Environmental Assisted Cracking

8/9/2019 Environmental Assisted Cracking

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Manipal Institute of Technology, Manipal

Seminar Report onEnvironmental assisted Cracking

Date: 24/2/2009 SUBMITTED BY:

Shivaprasad.P

080922004.

M.Tech CAMDA.


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Introduction:It was argued that fracture-toughness parameter K IC, value represents the lowest possible materialtoughness corresponding to the maximum allowable stress-intensity factor that could be appliedshort of fracture. Yet, failures are known to occur when the initial stress-intensity-factor level isconsiderably below K IC . These failures arise because cracks are able to grow to criticaldimensions with the initial stress-intensity level increasing to the point where K = K IC .Such crack extension can occur by a number of processes. Subcritical flaw growth mechanismsinvolving a cooperative interaction between a static stress and the environment include stresscorrosion cracking (SCC), hydrogen embrittlement (HE), and liquid-metal embrittlement (LME).Fracture mechanics tests can provide a characterization of the phenomenology of EAC such asthe rate of crack advance and the associated crack velocity dependence on temperature, pressure,and concentration of aggressive species. Surface chemistry and electrochemistry studies areneeded to identify the rate limiting processes, whereas metallurgical investigations are importantto identify what alloy compositions and microstructures are susceptible to the cracking processand what fracture micro mechanisms are operative. These processes are mutually dependent onone another. By contrast, final fracture can result from several mutually independent fracturemechanisms; in this instance, the fastest process will dominate the fracture mode.

Embrittlement models:These are the models which describe the SCC, HE,LME process. The need for so many modelsis to attests to the complexity of EAC phenomena. Yet, certain clear similarities and differencesin proposed mechanisms are becoming apparent and have led some investigators to conclude thatthese embrittling processes are often interrelated consequently,EAC may occur by either SCC orHAC processes or by both. The latter condition is illustrated by the iron plus water system; inthis instance, the chemical reaction between Fe and H 20 involves the liberation of hydrogen,which then introduces the basis for HAC.

Hydrogen-Embrittlement Models: The embrittlement of metal or alloy by atomic hydrogen involves the ingress of hydrogen into acomponent, an event that can seriously reduce the ductility and load-bearing capacity, causecracking and catastrophic brittle failures at stresses below the yield stress of susceptiblematerials . Hydrogen embrittlement occurs in a number of forms but the common features are anapplied tensile stress and hydrogen dissolved in the metal.Hydrogen can also be picked up from the electrode cover material or from residual water duringwelding. After diffusing into the base plate while the weld is hot, embrittlement occurs upon-cooling by a process referred to as cold cracking in the weld heat affected zone.Hydrogen may also enter the material as a result of electroplating (i.e., cathodic charging), whichcontributes to early failure. It is ironic that the electroplating process, designed to protect amaterial against aqueous environments and SCC, actually undermines fracture resistance of thecomponent by simultaneously introducing another cracking process.Hydrogen pickup and associated embrittlement can also be introduced into the metal whenever asample under stress is exposed to a hydrogen gas atmosphere. It should be noted thatembrittlement does not occur as a result of prior exposure to Hydrogen gas in the absence of stress. Hydrogen can diffuse rapidly through the lattice because of its small size. Calculationshave shown that hydrogen transport rates in association with dislocation motion can be severalorders of magnitude greater than that associated with lattice diffusion. Hence Hydrogen


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embrittlement is considered as a major contributing factor in a cracking process even though theEAC rate is greater than the rate of hydrogen diffusion through the lattice.Hydrogen tends to accumulate at grain boundaries, inclusions, voids, dislocation arrays, andsolute atoms. HAC is controlled by those hydrogen accumulation sites that are most sensitive tofracture. From Fig.2 it is observed that the cracking process can involve cleavage, intergranular,or ductile (micro void coalescence) fracture micro mechanisms.

