failure analysis of heat treated steel components

General Aspects of Failure AnalysisWaldek Wladimir Bose-Filho and Jose Ricardo Tarpani,Universidade de Sao PauloMarcelo Tadeu Milan, Instituto de Materiais Tecnologicos doBrasil Ltda.

FAILURE ANALYSIS is the process ofcollecting, examining, and interpreting damageevidence. The objective is to understand thepossible conditions leading to a failure andperhaps prevent similar failures in the future.A failure analysis should provide a well-documented chain of evidence that eitherexcludes or supports possible interpretation ofthe damage evidence. Clear-cut conclusionsdo not always occur, and the tendency ofdeveloping preconceived interpretations shouldbe avoided.Various publications (e.g., Ref 16) describe

the guidelines and methods of failure analysis,and this chapter briefly outlines some of thebasic aspects of failure analysis. The first sectiondescribes some of the basic steps and majorconcerns in conducting a failure analysis. Thisis followed by a brief review of failure typesfrom fracture, distortion, wear, and corrosion.Fracture is a common damage feature, becausethe vast majority of mechanical failures involvecrack propagationtypically classified as duc-tile, brittle, and fatigue, as briefly describedin more detail. Distortion, wear, and corrosionalso can be important damage factors in failureanalysis.

General Guidelines of Failure Analysis

For a complete evaluation, the sequence ofstages in the investigation and analysis of fail-ure, as detailed in Ref 5, is as follows (Ref 2):

1. Collection of background data and selectionof samples

2. Preliminary examination of the failed part3. Nondestructive and mechanical testing4. Selection, identification, preservation, and/

or cleaning of specimens

5. Macroscopic examination and analysis andphotographic documentation

6. Microscopic examination and analysis7. Selection, preparation, examination, and

analysis of metallographic specimens8. Determination of failure mechanism9. Chemical analysis10. Fracture mechanics analysis11. Testing under simulated service conditions12. Analysis of all the evidence, formulation of

conclusions, and writing the report

These stages or steps are briefly outlined asfollows.Collection of Background Data and

Selection of Samples. There are basicallythree fundamental principles to be carefullyfollowed when collecting damage evidencefrom a fractured material (Ref 2):

Locate the origin(s) of the fracture. Thewhole fracture surface should be visuallyinspected to identify the location of thefracture-initiating site(s) and to isolate theareas in the region of crack initiation thatwill be most fruitful for further micro-analysis. Where the size of the failed partpermits, visual examination should be con-ducted with a low-magnification wide-fieldstereomicroscope having an oblique sourceof illumination (Ref 3).

Do not put the mating pieces of a fractureback together, except with considerable careand protection. Protection of the surfaces isparticularly important if electron micro-scopic examination is to be part of the pro-cedure (Ref 2). Appropriate packaging offailed components for shipping is equallyimportant. Wrapping them directly into aplastic bag, or placing pieces directly intoa plastic bottle or container, can intro-duce unwanted hydrocarbon contaminants.

Failure Analysis of Heat Treated Steel Components

L.C.F. Canale, R.A. Mesquita, and G.E. Totten, editors, p 111-132

DOI: 10.1361/faht2008p111

Copyright 2008 ASM International

All rights reserved.

www.asminternational.org

Fingerprints on the failed surfaces can alsointroduce contamination (Ref 4);

Do not conduct a destructive testing withoutconsiderable thought. Alterations such ascutting, drilling, and grinding can ruinan investigation if performed prematurely.Destructive testing must be performed onlyafter all possible information has beenextracted from the part in the original con-dition and after all significant features havebeen carefully documented by photography(Ref 2).

Preliminary Examination of the FailedPart. In addition to locating the failure origin,visual analysis is necessary to reveal stress con-centrations, material imperfections, presence ofsurface coatings, case-hardened regions, welds,and other structural details that contribute tocracking. A careful macroexamination is neces-sary to characterize the condition of the fracturesurface so that the subsequent microexaminationstrategy can be determined. Corrodents oftendo not penetrate the crack tip, and this regionremains relatively clean. The visual macro-analysis will often reveal secondary cracks thathave propagated only partially through a crac-ked member. These part-through cracks can beopened in the laboratory and are often in muchbetter condition than the main fracture (Ref 3).Nondestructive and Mechanical Testing.

A wide variety of nondestructive testing isavailable, including dye penetrant, ultrasonics,x-ray, and eddy current, which can help inthe failure analysis task in order to unveileven subtle and/or internal defects in a part.Mechanical property tests are also ready to use,ranging from a sample hardness test to elevated-temperature tensile and impact testing. Thesetests are often used to determine if degradation isrelated to fabrication or to the service environ-ment. Sometimes, a standard test can be adaptedto simulate manufacturing or in-service condi-tions more closely (Ref 4).Selection, Identification, Preservation,

and/or Cleaning of Specimens. Unless afracture is evaluated immediately after it isproduced, it should be preserved as soon aspossible to prevent attack from the environment.The best way to preserve a fracture is to dry itwith a gentle stream of dry compressed air, thenstore it in a desiccator, a vacuum storage vessel,or a sealed plastic bag containing a desiccant.However, such isolation of the fracture is oftennot practical. Therefore, corrosion-preventive

surface coatings must be used to inhibit oxida-tion and corrosion of the fracture surface. Theprimary disadvantage of using these surfacecoatings is that fracture surface debris, whichoften provides clues to the cause of fracture, maybe displaced during removal of the coating.However, it is still possible to recover the sur-face debris from the solvent used to removethese surface coatings by filtering the spentsolvent and capturing the residue. In regard tocleaning techniques, fracture surfaces exposedto various environments generally contain un-wanted surface debris, corrosion or oxidationproducts, and accumulated artifacts that must beremoved before meaningful fractography canbe performed. Before any cleaning proceduresbegin, the fracture surface should be surveyedwith a low-power stereobinocular microscope,and the results should be documented with ap-propriate sketches or photographs. Low-powermicroscope viewing will also establish theseverity of the cleaning problem and should alsobe used to monitor the effectiveness of eachsubsequent cleaning step. It is important toemphasize that the debris and deposits on thefracture surface can contain information that isvital to understanding the cause of fracture. Themost common techniques for cleaning fracturesurfaces, in order of increasing aggressiveness,are (Ref 3):

Dry air blast or soft organic-fiber brushcleaning

Replica stripping Organic-solvent cleaning Water-based detergent cleaning Cathodic cleaning Chemical-etch cleaning

Macroscopic Examination and Analysisand Photographic Documentation. Moreoften than not, the investigation starts with alow-magnification, if any, observation of thefailed part. This visual examination can oftenquickly answer questions such as: What was themode of failure? Did it crack, or was there auniform or pitting corrosion failure? Did theprotective oxide film break down? Were thewelds visibly contaminated? A variable magni-fication stereoscope equipped with a ring lightand directional fiberoptic lighting is a powerfultool for macroscopic visual examination.Contemporary stereoscopes can operate over arange of 2.5 to 50 (Ref 4).Microscopic Examination and Analysis.

Once the area of interest is isolated, a smaller

112 / Failure Analysis of Heat Treated Steel Components

portion can be cut from the sample and mountedfor metallographic polishing and microscopicexamination. The microstructure of specimensmay be enhanced by a wide variety of metallo-graphic techniques that include, for example,heat tinting, stain etching, anodizing, and illu-mination by bright-field and polarizing light.Optical microscopic examination generally be-gins at 50 magnification and continues through1000 or even 1500 . Higher levels are bestsupplemented by differential interference con-trast lighting, which allows theoretical resolu-tion of features as fine as one-third of amicrometer. Features that are important torecognize include the uniformity and size of thegrain structure, the size distribution and shape ofintermetallic particles, and inclusions. Scanningelectron microscopy (SEM) is most usefulwhere extreme depth of focus and high magni-fications are needed. Fractures generally arecomplex, undulating surfaces that are difficult toimage, and an optical microscope can only focuson a very narrow region because of the veryshallow depth of field. However, the SEM excelsat imaging fracture surfaces, and it can beoperated in many different modes. The mostcommon mode is secondary electron imaging,which provides a detailed, high-depth-focusimage that is easy to interpret. BackscatteredZ contrast is used to identify regions ofimpurities within a matrix. High-atomic-numberspecies produce a light appearance, whereaslow-atomic-number species create a darkerappearance. The topographic backscatteredmode enhances the surface topography of thesample and accentuates height or elevation dif-ferences on a fracture surface. The characteristicx-rays can be detected and analyzed according totheir energy. This is called energy-dispersivex-ray analysis. The x-ray wavelength corre-sponds to the presence of a specific element, andits amplitude corresponds to the quantity ofsuch element. This technique allows quantita-tive characterization of elements within a givenphase. Bulk chemistry is typically analyzedduring failure analysis to verify conformancewith industry-accepted chemical limits. In thecase of reactive metals, light elements canembrittle them due to improper processing orservice conditions (Ref 4).Selection, Preparation, Examination, and

