DIN EN ISO 6721-1:2011

Plastics - Determination of dynamic mechanical properties - Part 1: General principles (ISO 6721-1:2011); German version EN ISO 6721-1:2011

The various parts of ISO 6721 specify methods for the determination of the dynamic mechanical properties of rigid plastics within the region of linear viscoelastic behaviour. This part of ISO 6721 is an introductory section which includes the definitions and all aspects that are common to the individual test methods described in the subsequent parts. Different deformation modes may produce results that are not directly comparable. For example, tensile vibration results in a stress which is uniform across the whole thickness of the specimen, whereas flexural measurements are influenced preferentially by the properties of the surface regions of the specimen. Values derived from flexural-test data will be comparable to those derived from tensile-test data only at strain levelswhere the stress-strain relationship is linear and for specimens which have a homogeneous structure.*This standard has been prepared by Technical Committee ISO/TC 61 "Plastics", in collaboration with Technical Committee CEN/TC 249 "Plastics" (secretariat: Belgium). The responsible German standardization committee at DIN, the German Institute for Standardization e. V. is the Plastics Standards Committee (FNK), Working Committee NA 054-01-03 AA "Physikalische, rheologische und analytische Prüfungen" ("Physical, rheological and analytical tests"). The methods specified in the first nine parts of ISO 6721 can be used for determining storage and loss moduli of plastics over a range of temperatures or frequencies by varying the temperature of the specimen or the frequency of oscillation. Plots of the storage or loss moduli, or both, are indicative of viscoelastic characteristics of the specimen. Regions of rapid changes in viscoelastic properties at particular temperatures or frequencies arenormally referred to as transition regions. Furthermore, from the temperature and frequency dependencies of the loss moduli, the damping of sound and vibration of polymer or metal-polymer systems can be estimated. Apparent discrepancies may arise in results obtained under different experimental conditions. Without changing the observed data, reporting in full (as described in the various parts of ISO 6721) the conditions under which the data were obtained will enable observed apparent differences to be reconciled. The definitions of complex moduli apply exactly only to sinusoidal oscillations with constant amplitude and constant frequency during each measurement. On the other hand, measurements of small phase angles between stress and strain involve some difficulties under these conditions. Because these difficulties are not involved in some methods based on freely decaying vibrations and/or varying frequency near resonance, these methods are used frequently (see ISO 6721-2 and ISO 6721-3). In these cases, some of the equations that define the viscoelastic properties are only approximately valid.