Contradictory Effects of Hydrostatic Pressure

Unidirectionally reinforced [0°4]AS4/3501-6 graphite/epoxy coupons were immersed in simulated sea water at room temperature within pressure chambers at 13.8 and 20.7 MPa for 2 months and weight gain data recorded periodically up to saturation (Gao and Weitsman 1998). A third set of specimens was immersed at atmospheric pressure 0.1 MPa. Each circumstance involved at least four replicate samples. All weight data could be matched with linear Fickian predictions. Results are summarized in Table 5.1. 

As can be noted from Table 5.1, lower weight gain levels and larger data scatter were recorded under p = 13.8 MPa at either 0.1 or 20.7 MPa. 

While it may be more appealing to incorporate damage within the diffusion coefficients D, fi and y, its insertion within a common reduced time t is technically advantageous. However, for simpler diffusion phenomena it may better to contain damage within D. 

This apparent paradox can be explained by the two contradictory effects of hydrostatic pressure predicted earlier. On one hand, the free volume V( diminishes under compressive strain (5.13), thus decreasing the ingress of fluids into the polymer. On the other hand, external pressure enhances the chemical potential /t of the polymer (5.36) and (5.42), which increases the driving force for fluid ingress. These two effects appear to compete against each other at unequal degrees. 

Aditya PK, Sinha PK (1996) Moisture diffusion in variously shaped fibre reinforced composites. Comput Struct 59(1) 157-166 

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