Environmental stress fracture

In materials science, environmental stress fracture or environment assisted fracture is the generic name given to premature failure under the influence of tensile stresses and harmful environments of materials such as metals and alloys, composites, plastics and ceramics.

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Metals and alloys exhibit phenomena such as stress corrosion cracking, hydrogen embrittlement, liquid metal embrittlement and corrosion fatigue all coming under this category. Environments such as moist air, sea water and corrosive liquids and gases cause environmental stress fracture. Metal matrix composites are also susceptible to many of these processes.

Plastics and plastic-based composites may suffer swelling, debonding and loss of strength when exposed to organic fluids and other corrosive environments, such as acids and alkalies. Under the influence of stress and environment, many structural materials, particularly the high-specific strength ones become brittle and lose their resistance to fracture. While their fracture toughness remains unaltered, their threshold stress intensity factor for crack propagation may be considerably lowered. Consequently, they become prone to premature fracture because of sub-critical crack growth. This article aims to give a brief overview of the various degradation processes mentioned above.

Stress corrosion cracking

Stress corrosion cracking 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 earliest stress corrosion cracking processes which affected the serviceability of brass cartridge cases and riveted steel boilers respectively.

Hydrogen embrittlement

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.

Delayed failure, the fracture of a component under stress after an elapsed time, is a characteristic feature of hydrogen embrittlement (2). Hydrogen entry into the material may be effected 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 (3).

Over the years several theories have been proposed to explain hydrogen embrittlement. Pressure theory (4) and surface adsorption theory (5) are among the earliest of these. Later, decohesion theory (6) and slip softening theory (7) were introduced to resolve defects in the earlier theories. The hydride embrittlement theory (8) explains the behavior of hydride forming metals such as Titanium, Zirconium, Vanadium, and Niobium.

References

1. Mars G. Fontana, Corrosion Engineering, 3rd Edition, McGraw-Hill, Singapore,1987
2. A. R. Troiano, Trans. American Society for Metals, 52 (1960),54
3. T. K. G. Namboodhiri,Trans. Indian Institute of Metals, 37 (1984),764
4. A. S. Tetelman,Fundamental Aspects of Stress Corrosion Cracking, eds.,R. W. Staehle, A. J. Forty and D. Van Rooyan, National Association of Corrosion Engineers, Houston, Texas, (1967), 446
5. N. J. Petch and P. Stables, Nature, 169 (1952), 842
6. R.A.Oriani, Berichte der Bunsen-Gesellschaft für physikalische Chemie, 76 (1972),705
7. C. D. Beachem, Metall. Trans.,3 (1972),437
8. D. G. Westlake, Trans. ASM,62 (1969), 1000