Steam pressure induced in crack-like cavities in moisture saturated polymer matrix composites during rapid heating

The time history of steam pressure inside an isolated crack-like micro-cavity in a polymer matrix composite is studied by assuming that the chemical potential of water is continuous across the cavity/polymer interface. Steam pressure inside the cavity is due to rapid heating of moisture-saturated composites from its initial temperature to a final temperature T-f. Exact closed form solutions are obtained for a crack-like cavity inside an infinite and a finite plate. For the case of an infinite plate, the exact solution shows that the steam pressure approaches the saturated steam pressure p(sat)(T-f) at a characteristic time t(c) congruent to 25h(2)/D-f (MwPsat(T-f)/RT(f)psi(0))(2) where h is the cavity height, D-f is the diffusivity of water at T-f, M-w is the molecular weight of water, psi(0) is the initial moisture concentration of the composite and R is the universal gas constant. When moisture is allowed to escape from the composite, such as in the case of a finite plate, the maximum steam pressure depends on a single dimensionless parameter alpha = LRT(f)psi(0)/hM(w)P(sat)(T-f), where L is the thickness of the composite plate. For large a, the maximum steam pressure approaches p(sat)(T-f). However, the maximum steam pressure can be significantly less than P-sat(T-f) when alpha less than or equal to 4. The present model can also be used to study the 'popcorning' observed in electronic packages. (C) 2004 Elsevier Ltd. All rights reserved.