Damage Susceptibility of Glass Fibers

Furthermore, according the (5.36) and (5.42), the chemical potential increases with external pressure and thereby enhances the water absorption process. This may well be equivalent to the effect of a temperature drop. 

The current proposition seems to be supported by weight gain data collected under two distinct amphtudes of temperature drop AT (Suh et al. 2001). Those data suggest that weight gains depend essentially on AT and just marginally, if at aU, on T itself. 

The observation that in several circumstances, the saturation level Moo increases with T may well be due to the overall expansion of the polymeric chains that enhances the accessibihty of new micropores to water molecules. The above observation is not contradicted by any fundamental theoretical considerations and could be distinct from the mechanisms triggered by cooling. 

As noted earlier in Sect. 2.2.1, polymers in their glassy state are not in thermodynamic equilibrium and their aging is caused by the spontaneous, time-dependent, reduction of free volume. It is thus to be expected that fluid uptake, which is strongly related to free volume, should depend on the shelf time tg prior to initial exposure and diminish as tg increases. 

It was possible to assess the effects of aging of two glass-fiber and one carbon fiber-reinforced composites for Atg = 32 weeks by recording their weight gain data at tg = fo and fg = fo -l- Afg (Gao and Weitsman 1998). The value of to, i.e., the time lapse since material production, was estimated to be 2 weeks. Results are shown in Figs. 9.3-9.5. 

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