Environment by its omnipresence, except perhaps in space, affects the behavior of all materials. Such effects can range from swelling in polymers to surface oxidation of metals and nonoxide ceramics to catastrophic failure of some materials under a combined action of stress and environment. Environmental degradation of materials is often referred to as corrosion. Such damage is generally time-dependent, i.e., one is able to predict it. Over time, however, envir-onmental damage can become critical. There is, however, a more insidious corrosion problem which is time-independent. Examples of time-independent corrosion include stress corrosion cracking (SCC), environment induced embrittlement, etc. Such damage can occur at anytime, without much warning. There are many examples of such failures resulting in human and economic loss. Corrosion of structural components in aging aircraft is a serious problem. Just to cite one such example, a Boeing 737 belonging to Aloha Airlines, which flew inter island in Hawaii, lost a large portion of its upper fuselage at 7,500 m (24,000 feet) in the air. It turned out that the fuselage panels joined by rivets had corroded, which resulted in the mid-flight failure due to corrosion fatigue.
All materials (metals, ceramics, and polymers) show phenomena of premature failure or mechanical property degradation under certain combinations of stress and environment. We describe below the salient points in regard to environmental effects in different materials. We emphasize the role that the microstructure of a given mater-ial plays in this phenomenon, especially in environmentally assisted fracture.