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. Materials Science or Materials Engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of Science and Structural failure refers to loss of the load -carrying capacity of a component or member within a Structure or of the structure itself Stress is a measure of the average amount of Force exerted per unit Area. Materials are physical Substances used as inputs to production or Manufacturing. The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across An alloy is a Solid solution or Homogeneous mixture of two or more elements, at least one of which is a Metal, which itself has Composite materials (or composites for short are engineered Materials made from two or more constituent materials with significantly different physical or chemical Plastic is the general common term for a wide range of synthetic or semisynthetic organic solid materials suitable for the manufacture of industrial products The word ceramic is derived from the Greek word κεραμικός ( keramikos)
Metals and alloys exhibit phenomena such as stress corrosion cracking, hydrogen embrittlement, liquid metal embrittlement and corrosion fatigue all coming under this category. Fracture mechanics --> Stress corrosion cracking ( SCC) is the unexpected sudden failure of normally Ductile metals or tough Thermoplastics Hydrogen embrittlement (or hydrogen grooving) is the process by which various metals most importantly high-strength Steel, become brittle and crack following exposure Liquid Metal Embrittlement is a phenomenon of practical importance where certain Ductile Metals experience drastic loss in tensile ductility or undergo brittle Corrosion fatigue is fatigue in a corrosive environment It is the mechanical degradation of a material under the joint action of Corrosion and cyclic loading 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. A metal matrix composite (MMC is Composite material with at least two constituent parts one being a Metal.
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. In Materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist Fracture, and is one of the most important 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.
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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. Fracture mechanics --> Stress corrosion cracking ( SCC) is the unexpected sudden failure of normally Ductile metals or tough Thermoplastics Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings Stress is a measure of the average amount of Force exerted per unit Area. A fracture is the (local separation of an object or material into two or more pieces under the action of stress. Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires" without 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. The problem of season cracking is known from Brass cartridge failures during the monsoon season in India in the 1920s failures which were caused by attack of the metal by ammonia
Hydrogen embrittlement
Small quantities of hydrogen present inside certain metallic materials make the latter brittle and susceptible to sub-critical crack growth under stress. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 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 (or hydrogen grooving) is the process by which various metals most importantly high-strength Steel, become brittle and crack following exposure
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. Melting is a process that results in the phase change of a substance from a Solid to a Liquid. Casting is a manufacturing process by which a liquid material is (usually poured into a mold which Welding is a fabrication process that joins materials usually Metals or Thermoplastics by causing coalescence. A product's service life is its expected lifetime or the acceptable period of use in service 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. The yield strength or yield point of a Material is defined in Engineering and Materials science as the stress at which a material Hydrogen may cause serrated yielding in certain metals such as Niobium, Nickel and some steels (3). Niobium (naɪˈoʊbiəm or columbium (/kəˈlʌmbiəm/ is a Chemical element that has the symbol Nb and Atomic number 41 Nickel (ˈnɪkəl is a metallic Chemical element with the symbol Ni and Atomic number 28 Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0
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. Titanium (taɪˈteɪniəm is a Chemical element with the symbol Ti and Atomic number 22 Zirconium (zɚˈkoʊniəm /ˌzɝˈkoʊniəm/ is a Chemical element with the symbol Zr and Atomic number 40 Vanadium (vəˈneɪdiəm is a Chemical element that has the symbol V and Atomic number 23 Niobium (naɪˈoʊbiəm or columbium (/kəˈlʌmbiəm/ is a Chemical element that has the symbol Nb and Atomic number 41
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 Silver Bridge was an Eyebar Chain Suspension bridge built in 1928 and was named for the color of its Aluminum paint The crack was one 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.
See also
References
- Mars G. Forensic Engineering is the investigation of materials, products, Structures or components that fail or do not operate/function as Fracture mechanics is the field of Mechanics concerned with the study of the formation of cracks in materials Structural failure refers to loss of the load -carrying capacity of a component or member within a Structure or of the structure itself Fontana, Corrosion Engineering, 3rd Edition, McGraw-Hill, Singapore,1987
- A. R. Troiano, Trans. American Society for Metals, 52 (1960),54
- T. K. G. Namboodhiri,Trans. Indian Institute of Metals, 37 (1984),764
- 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
- N. J. Petch and P. Stables, Nature, 169 (1952), 842
- R. A. Oriani, Berichte der Bunsen-Gesellschaft für physikalische Chemie, 76 (1972),705
- C. D. Beachem, Metall. Trans. ,3 (1972),437
- D. G. Westlake, Trans. ASM,62 (1969), 1000
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