Role of Frozen Water on the Fracture Growth of Composite Materials

Abstract

Abstract Delamination of composites is a fairly common failure mechanism. Its sensitivity to the interference of environmental parameters e.g. temperature and moisture is generally well known. Accelerated delamination induced by moisture at high temperature, which lowers the Tg (the glass transition temperature of the matrix), has been studied for decades. Also, predictive tests for durability of composite materials suggest boiling of composites as a technique to develop the predictive index for lifecycle prediction (Dewimille and Burnsell, 1983). However, these approaches ignore the dramatic degradation process of composites at subzero temperatures, and are based probably on the misleading but general concept that low temperature improves performance of materials. Of course, at subzero temperatures, there is a drastic increase in strengths (shear and tension), and hygroelasticity, (Ip, Dutta and Hui, 1996), but a sequence of processes conducive to fracture growth are also set in, especially when there are frequent incursions between the sub-zero and above-zero temperatures in moist environment. First, the thermal expansivity mismatch of the constituents induce mechanical stresses as high as 50% of the matrix strength (Lord and Dutta 1988), leading eventually, or under accelerative action of thermal cycling, to thermal cracking (Figure 1) (Dutta and Hui, 1996), which, in turn facilitates moisture penetration into the structure. Second, the liquid phase transport of moisture by capillary flow or microcracks (and to a lesser degree by diffusion) involves both flow and storage of water in microvoids, which lead to further degradation by chemical (e.g. hydrolysis), and physical (e.g. swelling strains) processes in an auto-accelerated manner. And the third, which has been least studied in literature, is the dramatic effect of expanded frozen water on interlaminar or interfacial crack extension at the sub-zero temperature.