The hydrogen-embrittling process, depends on three major factors: The original location and form of the hydrogen. The transport reactions involved in moving the hydrogen from its source to the' locations

where it reacts with the metal to cause embrittlement. The embrittling mechanism.

Now what is embrittling mechanism?There are number of theories that have been proposed. According to one model, called

the "planar pressure mechanism," the high pressures developed within internal hydrogengas pores of charged material cause cracking. Although this mechanism appears valid forhydrogen-charged steels, it cannot be operative for the embrittlement of steel by low-pressurehydrogen atmospheres. In the latter situation, there would be no thermodynamic reason for a lowgas pressure external atmosphere to produce a high gas pressure within the solid.Different type HE model was proposed by Beachem and discussed by

Hirth, among others. Beachem suggested that the presence of hydrogen in the metal latticegreatly enhances dislocation mobility at very low applied stress levels. Brittle behavior is thenenvisioned to occur as a result of extensive but highly localized plastic flow, which can occur atvery low shear stress levels.

Fig1. Variousprocesses involvedin the hydrogenembrittlement of ferrous alloys.


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Fig 2. Flow diagramdepicting hydrogensources, transportpaths, destinations,and induced fracturemicromechanisms.

Fig 3. Schematic representationof different hydrogen-inducedfracture paths as a function of stress level. (a) High K levelgenerates microvoidcoalescence;(b) intermediate K levelgenerates transgranular fractureby a quasi cleavagemechanism;(c) low K level leads tointergranular fracture path.


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Stress Corrosion Cracking Models :Stress corrosion cracking is a failure mechanism that is caused by environment, susceptiblematerial, and tensile stress. Stress corrosion cracking is an insidious type of failure as it canoccur without an externally applied load or at loads significantly below yield stress. Thus,catastrophic failure can occur without significant deformation or obvious deterioration of thecomponent. Pitting is commonly associated with stress corrosion cracking phenomena.The film rupture model, involving anodic dissolution at the crack tip, is capable of explainingmost examples of intergranular SCC. The principal feature of this model is that the protectivesurface film in the vicinity of the crack tip is ruptured by localized plastic flow. Consequently, anelectrolytic cell is created with the bare metal at the crack tip serving as the anode and theunbroken protective surface film serving as the cathode. The exposed bare metal is thensubjected to rapid anodic dissolution, thereby allowing the crack to advance, Since the protectivefilm is generally regarded to be passive in character, the rate of anodic dissolution and associatedcrack extension will depend, in part, on the repassivation rate.

When the passivation rate is low, the crack tip becomes blunt because of excessive dissolution on the crack sides; when the passivation rate is high, the amount of crack-tip penetration per film-rupture event is minimized.

Fig 4.Diagram showing film-rupture model.Localized plastic flow at crack (a)results in numerous film-rupture eventsassociated with transient anodic dissolution(b).

Liquid-Metal Embrittlement:Liquid metal embrittlement is the decrease in ductility of a metal caused by contact with liquidmetal. The decrease in ductility can result in catastrophic brittle failure of a normally ductilematerial. Very small amounts of liquid metal are sufficient to result in embrittlement. Whenmany ductile metals are coated with a micron-thin layer of certain liquid metals.Intergranular or transgranular cleavage fracture are the common fracture modes associated withliquid metal embrittlement. However reduction in mechanical properties due to decohesion canoccur. This results in a ductile fracture mode occurring at reduced tensile strength.Fracture times are extremely short, with crack velocities as high as 500 cm/s being reported foraluminum alloys and brass in the presence of liquid mercury (Hg).


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Liquid-metal embrittlement is a result from liquid-metal chemisorptions induced reduction in thecohesive strength of atomic bonds in the region of a stress concentration.The liquid-metal atom (L) is believed to reduce the interatomic bond strength between solidatoms, S 1 and S 2 at the crack tip, thereby causing bond rupture to occur at reduced stress levels.Once the S 1-S2 bond is broken, liquid-metal atoms then reduce the strength of the atomic bondbetween solid atoms S 1 and S 3 with local fracture continuing at a rapid pace.