Analysis of Metallographic Specimens. Oneof the worst things that can happen to the sampleis inadequate handling, examination, or pack-aging. It is imperative that the sample remains in

an undisturbed state prior to analysis, becausethe culprit is often found in minute surfacefeatures or traces of impurities. Fracture surfacesmust remain untouched so that high-magnifica-tion images can accurately determine the failuremode. The sample must be removed carefully.Important evidence can be destroyed by over-heating or by allowing adjacent fracture surfacesto fret or rub together during sectioning. Theideal method would be to unbolt the componentor to provide adequate support so that a slow-speed saw can be used to cut out the component.However, sawing lubricants can mask ordestroy residual chemicals or elements on thefailed surface, so precautions become extremelynecessary. If the component has failed in themiddle of a large area, more aggressive cutting/sectioning techniques may be warranted, butkeep a good distance from the failed region(Ref 4).Determination of Failure Mechanism

(with Adapted Text from Ref 7). A thoroughinvestigation should ensure that all damage isfound and documented, because multiple modesand mechanisms may be present in most real-world failure analyses. It is also important torecognize that many unique mechanisms may bedriven by more than one environmental factor,such as stress, temperature, corrosion, wear,radiation, or electrical factors.The term failure mechanism, or damage

mechanism, is meant to convey the specificseries of events that describe both how thedamage was incurred and the resulting con-sequences. Examples of damage mechanismsinclude high-temperature creep, hydrogenembrittlement, stress-corrosion cracking, andsulfidation. A failure or damage mechanismdescribes how damage came to be present.This definition of failure mechanism also

should not be confused with the description ofthe physical characteristics of damage observed.For example, intergranular fracture, buckling,transgranular beach marks, and pits can all bethought of as damage modes. The term damagemode or failure mode is best used to describewhat damage is present.Much confusion has occurred because of

the tendency of engineers to use the termsmechanism and mode interchangeably; in doingso, it is unclear that two distinct characteristicsneed to be assessed. Sometimes this occursbecause, within a given system, the samewording is used to describe both the failuremode and mechanism. For example, pitting

General Aspects of Failure Analysis / 113

describes a damage mode because the surface ofa material is pitted. In certain systems, pitting isalso a possible damage mechanism. In boilertubing, for example, a pitting damage mecha-nism describes a specific localized corrosionmechanism where pits form through dissolutionof metal either from low-pH or high-oxygenconditions. The metal under the pit surfaces isunaffected. In this system, pitting is a specificdamage mechanism, but many other damagemechanisms also result in a pitting damagemode in boiler tubing, including hydrogendamage, phosphate corrosion, and causticgouging.It is helpful to be as specific as possible in

differentiating damage mechanisms in a system.For example, fatigue is often identified as botha damage mode and a damage mechanism. Afatigue damage mode is the observable damagethat occurs under fatigue loading cycles (e.g.,the presence of beach marks). Classifying fati-gue as a damage mechanism is not necessarilycomplete because it does not point to the specificenvironment that results in a fatigue damagemode. Instead, specific mechanisms that canresult in a fatigue damage mode must beexamined. Examples include corrosion fatigue,thermomechanical fatigue, creep-fatigue inter-action, and mechanical fatigue.Determination of damage mechanisms starts

by characterizing the component(s) being ex-amined. It is impossible to know what is dif-ferent about a failure without first understandingwhat is expected from unfailed components.In general, the analyst should obtain as muchinformation as possible about a part and itsbackground during the course of an investiga-tion. Some key questions worth evaluatinginclude:

What was the part supposed to do? How wasit supposed to work?

How was the part made? What processeswere involved in its manufacture (e.g.,forming, joining, and heat treatment)? Whatproperties were expected at the time ofmanufacture?

What were the specified dimensions andtolerances for the as-manufactured part?

How was the part installed? To what service environment(s) was the part

exposed? Typical environments to examineinclude operating temperatures, stresses(steady state or slowly rising and cyclic),oxidizing/corrosive environments, and wear

environments. What properties were re-quired during service? How were propertiesexpected to change from service exposure?

How was the part inspected during serviceintervals? What information was foundduring these inspections?

What material characteristics were specifiedfor the part (e.g., composition, strength,hardness, impact, and stress-rupture proper-ties)? What specifications, industry stan-dards, and contracts govern these properties?

What were the various ways the part couldfail?

The last item is a key question to repeatedlyask throughout a failure investigation. The list ofvarious damage mechanisms by which a part canfail can be narrowed down through two basicconcepts (Ref 7). Limiting conditions that refinethe scope of explanations for observed damagecan be defined by using the following two rulesof thumb:

When the impossible is eliminated, whateverremains, however improbable, must beconsidered (Sherlock Holmes rule).

When two or more explanations exist for asequence of events, the simple explanation ismore likely to be the correct one (Occamsrazor).

Chemical Analysis. In a failure investiga-tion, routine analysis of the material is usuallyrecommended. There are two main categories ofchemical analysis that are often used by failureanalysts:

Bulk composition evaluation: often per-formed in order to determine whether thecorrect alloy was used in the subject com-ponent

Microchemical analysis: to find evidence ofcontamination, to evaluate the compositionof microphases revealed on ametallographicspecimen, or to evaluate corrosion products

Often, chemical analysis is done last, becausean analysis usually involves destroying a certainamount of material. There are instances wherethe wrong material was used, under which con-ditions the material may be the major cause offailure. In many cases, however, the difficultiesare caused by factors other than material com-position.Extreme care must be used in interpretation of

chemical analysis work performed as part of afailure investigation. Minor deviations from


specified composition must not be interpreted asthe sole cause of a failure, without much addi-tional supporting evidence. In most instances,slight deviations from specified compositionsare not likely to be ofmajor importance in failureanalysis. However, small deviations in alumi-num content can lead to strain aging in steel, andsmall quantities of impurities can lead to temperembrittlement. In specific investigations, parti-cularly where corrosion and stress corrosionare involved, chemical analysis of any deposit,scale, or corrosion product, or a substance withwhich the affected material has been in contact,is required to assist in establishing the primarycause of failure.Where analysis shows that the content of a

particular element is slightly greater than thatrequired in the specifications, it should not beinferred that such deviation is responsible for thefailure. Often, it is doubtful whether such adeviation has played even a contributory part inthe failure. For example, sulfur and phosphorusin structural steels are limited to 0.04% inmany specifications, but rarely can a failure inservice be attributed to sulfur content slightly inexcess of 0.04%. Within limits, the distributionof the microstructural constituents in a materialis of more importance than their exact pro-portions. An analysis (except a spectrographicanalysis restricted to a limited region of thesurface) is usually made on drillings represent-ing a considerable volume of material andtherefore provides no indication of possiblelocal deviation due to segregation and similareffects.Also, certain gaseous elements, or inter-

stitials, normally not reported in a chemicalanalysis, have profound effects on the mechan-ical properties of metals. In steel, for example,the effects of oxygen, nitrogen, and hydrogenare of major importance. Oxygen and nitrogenmay give rise to strain aging and quench aging.Hydrogen may induce brittleness, particularlywhen absorbed during welding, cathodic clean-ing, electroplating, or pickling. Hydrogen isalso responsible for the characteristic halos orfisheyes on the fracture surfaces of welds insteels, in which instance the presence of hydro-gen often is due to the use of damp electrodes.These halos are indications of local rupturethat has taken place under the bursting micro-stresses induced by the molecular hydrogen,which diffuses through the metal in the atomicstate and collects under pressure in pores andother discontinuities. Various effects due to gas

absorption are found in other metals and alloys.For example, excessive levels of nitrogen insuperalloys can lead to brittle nitride phases thatcause failures of highly stressed parts.Various analytical techniques can be used

to determine elemental concentrations and toidentify compounds in alloys, bulky deposits,and samples of environmental fluids, lubricants,and suspensions. Semiquantitative emissionspectrography, spectrophotometry, and atomic-absorption spectroscopy can be used to deter-mine dissolved metals (as in analysis of analloy), with wet chemical methods used wheregreater accuracy is needed to determine theconcentration of metals. Combustion methodsordinarily are used for determining the con-centration of carbon, sulfur, nitrogen, hydrogen,and oxygen.Wet chemical analysis methods may be

employed for determining the presence andconcentration of anions such as Cl, NO3

, andS. These methods are very sensitive.X-ray diffraction identifies crystalline com-

pounds either on the metal surface or as a massof particles and can be used to analyze corrosionproducts and other surface deposits. Minor andtrace elements capable of being dissolved can bedetermined by atomic-absorption spectroscopyof the solution. X-ray fluorescence spectro-graphy can be used to analyze both crystallineand amorphous solids, as well as liquids andgases.Stress Analysis and Fracture Mechanics

Analysis. When confronted with a cracked,fractured, or deformed component, the failureanalyst will usually seek to answer some basicquestions:

Were the loads and stresses encountered bythe part at the level anticipated duringdesign? Or did some unexpected condi-tion(s) contribute to the failure?

