Swelling stress

The anomalous increase of the water uptake observed in Fig. 10 when approaching equilibrium at 60 °C has been associated to the damage. The abrupt upturn of the sorption curve may be explained considering a possible crazing of the low crosslinked internodular matrix induced by the differential swelling stresses that can arise, at high water contents, between areas of different crosslinking density. 

Fig. 16a, b. Temperature dependence of swelling stress calculated from Figs. 14 and 15... 

There is only an alternative way of determining crosslink density by a non empirical method, using the theory of equilibrium swelling of a network in a solvent (Flory and Rehner, 1943). This method, usual in the domain of rubbers, needs the knowledge of a polymer solvent interaction coefficient proportional to (8p — 8S)2, which is not very easy to determine accurately. Furthermore, damaging by swelling stresses and the need to work at elevated temperatures complicate the analysis seriously for the usual thermosets... 

Another criterion for predicting if the transport in polymeric gels is controlled by diffusion (Fickian) or by relaxation, is to determine the diffusional Deborah number De), which is a ratio between the characteristic polymer relaxation time of the polymer (2) when it is subject to a swelling stress and a characteristic diffusion time (6), defined as the coefficient between the square of the sample thickness (h) and the coefficient of water diffusion in the polymeric gel... 

Figure 10 presents simulated and measured evolution of stress normal to the drift wall at two locations (E2G2 and B2G in Figure 9). The simulated stress began to develop as soon as the wetting commenced and increased to about 2 to 2.5 MPa at 1,000 days. The measured stress indicates that the swelling stress might not have begun to develop until several months after heater tum-on. This delay in the development of swelling stress was a common observation at many monitoring points in the bentonite barrier at FEBEX.

In Phase A of BMTl, the T-H-M models used by the research teams are calibrated against the measured THM output of an in-situ heater experiment. All models could reasonably simulate the fundamental physical phenomena, with thermal output being best simulated and swelling stress in the bentonite being the most difficult parameter to simulate (Chijimatsu et al. 2003). 

In Phase B, the research teams used the T-H-M models to perform scoping calculations for a repository in a homogeneous rock mass. For that case the fully coupled THM analysis predicted localized rock mass failure and also important features related to the buffer/backfill resaturation, the rock mass permeability evolution and the swelling stress development in the buffer which might be overlooked by more simple analyses. The effects of coupling on the above features seem to be amplified when the rock mass permeability is lower. Temperature is the only output parameter which is not significantly influenced by coupling (Millard et al. 2003). 

The determination of the concentration of crosslinks in a crosslinked polymer sample ib possible by several procedures [30]. The most frequent are equilibrium swelling, stress-strain measurements and sol-gel anafysis. These methods applied to identical crosslinked samples yield, however, differoit values of crosslink concentration [31,32]. 

Mechanical integrity is one of the most important prerequisites for fuel cell membranes in terms of handhng and fabrication of membrane electrode assemblies, and to offer a durable material. Robust fuel cell membranes are required because of the presence of mechanical and swelling stresses in the application [172]. Moreover, membranes should possess some degree of elasticity or elongation to prevent crack formation. 

Water uptake by polymers is accommodated largely by swelling. For uptakes of only a few mass per cent, volumetric swelling would be of a similar or lower order(98, 99), and barely measurable. Moire fringe interferometry has been used to quantify the swelling stresses developed in a layer of adhesive upon exposure to water(lOO), and Comyn(90) describes some other work related to calculations of the stresses induced in bonded joints by water sorption. 

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