BS ISO 6721-6:2019 pdf free.Plastics – Determination of dynamic mechanical properties.
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.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.BS ISO 6721-6 pdf download.