Was the material in the area of the crackingor deformation capable of meeting the con-ditions anticipated during design? Was theresome deficiency or discontinuity that con-tributed to the failure, or was there a localstress raiser at the critical location? Was thistaken into account by the designer?

In general, there are two types of conditions thatmay lead to structural failure:

Net-section instability, where the overallstructural cross section can no longer sup-port the applied load


The critical flaw size (ac) is exceededby some preexisting discontinuity or whensubcritical cracking mechanisms (for exam-ple, fatigue, stress-corrosion cracking, orcreep) reach the critical crack size

Failures due to net-section instability typi-cally occur when a damage process such ascorrosion or wear reduces the thickness of astructural section. This type of failure can beevaluated by traditional stress analysis or finiteelement analysis (FEA), which are effectivemethods in evaluating the effects of loading andgeometric conditions on the distribution of stressand strain in a body or structural system.However, stress analyses by traditional

methods or FEA do not easily account forcrack propagation from preexisting cracks orsharp discontinuities in the material. When apreexisting crack or discontinuity is present,the concentration of stresses at the crack tipbecomes asymptotic (infinite) when using theconventional theory of elasticity. In this regard,fracturemechanics is a useful tool, because it is amethod that quantifies stresses at a crack tipin terms of a stress-intensity parameter (K).The fracture mechanics of cracking from a dis-continuity or crack in a statically loaded com-ponent has two possible situations:

The crack reaches a critical length with rapid(brittle) separation.

The crack blunts, redistributing the stressstate, with continued loading creating a tearzone (and sharpened crack-tip radius) infront of the crack. In steels, this tear zonecan then cause the critical crack length to beexceeded, such that unstable cleavage frac-ture occurs or unstable microscale ductilefracture is induced.

Which event occurs depends on the temperatureand the loading rate, but in either event, crackpropagation is unstable (i.e., does not require anincreasing load after creation of the tear zone).Fracture mechanics is a tool to help evaluate theimplications of preexisting discontinuities orcracks.Testing under Simulated Service Con-

ditions. During the concluding stages of aninvestigation, it may be necessary to conducttests that simulate the conditions under whichfailure is believed to have occurred. Often,simulated-service testing is not practical be-cause elaborate equipment is required, and evenwhere practical it is possible that not all of the

service conditions are fully known or under-stood. Corrosion failures, for example, aredifficult to reproduce in a laboratory, and someattempts to reproduce them have given mis-leading results. Serious errors can arise whenattempts aremade to reduce the time required fora test by artificially increasing the severity of oneof the factorssuch as the corrosive medium orthe operating temperature. Similar problems areencountered in wear testing.On the other hand, when its limitations are

clearly understood, the simulated testing andstatistical experimental design analysis of theeffects of certain selected variables encounteredin service may be helpful in planning correctiveaction or, at least, may extend service life. Mostof the metallurgical phenomena involved infailures can be satisfactorily reproduced on alaboratory scale, and the information derivedfrom such experiments can be helpful to theinvestigator, provided the limitations of the testsare fully recognized.Analysis of All the Evidence, Formulation

of Conclusions, and Writing the Report.Before starting this final step, some questionsmust already be answered: Fracture surface:a. What is the fracture mode?b. Is the origin of the fracture visible?c. What is the relation between the fracture

direction and the normal or expected fra-cture directions?

d. How many fracture origins are there?e. Is there evidence of corrosion, paint, or

some other foreign material on the fracturesurface?

f. Was the stress unidirectional or was itreversed in direction?

The surface of a part:a. What is the contact pattern on the surface

of the part?b. Has the surface of the part been deformed

by loading during service or by damageafter fracture?

c. Is there evidence of damage on the surfaceof the part by manufacturing, assembling,repairing, or service?

Geometry and design:a. Are there any stress concentrations related

to the fracture?b. Is the part intended to be relatively rigid,

or is it intended to be flexible, like aspring?

c. Does the part have a basically flawlessdesign?


d. How does the partand its assemblywork?

e. Is the part dimensionally correct? Manufacturing and processing:a. Are there internal discontinuities or

stress concentrations that could cause aproblem?

b. If it is a wrought metal, does it containserious seams, inclusions, or forging pro-blems, such as end grains, laps, or otherdiscontinuities, that could have an effecton performance?

c. If it is a casting, does it contain shrinkagecavities, cold shuts, gas porosity, orother discontinuities, particularly near thesurface of the part?

d. If a weldment was involved, was thefracture through the weld itself or throughthe heat-affected zone in the parent metaladjacent to the weld? If through the weld,were these problems something like gasporosity, undercutting, underbead crack-ing, or lack of penetration? If through theheat-affected zone adjacent to the weld,how were the parent metal propertiesaffected by the heat of welding?

e. If the part was heat treated, was the treat-ment properly performed?

Material properties:a. Are the mechanical properties of the metal

within the specified range, if this can beascertained?

b. Are the properties of the metal suitable forthe application?

c. Residual and applied stress relationship.The residual-stress system that was withinthe part prior to fracture can have a pow-erful effectgood or badon the perfor-mance of a part.

d. What was the influence of adjacent partson the failed part?

e. Were fasteners tight? Assembly:a. Is there evidence of misalignment of the

assembly that could have had an effect onthe fractured part?

b. Is there evidence of inaccurate machin-ing, forming, or accumulation of toler-ances?

c. Did the assembly deflect excessively understress?

Service conditions: It is important to deter-mine if there were any unusual occurrences,such as strange noises, smells, fumes, orother happenings, that could help explain the

problem. The following questions shouldalso be considered:a. Is there evidence that the mechanism was

overspeeded or overloaded?b. Is there evidence that the mechanism was

abused during service or used under con-ditions for which it was not intended?

c. Did the mechanism or structure receivenormal maintenance with the recom-mended materials?

d. What is the general condition of themechanism?

Environmental reactions: The problemsrelated to the environment can arise anywherein the history of the part: manufacturing,shipping, storage, assembly, maintenance,and service. None of these stages should beoverlooked in a thorough investigation thatasks:a. What chemical reactions could have taken

place with the part during its history?b. To what thermal conditions has the part

been subjected during its existence? Report writing: Finally, the report analyzing

the failure should be written in a clear,concise, logical manner. It should be clearlystructured with sections covering the fol-lowing (Ref 6):a. Description of the failed itemb. Conditions at the time of failurec. Background history important to the

failured. Mechanical and metallurgical study of the

failuree. Evaluation of the material qualityf. Discussion of any anomaliesg. Discussion of the mechanism or possible

mechanisms that caused the failureh. Recommendations for the prevention of

future failures or for action to be takenwithsimilar pieces of equipment

Irrelevant data should be omitted, and,depending on the nature of the problem and thedata, not every report will need full treatmentsfor every one of the sections listed previously.Many times, the readership may include pur-chasing, operating, or accounting personnelwho are not technically trained. If this is thesituation, the report should be written so that itis comprehensible to these persons. At least,those sections of the report that bear on theirdecision-making or information needs should bewritten in language that is accessible to them.Frequently, a cover letter summarizing the mostimportant findings and the suggested action is a


good vehicle for reaching top executives whoare not as interested in the technical specifics butneed key findings and recommendations as abasis for decision making. Followup on therecommendations is frequently a difficult taskbut should be undertaken for the more criticalfailures. Cooperation between the investigator,the designer, the manufacturer, and the user iscritical in developing good, workable changes.

