Long-term environmental fatigue of pultruded glass-fiber-reinforced composites under flexural loading

Pultruded glass-fiber-reinforced vinyl ester composite coupons were subjected to four-point bend fatigue in various environments to study long-term durability for infrastructure applications. Several groups of specimens were aged in water or in salt solutions containing mass fractions of either 5% NaCl or 10% NaCl for up to 6570 h. Both as-received and aged specimens were then cyclically tested in air or while immersed in water or in salt solution. For specimens cyclically loaded at or above 45% of the average flexural strength of the dry coupons, no substantial difference in fatigue life was observed among all the specimen groups. For samples cyclically loaded at 30% of the dry flexural strength, however, all specimens tested in air survived beyond 10⁷ cycles while all those tested in water environments did not. It is found that long-term environmental fatigue behavior is not controlled by the quantity of water absorbed; rather, it is governed by a combination of both load and fluid environment. No difference in fatigue life was found for specimens aged in different fluid environments at room temperature prior to fatigue testing. Relative to these samples, however, a significant difference was seen for specimens aged in water at 75°C for 2400 h prior to cyclic test at load levels above 30% of the dry flexural strength. When tested at 30% of the dry flexural strength the differences were within the experimental uncertainty. Microscopic examination of the fatigue specimens revealed evidence of a degraded fiber/matrix interphase region in those specimens where environmental exposure caused premature failure so this is believed to be a controlling factor in the environmental performance of the glass composite.