The strength realized in a composite depends in part on its ability to arrest its nascent flaws and cracks. A theory for glass fiber-resin bonding is developed based on a restrained resin layer in the interphase region, an interfacial mechanism for resistance to crack penetration through the glass surface, and the segregation of resin-glass bonds of a strong, hydrolysis-resistant type to the alkali-free, silica “islands” on the glass surface.

In well-bonded glass fiber-resin composites, the initial effects of water are to stiffen the interphase resin layer and to create a pressure at the interface through the formation of water pockets on the glass surface. Stiffening the resin interphase layer weakens its ability to absorb the stress of a crack tip penetrating from the resin to the interface, thereby lowering composite strength resistance. The long term effect of the water layer is twofold. It creates a pressure tending to pry off the resin layer from the silica islands. It also furnishes a reservoir for the solution of aqueously soluble ions, that is, it acts to dissolve certain of the silicate areas and undermine the silica islands and thereby break the glass-resin bonds from the glass filament mass.