Fracture

The process of fracture, in general terms, canbe described in terms of the mechanisms ofcrack initiation and/or crack extension (growth).Different mechanisms may occur for crackinitiation and the subsequent process of crackgrowth. For example, crack extensionmay occurby the brittle mechanism of cleavage, eventhough extensive elongation accompanied orpreceded crack initiation. The fracture may beclassified as either ductile or brittle, dependingon whether the mechanism is describing crackinitiation or crack growth, respectively. Like-wise, the low-energy catastrophic fracture of ahigh-strength aluminum alloy by microvoidcoalescence is also difficult to classify because,although the fracture energy is low and failureinitiates by fracture or decohesion of brittleparticles, the growth and coalescence of themicrovoids occurs by plastic deformation.Another difficulty is that cleavage fracture maybe initiated by dislocation interactions that, bydefinition, involve plasticity. This is why frac-tures are sometimes difficult to logically classify(Ref 5). Therefore, it is helpful to be clearwhether fracture mechanisms are describing theprocess of crack initiation or extension. Crackextension also can be multimode over time (e.g.,fatigue crack growth followed by overload).In terms of fracture appearances (or fracture

modes, defined earlier in the section Deter-mination of Failure Mechanism in this chap-ter), a general summary of the visual andmicroscopic aspects of fracture surfaces formetallic materials is provided in Table 1 (Ref 8).Several analytical procedures are available fordistinguishing among the various types of frac-ture. For example, the presence or absence ofplastic macrodeformation can be determinedwith the unaided eye or by use of a steel scale, amachinists micrometer, or a machinists ormeasuring microscope. Differences in somedimensional attribute of parts (such as width or

thickness) at andwell away from the fracture canserve to define macrodeformation after assur-ance that both points of measurement had thesame dimension before fracture.Fracture-surface matching is also used to

determine the presence or absence of plasticdeformation. It is very important, however, toresist the temptation to fit the matching fracturesurfaces together, because this almost alwaysdestroys (smears) microscopic features. Thefracture surfaces should never actually touchduring fracture-surface matching.The origin of a fracture may be indicated by a

discoloration or by the topography of the frac-ture surface. A discolored area on a fracturesurface may be produced by a preexisting crackwhose surfaces have been corroded or oxidized.For example, the surfaces of a quench crack canbe oxidized during a subsequent tempering heattreatment; the oxide film gives a bluish-blackcolor to the surfaces of the crack. Topographicalfeatures that often reveal the origin of a fractureare either chevron or river patterns or a setof diverging ledges. If the fracture surface isessentially featureless, the presence of a shearlip can be used to locate, within limits, the originof a fracture. For example, a shear lip is notformed at the origin of a stress-corrosion crack,but when the crack begins to propagate rapidly, ashear lip is formed wherever the crack frontexits from the interior to the free surface. Beachmarks, which are associated with fatigue-initiated fractures, also provide a definite indi-cation of the crack origin; however, it should benoted that fracture surfaces having an appear-ance similar to that of the beach-mark patterncan be produced by stress corrosion.Generally, cyclic loading produces only a

single crack, which is usually located at a site ofstress concentration or of a metallurgical defect,whereas additional cracks, formed indepen-dently of themain crack and at a distance from it,may be observed on the surface of a structuralor machine component subjected to corrosionfatigue or stress corrosion.On the microscopic level, striations on the

fracture surface are unique to fatigue, and thecrack path, although normally transgranular, canbe intergranular. For example, intergranularfatigue cracking can occur in the case of a car-burized steel or in a material that has a highdensity of second-phase particles at the grainboundaries.Corrosion-fatigue and stress-corrosion cracks

may propagate transgranularly, intergranularly,


Table 1 Fracture mode identification chart

Method

Instantaneous failure mode(a) Progressive failure mode(b)

Ductile overload Brittle overload Fatigue Corrosion Wear Creep

Visual, 1 to

50

(fracture

surface)

Necking ordistortion in

direction

consistent with

applied loads

Dull, fibrousfracture

Shear lips

Little or nodistortion

Flat fracture Bright or coarsetexture,

crystalline,

grainy

Rays orchevrons

point to origin

Flat progressivezone with beach

marks

Overload zoneconsistent with

applied loading

direction

Ratchet markswhere origins

join

Generalwastage, rough-

ening, pitting, or

trenching

Stress-corrosionand hydrogen

damage may

create multiple

cracks that

appear brittle

Gouging,abrasion,

polishing,

or erosion

Galling or storingin direction of

motion

Roughened areaswith compacted

powdered debris

(fretting)

Smooth gradualtransitions in

wastage

Multiple brittle-appearing fissures

External surfaceand internal

fissures contain

reaction scale

coatings

Fracture afterlimited

dimensional

change

Scanning

electron

microscopy,

20 to

10,000

(fracture

surface)

Microvoids(dimples)

elongated

in direction of

loading

Single crack withno branching

Surface slip bandemergence

Cleavage orintergranular

fracture

Origin area maycontain an

imperfection

or stress

concentrator

Progressivezone: worn

appearance,

flat, may show

striations at

magnification

above 500

Overload zone:may be either

ductile or brittle

Path of penetra-tion may be

irregular,

intergranular, or

a selective phase

attacked

EDS mayhelp identify

corrodent(c)

Wear debris and/orabrasive can be

characterized as to

morphology and

composition

Rolling-contactfatigue appears

like wear in early

stages

Multipleintergranular

fissures covered

with reaction scale

Grain faces mayshow porosity

Metallographic

inspection,

50 to 1000

(cross

section)

Grain distortionand flow near

fracture

Irregular,transgranular

fracture

Little distortionevident

Intergranular ortransgranular

May relate tonotches at

surface or brittle

phases internally

Progressivezone: usually

transgranular

with little

apparent

distortion

Overload zone:may be either

ductile or brittle

General orlocalized surface

attack (pitting,

cracking)

Selective phaseattack

Thickness andmorphology of

corrosion scales

May showlocalized

distortion at

surface consistent

with direction of

motion

Identify embeddedparticles

Microstructuralchange typical of

overheating

Multiple inter-granular cracks

Voids formed ongrain boundaries

or wedge-shaped

cracks at grain

triple points

Reaction scalesor internal

precipitation

Some cold flowin last stages of

failure

Contributing

factors

Load exceeded thestrength of the part

Check for properalloy and proces-

sing by hardness

check or destruc-

tive testing,

chemical analysis

Loading directionmay show failure

was secondary

Short-term,high-temperature,

high-stress rupture

has ductile

appearance

(see creep)

Load exceededthe dynamic

strength of the

part

Check for properalloy and

processing as

well as proper

toughness, grain

size

Loadingdirection may

show failure was

secondary or

impact induced

Lowtemperatures

Cyclic stressexceeded the

endurance limit

of the material

Check for properstrength, surface

finish, assembly,

and operation

Prior damage bymechanical or

corrosion modes

may have

initiated

cracking

Alignment,vibration,

balance

High cycle lowstress: large

fatigue zone;

low cycle high

stress: small

fatigue zone

Attack morphol-ogy and alloy

type must be

evaluated

Severity ofexposure

conditions may

be excessive;

check: pH,

temperature,

flow rate,

dissolved

oxidants, elec-

trical current,

metal coupling,

aggressive

agents

Check bulkcomposition and

contaminants

For gouging orabrasive wear:

check source of

abrasives

Evaluate effec-tiveness of lubri-

cants

Seals or filters mayhave failed

Fretting inducedby slight looseness

in clamped joints

subject to

vibration

Bearing or materi-als engineering

design may reduce

or eliminate

problem

Watercontamination

High velocitiesor uneven flow

distribution,

cavitation

Mild overheatingand/or mild

overstressing at

elevated

temperature

Unstable micro-structures and

small grain size

increase creep

rates

Ruptures occurafter long

exposure times

Verify properalloy

(a) Failure at the time of load applicationwithout prior weakening. (b) Failure after a period of timewhere the strength has degraded due to the formation of cracks, internal

defects, or wastage. (c) EDS, energy-dispersive spectroscopy. Compiled by C.R. Morin, S.L. Meiley, and Z.B. Flanders, Packer Engineering Associates, Inc.


or by a combination of both modes. A distin-guishing feature of stress corrosion is thebranching of the main crack. If corrosion pitsor corrosion products are found only on theslow-growth region of a fracture surface, theenvironment was in all probability sufficientlycorrosive to affect the fracture mechanism.However, if evidence of corrosion is found onboth the slow-growth and fast-growth areas,some corrosion took place subsequent to frac-ture, and the environment may or may not haveinfluenced fracture.

Ductile Fracture

Ductile fracture takes place when a materialcapable of undergoing plastic deformation issubjected to stresses that culminate in its rup-ture. Macroscopically, the ductile fracture pro-cess presents some peculiarities that allow it tobe identified immediately. The first feature is thepresence of plastic deformation that may beaccompanied by neck formation. In tensiletestpieces of ductile materials, besides necking,the fracture surface presents a fibrous aspect anda cup-cone geometry, as seen in Fig. 1.

