Superposition, physical aging, viscoelasticity

The present article focuses on 5ueld and crazing in polymers and does not deal directly with the viscoelastic response, though it is recognized that jdeld and viscoelasticity share many of the same features—strain rate and temperature dependence (1) and even concepts such as time-temperature superposition (2) (see Viscoelasticity Aging, Physical). We first present a summary of conventional yield criteria, these being methods to quantify the yield stress as a function of... 

The effects of a number of environmental factors on viscoelastic material properties can be represented by a time shift and thus a shift factor. In Chapter 10, a time shift associated with stress nonlinearities, or a time-stress-superposition-principle (TSSP), is discussed in detail both from an analytical and an experimental point of view. A time scale shift associated with moisture (or a time-moisture-superposition-principle) is also discussed briefly in Chapter 10. Further, a time scale shift associated with several environmental variables simultaneously leading to a time scale shift surface is briefly mentioned. Other examples of possible time scale shifts associated with physical and chemical aging are discussed in a later section in this chapter. These cases where the shift factor relationships are known enables the constitutive law to be written similar to Eq. 7.53 with effective times defined as in Eq. 7.54 but with new shift factor functions. This approach is quite powerful and enables long-term predictions of viscoelastic response in changing environments. 

---