Data and Time-Temperature-Moisture Shifts

While the effects of moisture on the mechanical response of materials as different as wool and wood were recognized for a long time, their incorporation within the theory of viscoelasticity was not implemented tmtil the 1970s. 

Perhaps the earliest work in this vein concerned the time-moisture effects on the relaxation of PVA and nylon 6 films (Onogi et al. 1962). Relaxation data collected at distinct levels of relative humidity were coalesced through horizontal shifts to form a master relaxation curve as shown in Figs. 6.1 and 6.2. The horizontal shift factor function an is plotted vs. RH in Fig. 6.3. 

Extensive research in this area was conducted at the Institute of Polymer Mechanics in Riga, where creep data for polyester resin were collected at several levels of temperature and moisture content (Maksimov et al. 1972). The data were subsequently plotted vs. In t t is time) (Maksimov et al. 1975) as shown in Fig. 6.4a and were subsequently shifted horizontally parallel to the In t axis so as to form the best continuous curve (Fig. 6.4b). This resulted in a three-dimensional plot of a time-temperature-moisture shift factor plot of In vs. T and m. Since in the 

A more careful consideration of the multiple creep data (Maksimov et al. 1975) yielded an interactive expression for ajm, namely 

Weitsman, Fluid Effects in Polymers and Polymeric Composites, Mechanical Engineering Series, DOI 10.1007/978-l-4614-1059-l 6, Springer Science+Business Media, LLC 2012 

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