The fracture process begins in the center ofthe testpiece with microvoid nucleation alonggrain boundaries or from interfaces such as thosefound in base metal/inclusions boundaries. Asthe applied stress increases, microvoids growand coalesce, forming a crack in the center of thepart. This process, depicted in Fig. 2, ends up inrapid crack propagation by shearing of theremaining ligament of the neck region, at anangle of 45 in relation to the loading direction.It is important to emphasize that a cup-conegeometry will depend on the geometry anddimensions of the part and mechanical proper-ties of the material. Thin sheets, for instance,present neck formation and a fracture surfaceoriented at an angle of 45 in relation to theapplied load, as observed in Fig. 3. Ductilefracture takes place intergranularly, unless somesort of mechanism weakens the grain bound-aries. The microscopic aspect of the fracturesurface consists of several small ellipticalcavities, or microvoids, as depicted in Fig. 4.

Brittle Fracture

Brittle fracture occurs with little or no plasticdeformation. This type of fracture is often

Fig. 1 Ductile fracture showing the typical cup-cone geo-metry

Microvoids

Fig. 2 Schematic representation of the cup-cone geometryformation during the ductile fracture process

Fig. 3 Thin sheet testpiece of a low-carbon steel after fracture

Fig. 4 Microvoids on the fracture surface of AA6061-T1tensile testpiece


associated with materials of high strength andlow ductility or materials that were subjectedto an embrittlement process. The crack, oncenucleated, propagates very quickly in a directionperpendicular to the applied load. Figure 5 pre-sents an example of a gray cast iron testpiece thatpresented brittle fracture.Besides the mechanical properties, several

other factors may result in a brittle behavior,such as temperature, loading rate, presence ofstress concentrators, and dimensions. Low tem-peratures tend to reduce the ductility of metals,especially those possessing a body-centeredcubic structure, resulting in a typically brittlefracture. Figure 6 shows that as the temperaturedrops, the brittle aspect on the fracture surface ofimpact testpieces increases. The presence ofstress raisers or larger dimensions introduces amore severe triaxial stress state within thematerial, and thus, there is larger probability thatbrittle fracture will occur. However, it is known

that the superposition of high hydrostatic stres-ses on the material reduces the triaxiality levels,increasing ductility. High applied loading ratesare likely to make plastic deformation moredifficult because shearing processes are time-dependent, resulting in brittle behavior.Crack propagation by brittle fracture can

occur across the grains (transgranular) oralong the grain boundaries (intergranular). Inthe transgranular mode, the fracture processtakes place by cleavage along specific crystal-lographic planes. Figure 7 presents cleavageregions in a microalloyed low-carbon steel,which can be identified by flat regions on thefracture surface. Additionally, it is worth men-tioning that most parts of steels will presentalternate regions consisting of cleavage areasand microvoids, evidencing a mixed mode ofcrack propagation.In another situation, fracture can take place

intergranularly, because the grain boundary is a

Fig. 5 Tensile testpiece of gray cast iron presenting brittlefracture

Fig. 6 Fracture surfaces of SAE 4140 impact testpieces.Tested at room temperature, right, and at196 C, left

Fig. 7 (a) Cleavage region observed in low-carbon steel.(b) Magnification of the region delimited by the rec-

tangle in (a) showing an inclusion in the center of the cleavageregion


weaker path for crack propagation. Normally,this fracture mode will occur when someembrittlement process resulted in grain bound-aries being more susceptible to crack propaga-tion than the core of the grain, such as anunsuitable heat treating or by environmentalfactors. Figure 8 presents an example of inter-granular brittle fracture in an austenitic stainlesssteel SAE 316L, where grain boundaries canclearly be observed on the fracture surface.

Fatigue Fracture

According to the definition given by ASTME1823, fatigue is the process of progressivelocalized permanent structural change occurringin a material subjected to conditions that pro-duce fluctuating stresses and strains at somepoint or points and that may culminate in cracksor complete fracture after a sufficient number offluctuations. A material subjected to fatiguecan fracture at applied stresses much lower thanthose necessary to fracture the same materialunder monotonic conditions. The fluctuatingstresses can be originated from mechanical,thermal, or vibration loading conditions, and thephenomenon is responsible for more than 80%of mechanical failures of components. For morethan 150 years, the study of metals fatiguehas involved engineers, physicists, chemists,and mathematicians, and everyday this studybecomes more and more complex and impor-tant. The theory about fatigue is extremely vast,and for each question answered, another one,more instigating, appears, requiring a broadknowledge of materials science. In the followingtopics, a brief overview is given about the mainmechanisms and factors influencing the fatigue

life of a component during both the nucleationand crack propagation phases.Fatigue Crack Initiation. Generally, fatigue

cracks are initiated at free surfaces, where thereis no constraint to material deformation; how-ever, in some cases, cracks may be initiated inthe interior of the material where interfaces arepresent, such as the interface of a carburizedsurface layer and the base metal or the interfaceof an inclusion and the base metal, or from gasbubbles. In other cases, subsurface cracks werefound to nucleate below the surface where highcompressive residual stresses were introducedby shot peening or surface rolling.One of the classic models of fatigue crack

nucleation considers that when a material isunder loading (monotonic or cyclic), slips occurat the high-shear-stress planes, creating steps onthe material surface. Under cyclic loading,the formation of intrusions and extrusions isobserved, as schematically represented in Fig. 9.Slip band intrusions are excellent stress raisersthat can be sites of crack nucleation.Besides the applied stress amplitude, DS/2,

several other factors are likely to affect thenucleation of a fatigue crack, such as the meanstress, Sm, or load ratio, R; geometry and surfacefinishing of the part; mechanical properties; andenvironment. Here, the R ratio is defined as theratio between the minimum and maximum loadsduring the fatigue cycle.A large proportion of fatigue data found in the

literature refers to tests conducted at Sm= 0,that is, for a load ratio R=1. However, inmany engineering situations, the fluctuatingstresses are superimposed to a static stress.Larger mean stresses reduce the nucleation timebecause they facilitate the plastic deformationmechanism associated with this phenomenon. Inan S-N graph, this can be represented by curvesshifted to the left and down, as represented inFig. 10.

Fig. 8 Fractograph of SAE 316L showing intergranular brittlefracture

Metal Surface

Intrusion Extrusion

Fig. 9 Schematic representation of an intrusion formation onthe surface of a metallic material


The mechanism proposed in Fig. 9 isadequate to explain the initiation of crackson polished testpieces or components withoutthe presence of geometric discontinuities.However, in engineering components, there areseveral stress concentrators, such as scratches,notches, machining marks, corrosion pits, andmicroconstituents such as grain boundaries,triple points, and inclusions, that individually orsynergistically can reduce the initiation time.Since the initiation depends essentially on

plastic deformation mechanisms, high-strengthmaterials normally present a higher resistance tofatigue crack nucleation. In this sense, severalsurface-hardening treatments are employed toselectively reinforce the material, aiming toretard crack initiation and therefore to increasefatigue life.The chemical composition and/or the micro-

structure of the surface can be modified bythermochemical treatments, such as carburizingor nitriding, or by cold deformation processes,such as shot peening or surface rolling.Mechanical parts that necessarily present stressconcentrators, such as crankshafts, gears, andbolts, can be subjected to these treatmentsto increase the fatigue limit of the material.Figure 11 shows a micrograph of the transversesection of a bolt, where the thread was coldformed by surface rolling. As a consequence,surface grains are flattened due to the mechan-ical deformation imposed. In this case, besidesincreasing hardness and mechanical strength,the process avoids the introduction of harmfulmachining marks.Surface treatments may also increase fatigue

life by the introduction of compressive residualstresses on the surface of the material. As long asthe material remains in linear elastic conditions,the principle of stress superposition can be

employed to describe the actual stress state inmaterials containing residual stresses. There-fore, the effective stress, S0, is given by thesum of the applied stress, S, to the residual stress,Sres:

S0=S+Sres (Eq 1)

Similarly, the effective minimum and maximumstresses are defined, respectively, as:

S0max=Smax+Sres (Eq 2)

S0min=Smin+Sres (Eq 3)

Consequently, the effective stress amplitude,mean stress, and load ratio are given, respec-tively, by:

DS0

2=

S0max7S0min

2=

(Smax+Sres)7(Smin+Sres)

2

=Smax7Smin

2=DS

2(Eq 4)

S0m=S0max+S

0min

2=

(Smax+Sres)+(Smin+Sres)

2

=Smax+Smin

2+Sres=Sm+Sres (Eq 5)

R0=S0minS0max

=Smin+Sres

Smax+Sres(Eq 6)

