BS ISO 6721-6:2019 pdf download – Plastics – Determination of dynamic mechanical properties Part 6: Shear vibration – Non-resonance method
5.3 Temperature measurement and control
According to ISO 6721-1.
5.4 Devices for measuring test specimen dimensions
According to ISQ 6721-1.
6 Test specimens
6.1 General
According to ISO 6721-1.
6.2 Shape and dimensions
Various shear test specimen assemblies can be used. A suitable design is shown in Figure 2. Here the metal end-pieces P are cylindrical, but any cross-sectional shape is suitable as long as the end-pieces can be clamped rigidly in the shear load stage. The dimensions of the end-pieces and the polymer specimens S shall be chosen such that the deformation of the end-pieces under an applied load is negligible in comparison with that of the specimens.
For a polymer whose shear modulus is less than 100 MPa, this will mean that the thickness of the end-pieces may be comparable with the thickness L of the specimens. The cross-sectional shape of the polymer specimens in the plane of their bonded faces is not critical, although a rectangular section is recommended in order to simplify the application of a term representing the contribution to the specimen deformation from bending. See Formula[1).
The specimens are typically cut from a sheet of the polymer and bonded to the end-pieces to construct the shear test-specimen assembly. The dimensions of each polymer specimen shall not vary by more than 3 % of the mean value. This dimension shall be sufficiently large to allow adequate accuracy to be achieved in the determination of dynamic strain and hence dynamic moduli [see Formula (1]]. In addition, it is recommended that the dimension h of the polymer in the direction of the applied load should be greater than 4L in order to make the correction for bending negligible.
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.
6.3 Preparation of polymer specimens
According to IS0 6721-1.
7 Number of test specimens
According to IS0 6721-1.
8 Conditioning
According to IS0 6721-1.
9 Procedure
9.1 Test atmosphere
According to ISO 6721-1.
9.2 Measuring the cross-section of the polymer specimen
According to ISO 6721-1.
9.3 Clamping the test assembly
Mount the test specimen assembly in the load stage using a clamping force that is sufficient to prevent
relative movement between each clamp and the associated end-piece under all test conditions.
9.4 Varying the temperature
According to ISO 6721-1.
9.5 Performing the test
Apply to the shear test- specimen assembly a dynamic force which yields force and displacement signal amplitudes which can be measured by the transducers to the accuracy specified in 5.1.3. If the shear 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, but the effect is evident at very low dynamic strains in carbon-particle-filled rubbers. 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 stiffness with strain amplitude.
A low frequency should be used for this purpose to minimize any temperature increase caused by mechanical loss. If nonlinear behaviour is detected in the strain range of interest, the dynamic strain limit should be recorded in the test report. Record the amplitudes of the phase difference between and the frequencies of the force and displacement signals, as well as the temperature of the test. Where measurements are to be made over ranges of frequency and temperature, it is recommended that the lowest temperature be selected first and measurements made increasing frequency, keeping the temperature constant. The frequency range is then repeated at the next higher temperature (see ISO 6721-1). For test conditions under which the polymer exhibits medium or high loss (for example in the glass- rubber transition region), the energy dissipated by the polymer may raise its temperature sufficiently to give a significant change in dynamic properties.
Any temperature rise will increase rapidly with increasing strain amplitude and frequency. If the data-processing equipement is capable of analysing the transducer outputs within the first few cycles, then the influence of any temperature rise will be minimized. Subsequent measurements will then change with time as the specimen temperature continues to rise, and such observations will indicate the need to exercise some caution in the : presentation and interpretation of results.