- Stress corrosion cracking
Stress corrosion cracking (SCC) is the unexpected sudden failure of normally
ductilemetals or tough thermoplasticssubjected to a tensile stressin a corrosive environment, especially at elevated temperature (in the case of metals). SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure. [ASM International, "Metals Handbook (Desk Edition)" Chapter 32 (Failure Analysis), American Society for Metals, (1997) pp 32-24 to 32-26 ]
The stresses can be the result of the crevice loads due to
stress concentration, or can be caused by the type of assembly or residual stresses from fabrication (eg. cold working); the residual stresses can be relieved by annealing.
stainless steels and aluminium alloys crack in the presence of chlorides, mild steelcracks in the present of alkali (boiler cracking) and nitrates, copperalloys crack in ammoniacal solutions ( season cracking). This limits the usefulness of austenitic stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. Worse still, high-tensile structural steels crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially containing chlorides. With the possible exception of the latter, which is a special example of " hydrogen cracking", all the others display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions where fracture mechanicspredicts that failure should not occur. That is, in the presence of a corrodent, cracks develop and propagate well below "K"Ic. In fact, the subcritical value of the stress intensity, designated as "K"Iscc, may be less than 1% of "K"Ic, as the following table shows:
Polymers can also be attacked by certain reagents, and if under load, then cracks will grow just as in metals and alloys. Perhaps the oldest known example is the ozone crackingof rubbers, where traces of ozone in the atmosphere attack double bondsin the chains of the materials. Elastomers with double bonds in their chains include natural rubber, nitrile rubberand styrene-butadienerubber. They are all highly susceptible to ozone attack, and can cause problems like car fires (from rubber fuel lines) and tyre blow-outs. Nowadays, anti-ozonants are widely added to these polymers, so the incidence of cracking has dropped. However, not all safety-critical rubber products are protected, and since only ppbof ozone will start attack, failures are still occurring. Another highly reactive gas is chlorine, which will attack susceptible polymers such as acetal resinand polybutylenepipework. There have been many examples of such pipes and acetal fittings failing in properties in the USA as a result of chlorine-induced cracking. Essentially the gas attacks sensitive parts of the chain molecules (especially secondary, tertiary or allylic carbon atoms), oxidising the chains and ultimately causing chain cleavage. The root cause is traces of chlorine in the water supply, added for its anti-bacterial action, attack occurring even at parts per milliontraces of the dissolved gas.
Most step-growth polymers can suffer
hydrolysisin the presence of water, often a reaction catalysed by acidor alkali. Nylonfor example, will degrade and crack rapidly if exposed to strong acids. Polycarbonateis susceptible to alkali hydrolysis, the reaction simply depolymerising the material. Polyestersare prone to degrade when treated with strong acids, and in all these cases, care must be taken to dry the raw materials for processing at high temperatures to prevent the problem occurring.
Many polymers are also attacked by
UV radiationat vulnerable points in their chain structures. Thus polypropylenesuffers severe cracking in sunlightunless anti-oxidants are added. The point of attack occurs at the tertiary carbon atom present in every repeat unit, causing oxidation and finally chain breakage.
The subcritical nature of propagation may be attributed to the
chemical energyreleased as the crack propagates. That is,
:"elastic energy released + chemical energy = surface energy + deformation energy"
The crack initiates at "K"Iscc and thereafter propagates at a rate governed by the slowest process, which most of the time is the rate at which corrosive ions can diffuse to the crack tip. As the crack advances so "K" rises (because crack length appears in the calculation of stress intensity). Finally it reaches "K"Ic , whereupon fast fracture ensues and the component fails. One of the practical difficulties with SCC is its unexpected nature.
Stainless steels, for example, are employed because under most conditions they are 'passive', i.e. effectively inert. Very often one finds a single crack has propagated while the rest of the metal surface stays apparently unaffected.
SCC caused the catastrophic collapse of the
Silver Bridgein December 1967, when an eyebar suspension bridge across the Ohio river at Point Pleasant, WV, suddenly failed. The main chan joint failed and the whole structure fell into the river, killing 46 people in vehicles on the bridge at the time. Rust in the eyebar joint had caused a stress corrosion crack, which went critical as a result of high bridge loading and low temperature. The failure was exacerbated by a high level of residual stressin the eyebar. The disaster led to a nationwide reappraisal of the state of the nation's bridges. [Lewis, Peter Rhys, Reynolds, K, and Gagg, C, "Forensic Materials Engineering: Case studies", CRC Press (2004).]
nylon 6,6connector in a diesel fuelline fractured when a small drop of sulfuric acidleaked from the lead-acid batteryoverhead. It formed a small crack which grew until fuel started leaking. As the critical crack grew, leakage increased until the line parted and fuel fell unrestricted into the road, and caused several crashes to other motorists. The driver of the vehicle should have spotted the leak before it became critical. [Peter R Lewis and Sarah Hainsworth, "Fuel Line Failure from stress corrosion cracking", Engineering Failure Analysis,13 (2006) 946-962.]
acetal resinjunction in a water supply system suddenly fractured over a weekend, causing substantial damage to computers stored below in the building. The junction failed at injection mouldingdefects by chlorineattack of the polymer. The water supply contained only 5 ppm of chlorine, but was sufficient to trigger stress corrosion cracking.
Forensic materials engineering
Forensic polymer engineering
Environmental stress fracture
* [http://www.key-to-metals.com/Article17.htm Stress corrosion cracking of aluminum alloys]
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