Therefore, the presence of a residual-stressfield does not affect the stress amplitude butaffects the mean stress and the load ratio.A compressive residual stress reduces the meanstress and the load ratio, increasing the number

S/2

Nf

Increasing

Sm

Fig. 10 Mean stress effect on S-N fatigue curves

Fig. 11 Optical micrograph of the transverse section of athread fillet machined by surface rolling. The ma-

terial consists of duplex stainless steel


of cycles for crack nucleation and vice versa.In some situations, where high surface com-pressive residual stresses are found, such as inmaterials subjected to surface-hardening treat-ments, a crack may initiate below the surface,where the compressive residual-stress level islower. An example of subsurface crack nuclea-tion is observed in Fig. 12 for a surface-rolledductile cast iron subjected to bending-rotatingfatigue.Fatigue Crack Propagation. Basically, fati-

gue crack propagation can be divided into threestages: stage I (short cracks), stage II (longcracks), and stage III (final fracture).A fatigue crack, once initiated, propagates

along high shear-stress planes (45), as sche-matically represented in Fig. 13. This is knownas stage I or the short crack growth propagationstage. The crack propagates until it is deceler-ated by a microstructural barrier, such as a grainboundary, inclusions, or pearlitic zones, thatcannot accommodate the initial crack growthdirection. Therefore, grain refinement is capableof increasing fatigue strength of the material dueto the insertion of a large quantity of micro-structural barriers, that is, grain boundaries, thatmust be overcome in stage I of propagation.Surface mechanical treatments, such as shot

peening and surface rolling, contribute to theincrease in the number of microstructuralbarriers per unit of length due to the flattening ofthe grains.When the stress-intensity factor, K, increases

as a consequence of crack growth or higherapplied loads, slips start to occur in differentplanes close to the crack tip, initiating stage II ofpropagation. While stage I of propagation isorientated 45 in relation to the applied load,propagation in stage II is perpendicular to loaddirection, as depicted in Fig. 13. An importantcharacteristic of stage II propagation is thepresence of ripples on the fracture surface,known as striations, which are only visible withthe aid of a scanning electron microscope. Notall engineering materials exhibit striations. Theyare clearly seen in pure metals and many ductilealloys, such as aluminum alloys. In steels, theyare frequently observed in cold-worked alloys.Figure 14 shows examples of fatigue striationsin an interstitial-free steel and in aluminumalloys. The most accepted mechanism for theformation of striations on the fatigue fracturesurface of ductile metals (Ref 9) consists ofsuccessive blunting and resharpening of thecrack tip, as represented in Fig. 15.Finally, stage III is related to the unstable

crack growth as Kmax approaches KIc. At thisstage, crack growth is controlled by static modesof failure and is very sensitive to the micro-structure, load ratio, and stress state (plane-stress or plane-strain loading).Macroscopically, the fatigue fracture surface

can be divided into two distinct regions, asshown by Fig. 16. The first region corresponds tothe stable fatigue crack growth and presentsa smooth aspect due to the friction betweenthe crack-wake faces. Sometimes, concentricmarks, known as beach marks, can be seen onthe fatigue fracture surface as a result of suc-cessive arrests or decrease in the fatigue crackgrowth rate due to a temporary load drop or to anoverload that introduces a compressive residual-stress field ahead of the crack tip.The other region corresponds to the final

fracture and presents a fibrous and irregularaspect. In this region, the fracture can be eitherbrittle or ductile, depending on the mechanicalproperties of the material, dimensions of thepart, and loading conditions. The exact fractionof area of each region will depend on the appliedload level. High applied loads will result in asmall stable fatigue crack propagation area,as depicted in Fig. 16(a). On the other hand,

Fig. 12 Probable subsurface crack nucleation site in a sur-face-rolled ductile cast iron testpiece tested under

bending-rotating conditions

Stage I Stage II

Su

rfa

ce

Fig. 13 Stages I and II of fatigue crack propagation


if lower loads are applied, the fatigue crackwill have to grow longer before the appliedstress-intensity factor, K, reaches the fracturetoughness value of the material, resulting in asmaller area of fast fracture (Fig. 16b).Ratcheting marks are another macroscopic

feature that can be observed in fatigue fracturesurfaces. These marks originate when multiple

Fig. 14 Fatigue striations in (a) interstitial-free steel and (b)aluminum alloy AA2024-T42. (c) Fatigue fracture

surface of a cast aluminum alloy where a fatigue crack wasnucleated from a casting defect, presenting solidification den-drites on the surface. Arrow at top right indicates fatigue striations.

(a)

(b)

(c)

(d)

(e)

Fig. 15 Proposed mechanisms of striation formation in stageII of propagation. (a) No load. (b) Tensile load. (c)

Maximum tensile load. (d) Load reversion. (e) Compressive load.Source: Ref 9

Fig. 16 Fatigue fracture surface. (a) High applied load.(b) Low applied load


cracks, nucleated at different points, join to-gether, creating steps on the fracture surface.Therefore, counting the number of ratchet marksis a good indicator of the number of nucleationsites. Figure 17 presents in detail some ratchetmarks found on the fracture surface of a largeSAE 1045 rotating shaft, fractured by fatigue.Similar to the initiation phase, many factors

can affect long fatigue crack propagation rates.Among them, special attention should be givento the effects of load ratio and the presence ofresidual stresses.Increasing the load ratio has a tendency to

increase the long crack growth rates in allregions of the fatigue crack growth rate versusapplied stress-intensity factor range curve, orsimply, da/dN versus applied DK curve. Gen-erally, the effect of increasing load ratio is lesssignificant in the Paris regime than in near-threshold and near-failure regions (Fig. 18).

Near the threshold stress-intensity factor,DKth, the effects of R ratio are mainly attributedto crack closure effects, where crack faces comein contact at an appliedKcl that is higher than theminimum applied stress-intensity factor, Kmin.Several different mechanisms may contribute

to premature crack closure. One of them consistsof plasticity-induced closure, represented inFig. 19(a). As the crack grows, the material thathas been previously permanently deformedwithin the plastic zone now forms an envelope ofplastic zones in the wake of the crack front. Thisleads to displacements normal to the crack sur-faces as the restraint is relieved. This is no pro-blem while the crack is open; however, as theload decreases, the crack surfaces touch beforethe minimum load is reached, shielding thecrack. This type of premature contact can alsooccur due to the crack-wake roughness andirregularities (Fig. 19b) or by the presence ofcorrosion subproducts, such as oxides (Fig. 19c).As observed in Fig. 20, the effect of closure

produces a reduction in the effective DK rangebecause of the increase in the effective Kmin,reducing the driving force for fatigue crackgrowth. The effect is more significant near thethreshold region because the crack tip openingdisplacements are smaller and the crack facesare closer to each other. Additionally, for thesame applied DK, higher R ratios increase theapplied values of Kmax and Kmin, increasingDKeff.For most materials, the Paris regime is con-

sidered closure-free and Kmax-independent, and

Fig. 17 Ratcheting marks, indicated by the arrows, in an SAE1045 shaft fractured by fatigue

K

Increasing

Rda/dN

Near threshold

Final failure

Paris regime

Fig. 18 Schematic representation of the R ratio effect onfatigue crack growth curves. The near-threshold,

Paris regime, and final failure regions are also indicated on thecurves.

Plastic deformation

envelope

(a)

(b)

(c)

Premature contact points Oxides

Plastic zone

Crack tip

Fig. 19 Crack closure mechanisms induced by (a) plasticity,(b) roughness, and (c) oxide


the crack growth rates are generally very similarfor tests conducted under different R ratios. Nearthe final failure, the effects of R ratio are relatedto the higher monotonic fracture component asKmax approaches KIc. Therefore, for the sameapplied DK, Kmax values are higher for testsconducted under higher applied R ratios, andconsequently, da/dN values are higher.The effects of residual stress on fatigue crack

growth are related to alterations in the R ratioand in the applied DK. In other terms, the resi-dual stresses affect the two parameters thatcontrol the crack driving force, that is, Kmax andDKeff. When a crack is introduced in a platesubjected to a residual-stress field, a residualstress-intensity factor, Kr, arises that can eitherdecrease or increase the crack driving forceparameters.The superposition principle can also be

applied in terms of the stress-intensity factor,provided that the material remains linearlyelastic. In this sense,Kr can be added toKmax andKmin:

K0max=Kmax+Kr (Eq 7)

K0min=Kmin+Kr (Eq 8)

As a result, R0 and DK0 are defined as follows. IfK0min40, then:

R0=K0minK0max

=Kmin+Kr

Kmax+Kr(Eq 9)

DK0=K0max7K0min= Kmax+Kr 7 Kmin+Kr

=Kmax7Kmin=DK

(Eq 10)

If K0minj0, then:

R0=0 (Eq 11)

DK0=K0max=Kmax+Kr (Eq 12)

It is important to note that these equationsassume that the part of the fatigue cycle duringwhich the crack is closed at its tip (i.e., K050)makes no contribution to crack growth.

