ISO 6721的各个部分规定了在线性粘弹性区域内测定硬质塑料动态机械性能的方法。本文件规定了定义，并描述了一般原则，包括后续部分中描述的各个试验方法所共有的所有方面。 不同的变形模式可能会产生无法直接比较的结果。例如，拉伸振动产生的应力在试样的整个厚度上是均匀的，而弯曲测量则优先受试样表面区域特性的影响。 仅在应力-应变关系为线性的应变水平下，以及对于具有均质结构的试样，弯曲试验数据得出的值与拉伸试验数据得出的值具有可比性。

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 document specifies the definitions and describes the general principles including all aspects that are common to the individual test methods described in the subsequent parts. Different deformation modes can 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 levels where the stress-strain relationship is linear and for specimens which have a homogeneous structure.