Fig 5. Model for liquid-metal embrittlement. Liquid-metal atomL reduces interatomic bond strength between atoms S 1 and S 2. S1 and S 3. and so on.

Variables Affecting Environment-Assisted Cracking:EAC depends on a number of factors, including alloy chemistry and thermo mechanicaltreatment, the environment itself, temperature, and pressure.

1. Alloy Chemistry and Thermo mechanical Treatment:Many studies have been conducted to examine the relative EAC propensity of different familiesof alloys and specific alloys thermo mechanically treated to different specifications . Studiesindicate that overaging is the most effective way to accomplish improvement of EACresistance . Toughness is improved while strength decreases as a result of the overaging process.The effect of overaging on 7079 and 7178 aluminum alloys is shown in Fig.6. Although Stage Iin the 7079 alloy is shifted markedly to higher K levels, reflecting a sharp increase in K IEAC ,thegrowth rates associated with Stage II cracking remain relatively unchanged. Consequently, themajor problem of very high Stage II cracking rates in this material remains even after overaging.By contrast, preliminary data for the 7178 alloy show a marked decrease in Stage II crack growthrate with increasing aging time, while Stage I cracking is shifted to a much lesser extent. Itwould be most desirable to have the overaging treatment effect a simultaneous lowering of theStage II cracking rate and a displacement of the Stage I regime to higher K levels. This mayprove to be the case in other alloy systems.In general, K IEAC values tend to be greater in materials possessing higher K 1C levels and loweryield strength.If the relative degree of susceptibility to environment-assisted cracking is defined by the ratioK IEAC /K IC ., the generally observed trend is for K IEAC /K IC to decrease with increasing alloystrength. That is, K IEAC values drop faster than K 1C values with increasing strength.


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Most often, hydrogen and stress corrosion cracks follow an intergranular path in the importanthigh-strength steel, titanium, and aluminum alloys . Consequently, environment-assistedcracking in wrought alloys is usually of greater concern in the short transverse directionthan in other orientations. As such, EAC orientation sensitivity parallels K IC orientationdependence.

Fig.6. Effect of overaging on EAC (salt water) in 7xxx series aluminum alloys: (a) 7079 alloyshows pronounced shift of Stage I behavior to higher K levels while (daldt) II remains relativelyconstant; (b) 7178 alloy shows sharp drop in (daldt) lI .

2.Environment

The kinetics of crack growth and the threshold K IEAC level depend on the material-environmentsystem. The complex aspects of the material-environment interaction can be greatly simplifiedby treating the problem from the phenomenological viewpoint in terms of a single mechanism,environmental-assisted cracking.

This concept is supported by Speidel's results shown in Fig.7, which reveal parallel Stage I and IIresponses for the 7075 aluminum alloy in liquid mercury and aqueous potassium iodideenvironments. The liquid metal represents a more severe environment for this aluminum alloy, but the phenomenology is the same. Furthermore, we see that the alloy in the overaged conditionis more resistant to the liquid-metal EAC. However, with increasing moisture content, crackingdevelops with increasing speed. Consequently, EAC in aluminum alloys may take the form of stress corrosion cracking and liquid-metal embrittlement but not gaseous hydrogenembrittlement.


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Fig 7. Environment-assisted crackingwith liquid mercury and aqueous iodidesolution in 7075 aluminum alloy

3. Temperature and Pressure.

EAC processes involve chemical reactions, it is to be expected that temperature and pressurewould be important variables. Test results, such as those shown in Fig. , for hydrogen cracking ina titanium alloy show the strong effect of temperature on the Stage II cracking rate. These datacan be expressed mathematically :

Where is activation energy for the rate-controlling process.