Distortion

Distortion is the least serious mode of failure,but it can lead a part to failure or a structure tocollapse. It is easy to recognize but very difficultto prevent. This is due to the fact that distortiondoes not involve the part itself but its use anddesign. There are four reasons for distortion:yielding, buckling, creep, and residual stresses.Yielding. When a load is put on a part, and it

causes the part to be permanently distorted, it isunable to perform the intended function andtherefore must be considered failed. In a well-designed part, the stresses never exceed the yieldpoint, and the part deforms only elastically; thatis, when the load is released, the part returns toits original dimensions.In a good design, the part operates in the

elastic range, that is, below yielding point;beyond this, the part will be permanentlydeformed, and greater loads will cause the partto actually break. This point is considered to be avery basic point to design and applies when theload on a part is applied in a quasi-static way,such as the load on a building structure or thestress in the legs of a desk. A ductile failure is

K

Time

Kmax

Kmin

KapKeff K

Time

Kmax

Kap=Keff

Kcl

(b)

Kcl

Kmin

(a)

Fig. 20 Load ratio effect on DKeff in a fatigue cycle. (a) Kmin5Kcl. (b) Kmin4Kcl


onewhere there is a great deal of distortion of thefailed part. Commonly, a ductile part fails whenit distorts and can no longer carry the neededload. However, some ductile parts break intotwo pieces and can be identified because there isa great deal of distortion around the fractureface, similar towhat would happen if toomuch isplaced load on a low-carbon steel bolt.Buckling. The failure of an engineering

component is not always caused by materialsfracture. In many occasions, the componentdistortion may be sufficient to put it out offunction. The distortion can be elastic or plastic.The elastic distortions are temporary; however,they may be sufficient to cause interference onthe mobile parts. The plastic distortion is per-manent and can be a result of an overload orcreep deformation. The overload causes per-manent plastic deformation when the materialyield limit is overcome. This may happen in thepresence of stress concentrators, high tempera-ture, inadequate heat treatment, or incorrectmaterials selection for the component applica-tion. Compressive overloads may lead thematerial to overcome the buckling strengthlimit, such as the one shown in Fig. 21 for analuminum part. The buckling strength is essen-tially a design problem (not metallurgical), andthe load depends on the dimensions of the partand the Youngs modulus of the material (theonly materials factors involved).Creep is a time-dependent phenomenon that

causes a part failure if it is under both quasi-static load and temperatures higher than 0.3 Tm(absolute melting temperature). Creep strainmay produce sufficiently large deformation or

distortion that a part can no longer perform itsintended function. The two general types ofcreep processes are grain-boundary sliding andvoids at grain boundaries (cavitation creep).The creep processes are easily identified by

the local ductility and large numbers of inter-granular cracks that will depend on the tem-perature and strain rate imposed. In general, ahigh strain rate combined with high temperatureresults in ductile fracture, followed by a largeelongation and neck formation. Additionally,the grains near the fracture surface tend to beelongated. On the other side, the combination oflow strain rate and high temperature results inintergranular brittle fracture, with low elonga-tion or necking. Intergranular fracture in suchconditions normally initiates by grain-boundarysliding from triple points or at grain-boundaryintersections with second-phase particles, caus-ing cavities on the material microstructure, aspresented in Fig. 22.Once the crack nucleates, it propagates by

grain boundaries, and given that some sig-nificant plastic deformation may take place,the fracture surface tends to exhibit grains ofequiaxial shape. Therefore, to increase creepstrength, the material is normally heat treated toincrease the grain size, reducing the ratiobetween the grain surface area and volume. Inturbines that work at very high temperatures, thecreep mechanism must be considered. In thiscase, the component may be produced frommonocrystals that significantly increase thecreep resistance.Most creep curves show three distinct stages

(Fig. 23). After the elastic strain, there is a regionof increasing plastic flow at decreasing rate (firststage), followed by a region of approximatelyconstant strain rate (secondary stage), and finallya region of intense increase in the strain rate,which rapidly extends to fracture (third stage).

Fig. 21 Aluminum part that suffered buckling

Cavities

(b)(a)

Fig. 22 Intergranular crack formation at high temperature bygrain-boundary sliding at (a) triple points and

(b) inclusions


Residual stresses can play a significant rolein explaining or preventing failure of a compo-nent. One example of residual stresses prevent-ing failure is the use of shot peening processesthat increase the fatigue life of a component byinducing surface compressive stresses.Unfortunately, there are also processes or

processing errors that can induce excessivetensile residual stresses in locations that maypromote failure of a component. The internalstate of stress is caused by thermal and/ormechanical processing of the parts. Commonexamples of these are bending, rolling, orforging a part. Thermal residual stresses areprimarily due to differential expansion when ametal is heated or cooled. Two control factorsare thermal treatment (heating or cooling)and restraint. Both the thermal treatment andrestraint of the component must be present togenerate residual stresses. Residual stressescan result in visible distortion of a component.However, in the case of residual stresses, thedistortion can also be useful in estimating themagnitude or direction of these stresses.

Wear-Assisted Failure

Wear may be defined as damage to a solidsurface caused by the removal or displacementof material by the mechanical action of a con-tacting solid, liquid, or gas. It may cause sig-nificant surface damage, and the damage isusually thought of as gradual deterioration.While the terminology of wear is unresolved,the following categories are commonly used:adhesive wear, abrasive wear, erosive wear,fretting, cavitation, rolling, contact fatigue, andcorrosive wear.

Adhesive wear has been commonly identifiedby the terms galling or seizing. It is caused bythe material transference from one surface toanother during their relative movement due to asolid-state welding process. Figure 24 shows aschematic representation of this process. Highcontact pressure among the surface roughnessresults in local plastic deformation and pointsof microwelding. The movement between thesurfaces causes the rupture of the junctions,resulting in a rough peak in one surface and avalley on the other. Eventually, the tip of a peakmay break, and an abrasive particle is formed.Abrasive wear, or abrasion, is caused by the

displacement of material from a solid surfacedue to hard particles or protuberances slidingalong the surface. The particles may be foundfree between two surfaces or attached to one ofthem, and the wear level depends on the relativehardness between the particle and the surface(Fig. 25). The abrasion may also happen due tothe protuberances or sharp asperities on one ofthe surfaces in contact. The process of abrasiveerosion may be considered as abrasive wear.Erosion, or erosive wear, is the loss of

material from a solid surface due to relativemotion in contact with a fluid that contains solidparticles. In this case, the particle is found to bedispersed in a fluid or gas means, and it reachesthe surface under relatively high velocity(Fig. 25d). Figure 26 shows the microstructureof the transversal section of an H11 tool steelthat has been subject to abrasive erosion.Fatigue wear can be characterized by the

formation of cracks superficially and/or sub-superficially and the removal of posteriormaterial due to cyclic loading of solid surfaces.The sliding contact and/or rolling between solids

Time

Stage I Stage II Stage III0

t

t = creep rate

Fracture X

Fig. 23 Schematic strain-time curve at constant load andtemperature showing the three stages of creep

Adhesion

Particle

Fig. 24 Transference mechanism of a material from onesurface to another and the formation of an abrasive

particle in the process of adhesive wear


or the repetitive impact of solids and/or liquidsin a surface are responsible for the superficialfatigue. When two surfaces of this nature inter-act due to load application, the area effectivelyin contact may be very small, resulting in highcompressive and shear stresses that may lead tocrack nucleation. If only rolling is present, themaximum shear stress takes place just below thesurface, giving rise to cracks that propagateparallel to the surface and emerge at the surface,causing part of the material to separate from thecomponent, as shown in Fig. 27.However, pure rolling is not found in in-

service conditions. Normally, there is somesliding between the two surfaces, which altersthe stress field due to an increase in the shearcomponent, displacing the resulting stress closerto the surface. The cracks start to nucleate onthe component surface, propagating at a veryshallow angle, as shown in Fig. 28.

Fretting fatigue is considered a phenomenonwhere the damage is introduced by a conjunctionof events consisting of adhesion, oscillatorymovement of very low amplitude, oxidation, andabrasion. The small oscillatory movements maycause points of adhesion on the surface thateventually break, forming oxidized particles that

(b)

(d)

(a)

(c)

Fig. 25 Abrasive wear. (a) Free particle between two sur-faces. (b) Particle attached to one of the surfaces.

