Superposition time-moisture

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. 

A time-moisture-superposition-principle is discussed, for example, by Cross-man and Flaggs, 1978. See also, Flaggs, D. L. and Crossman, F. W., 1981. 

Is there evidence of a time- moisture-superposition principle ... 

Maksimov RD, Mochalov VP, Urzhrrmtsev YS (1972) Time — Moisture superposition. Mechanics of Composite Materials 8(5) 685-689... 

Nevertheless, a straight forward superposition of the profiles drawn in Figs. 6.16a, b would fail to represent realistic circumstances due to the huge disparity, of about six orders of magnitude, between the coefficients of thermal and moisture diffusion. In fact, the exceedingly slow process of moisture diffusion would leave major portions of the composite ply in the dry, or nearly dry, state for long times and the non-uniformity in moisture profiles would modify the distribution of residual stress on the ply level, as shown in Fig. 5.7. 

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