The apparent activation energy may then be compared with other data to suggest the nature of the rate-controlling process.It has been found that the apparent activation energies for the cracking of high-strength steel in water andhumidified gas are both about 38 kJ/mol, which corresponds to the activation energy for hydrogendiffusion in the steel lattice. The apparent activation energy for Stage II cracking in the presence of gaseous hydrogen is only 16 to 17kJ/mo1. Since the embrittling mechanism appears to be the same for theboth environments. The change in probably reflects differences in the rate-controllinghydrogen-transport process. In this regard, note that the cracking rate in gaseous hydrogenis higher than that in water.The increase in Stage II crack growth rate with increasing pressure can be describedmathematically


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Increased pressure enhances hydrogen transport, which in turn increases the cracking rate.

Life and crack-length calculations :Kinetic crack growth data can be integrated to provide estimates of component life and crack length as a function of time. The effective steady-state cracking rate is controlled by the slowestprocess acting in Regions I, II, and III. If one ignores the contribution of then the controlling crack growth rate is given by

or

rearrangement of terms, the time devoted to steady-state cracking is given by:

To solve above equation, expressions for are needed in terms of K and the crack length a . InStage I

WhereK = stress intensity factor

T = temperatureP = pressure

Since log da/dt-K plots are often linear,

where C 1and m are independent of K but may depend on T, P, and environment. For Region II

The lack of K dependence in and the fact that C 2 depends on T, P, and environment . C 2 can beevaluated by


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From above equations it is possible to calculate the length of a crack at any given time, once thevarious constants are determined from experimental data.One additional subtle point should be made regarding the life computation. It should berecognized that the life of a component or test specimen will depend on the rate of change of thestress-intensity factor with crack length dK/da . Consequently, for the same initial K level, thesample with the lowest dK/da characteristic will have the longest life. That is, changingspecimen geometry would alter the time to failure.

Fracture mechanics test methods:For determination of K IC : Precracked samples were placed in the environmental chamber andstressed in bending at different initial K levels by a loaded scrub bucket hung from the end of thecantilever beam.For each test condition associated with a different initial K value (always less than K IC the timeto failure was recorded.

FIG..8.Environment-assisted cracking teststand. Specimen is placed inenvironment chamber at A and loadedby weights placed in scrub bucket.

For determining K IEAC :EAC data have been obtained with a modified compact specimen configuration (Fig. 9)In thisinstance, a screw, engaged in the top half of the sample, bears against the bottom crack surface.This produces a crack-opening displacement corresponding to some initial load. In this manner,the specimen is self-stressed and does not require a test machine for application of loads. As thecrack extends by environment-assisted cracking, the load and, hence, the K level drop under the


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prevailing constant displacement condition. The crack finally stops when the K level drops belowK IEAC .Consequently, only one specimen is needed to determine K IEAC .Such a test is very easy toconduct and very portable, since the self-stressed sample can be carried to any environmentrather than vice versa.

FIG.9. Modified compact tensionsample with threaded bolt bearing on loadpin. Initial crack opening displacementdetermined by extent to which bolt is engaged.

Engage the screw thread to produce a given crack-opening displacement and place the specimenin the environment. Samples are .examined periodically to determine when the crack stopsgrowing. The K IEAC value is then defined by the residual applied load remaining after the crack has ceased growing and the final crack length as seen on the fracture surface.

Conclusion:Stress corrosion is caused by the combination of quasi static or cyclic stress and a corrosiveenvironment. If the material undergoes anodic dissolution at the crack tip, a stress corrosioncracking (SCC) mechanism dominates with the aid of the static or cyclic stress.

If K < K IEAC then failure is not expected in an aggressive or corrosive fluid If K IEAC


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Reference: Deformation and Fracture mechanics of Engineering materials by Richard. W. Hetrzberg

, John Wiley and Sons Inc Publication, 4 th edition 1995 FRACTURE MECHANICS by Nestor Perez 2 nd Edition Reprint 2004 Kluwer

Academic Publishers http://www.corrosion-doctors.org/ http://www.uni-saarland.de/fak8/wwm/research/phdbarnoush/ http://www.materialsengineer.com/A-failue.htm

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Environmental Assisted Cracking