(c) Sharp asperity. (d) Erosion

Fig. 26 Fractography showing an H11 tool steel that hassuffered abrasive erosion

Fig. 27 Schematic representation of contact fatigue underpure rolling between two surfaces

Fig. 28 Damage by contact fatigue in rolling combined withsliding conditions in gears produced from a quen-

ched and tempered AISI 8620 carburized steel. (a) Transversalsection. (b) Frontal view from a formed cavity


act as abrasives on the surface, since the small-amplitude movements avoid their dispersionapart from the source point. Figure 29 presents amicrograph from a plasma nitrided Cr-Mo-Vsteel, where a microcrack formed in the frettingregion.More than one mechanism can be responsible

for the wear observed on a particular part. Themost critical function provided by lubricants isto minimize friction and wear to extend equip-ment service life. Gear failures can be traced tomechanical problems or lubricant failure.Lubricant-related failures are usually traced tocontamination, oil film collapse, additivedepletion, and use of improper lubricant for theapplication. The most common failures are dueto particle contamination of the lubricant. Dustparticles are highly abrasive and can penetratethrough the oil film, causing plowing wear orridging on metal surfaces. Water contaminationcan cause rust on working surfaces of gears andeventually destroy metal integrity. To preventpremature failure, gear selection requires carefulconsideration of the following: gear tooth geo-metry, tooth action, tooth pressures, construc-tion materials and surface characteristics,lubricant characteristics, and operating environ-ment.

Environmentally Assisted Failure

Corrosion is chemically induced damageto a material that results in deterioration ofthe material and its properties. Corrosioncan seldom be totally prevented, but it can beminimized or controlled by proper choice ofmaterial, design, coatings, and occasionallyby changing the environment. Various types of

metallic and nonmetallic coatings are regularlyused to protect metal parts from corrosion.Corrosion may result in failure of the com-

ponent. Several factors should be consideredduring a failure analysis to determine the effectcorrosion played in a failure, such as type ofcorrosion, corrosion rate, the extent of the cor-rosion, and the interaction between corrosionand other failure mechanisms.Uniform, pitting crevice, galvanic, and stress-

corrosion cracking are the most common typesof corrosion. Uniform corrosion is characterizedby corrosive attack proceeding evenly over theentire surface area or a large fraction of the totalarea. General thinning takes place until failure.On the basis of tonnage wasted, this is the mostimportant form of corrosion.Stress-corrosion cracking necessitates a

tensile stress, which may be caused by residualstresses, and a specific environment to causeprogressive fracture of a metal. Aluminumand stainless steel are well known for stress-corrosion cracking problems. However, allmetals are susceptible to stress-corrosion crac-king in the right environment.Pitting corrosion is a localized form of cor-

rosion by which cavities or holes are producedin the material. Pitting is considered to bemore dangerous than uniform corrosion damagebecause it is more difficult to detect, predict, anddesign against. Corrosion products often coverthe pits. A small, narrow pit with minimaloverall metal loss can lead to the failure ofan entire engineering system. Pitting corrosion,which, for example, is almost a commondenominator of all types of localized corrosionattack, may assume different shapes.Crevice corrosion is a localized form of

corrosion usually associated with a stagnantsolution on the microenvironmental level. Suchstagnant microenvironments tend to occur increvices (shielded areas) such as those formedunder gaskets, washers, insulation material,fastener heads, surface deposits, disbondedcoatings, threads, lap joints, and clamps. Crevicecorrosion is initiated by changes in local chem-istry within the crevice.Galvanic corrosion (also called dissimilar-

metal corrosion or, wrongly, electrolysis) refersto corrosion damage induced when two dis-similar materials are coupled in a corrosiveelectrolyte. It occurs when two (or more) dis-similar metals are brought into electrical contactunder water. When a galvanic couple forms, oneof the metals in the couple becomes the anodeFig. 29 Fretting fatigue at the surface of a Cr-Mo-V steel


and corrodes faster than it would all by itself,while the other becomes the cathode and cor-rodes slower than it would alone.

REFERENCES

1. D. Dennies, How to Organize a FailureInvestigation, ASM International, 2005

2. D.J. Wulpi, Chapter 1: Techniques of FailureAnalysis, Understanding How ComponentsFail, 2nd ed., ASM International, 2000,p 111

3. C.R. Brooks and A. Choudhury, Chapter 1:Introduction,Metallurgical Failure Analysis,McGraw-Hill, 1993, p 172

4. R. Graham, Strategies for Failure Analysis,Adv. Mater. Process. Aug 2004, p 4550

5. D.A. Ryder, T.J. Davies, I. Brough, and F.R.Hutchings, General Practice in Failure

Analysis, Failure Analysis and Prevention,Vol 11,Metals Handbook, 9th ed., AmericanSociety for Metals, 1986, p 1546

6. G.F. Vander Voort, Conducting the FailureExamination, Prac. Fail. Anal.,Vol 1 (No 2),April 2001, p 1446 andFailure Analysis andPrevention, Vol 11, ASM Handbook, ASMInternational, 2002

7. A. Tanzer, Determination and Classificationof Damage, Failure Analysis and Prevention,Vol 11, ASM Handbook, ASM International,2002

8. G. Powell, Identification of Types of Failure,Failure Analysis and Prevention, Vol 11,Metals Handbook, 9th ed., American Societyfor Metals, 1986, p 7581

9. C. Laird, The Influence of MetallurgicalStructure on the Mechanisms of FatigueCrack Propagation, Fatigue Crack Propa-gation, STP 415, ASTM, p 131168


Failure in Steel ForgingMd. Maniruzzaman and Richard D. Sisson, Jr.,Worcester Polytechnic InstituteStephen R. Crosby, The Stanely WorksCharlie Gure (deceased)

IN-PROCESS OR SERVICE FAILURES offorgings may occur for a variety of reasons. Thestartingmaterial may be of insufficient quality tobe adequately formed without cracking, or theforging process may introduce various types ofdiscontinuities that cause failure during services.For example, well-known forging-related dis-continuities include:

Laps Bursts Flakes Segregation Cavity shrinkage Centerline pipe Parting-line grain flow Inclusions

Forging discontinuities are discussed in moredetail in the texts on forging (Ref 14).This article describes six case studies of

failures with steel forgings (summarized inTable 1). The case studies illustrate difficultiesencountered in either cold forging or hot forg-ing in terms of preforge factors and/or dis-continuities generated by the forging process.Tables 2 and 3 summarize these factors forcold and hot forging, respectively. Supporting

topics that are discussed in the case studiesinclude:

Validity checks for buster and blockerdesign

Lubrication and wear Mechanical surface phenomenon Forging process design Forging tolerances

As case studies were being selected, each ofthe aforementioned supporting topics wasreviewed for any impact that particular studyhad on the case being examined. It is a well-known fact that forging solutions have severalpossible avenues to follow. There is no uniquetheory in plasticity that leads to the solution.Most of the work reported here was performedusing the minimum amount of energy to createthe particular product. Factors unrelated to thedeformation process, such as chemistry, micro-structure, phase, grain size, segregation, andprior strain history, are not addressed here.Instead, factors directly related to the deforma-tion process itself are presented in this abbre-viated discussion.Wear, plastic deformation processes, and

laws of friction are introduced as a group of

Table 1 Failure analysis of steel forgings and components

Case study Defect Solution

Crankshaft underfill Unable to fill crankshaft flanges with existing press

capacity

Introduce creep stages for last increment of

displacements

Tube bending Unable to control exterior wall thinning and interior wall

thickening

Introduce induction heating and cooling to limit the

heated axial tube length prior to making the bend

Spade bit Unable to achieve center web thickness at programmed

force and sufficient flow to wings

Adjust the die angle to create more shear stress, enabling

full flow to the wings

Trim tear Forge material tore at trimline when forging was

trimmed immediately following finish forging

Introduce a delay time after forge and prior to trim,

allowing the forge material to cool and gain strength

Upset forging Cracking at circumferential bulge after upset Re-examine the strain and strain rate and process map

for stable flow

Flow-through laps

and avoidance

Material foldover at tops of rib and flange intersections

and cases of material flow under previously filled

flanges

Replace the input piece with a newly designed preform

piece, following the design procedures given in this

work

Failure Analysis of Heat Treated Steel Components

L.C.F. Canale, R.A. Mesquita, and G.E. Totten, editors, p 133-149

DOI: 10.1361/faht2008p133

Copyright 2008 ASM International

All rights reserved.

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failure analysis of heat treated steel components

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