour is detected in the strain range of interest, the dynamic strain limit should be recorded in the test report.BS ISO 6721-4 pdf download.