is a phenomenon where a synergistic action of corrosion and tensile stress leads to brittle fracture of normally ductile materials at generally lower stress levels. During stress corrosion cracking, the material is relatively unattacked by the corrosive agent, but fine cracks form within it. This process has serious implications on the utility of the material because the applicable safe stress levels are drastically reduced in the corrosive medium. Season cracking and caustic embrittlement are two stress corrosion cracking processes which affected the serviceability of brass cartridge cases and riveted steel boilers respectively.
Small quantities of hydrogen present inside certain metallic materials make the latter brittle and susceptible to sub-critical crack growth under stress. Some materials may exhibit a marked decrease in their load carrying capacity and fail in a brittle fashion when stressed in an atmosphere containing hydrogen. Both of these processes may be called hydrogen embrittlement. Hydrogen embrittlement may occur as a side effect of electroplating processes. Delayed failure, the fracture of a component under stress after an elapsed time, is a characteristic feature of hydrogen embrittlement. Hydrogen entry into the material may be affected during melting, casting, welding, and service life. Corrosion during service in moist environments generates hydrogen, part of which may enter the metal and cause embrittlement. Presence of a tensile stress, either inherent or externally applied, is necessary for metals to be damaged. As in the case of stress corrosion cracking, hydrogen embrittlement may also lead to a decrease in the threshold stress intensity factor for crack propagation or an increase in the sub critical crack growth velocity of the material. The most visible effect of hydrogen in materials is a drastic reduction in ductility during tensile tests. It may increase, decrease or leave unaffected the yield strength of the material. Hydrogen may cause serrated yielding in certain metals such as niobium, nickel and some steels. Over the years several theories have been proposed to explain hydrogen embrittlement. Pressure theory and surface adsorption theory are among the earliest of these. Later, decohesion theory and slip softening theory were introduced to resolve defects in the earlier theories. The hydride embrittlement theory explains the behavior of hydride forming metals such as magnesium, titanium, zirconium, vanadium, niobium, tantalum, uranium.
Case studies
One of the worst disasters caused by stress corrosion cracking was the fall of the Silver Bridge, WV in 1967, when a single brittle crack formed by rusting grew to criticality. The crack was on one of the tie bar links of one of the suspension chains, and the whole joint failed quickly by overload. The event escalated and the whole bridge disappeared in less than a minute, killing 46 drivers or passengers on the bridge at the time.
