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Analise de Falhas ASM

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CHAPTER 1 Techniques of Failure Analysis In study of any failure, the analyst must consider a broad spectrum of possibilities or reasons for the occurrence. Often a large number of fac- tors, frequently interrelated, must be understood to determine the cause of the original, or primary, failure. The analyst is in the position of Sherlock Holmes attempting to solve a baffling case. Like the great de- tective, the analyst must carefully examine and evaluate all evidence available,
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  CHAPTER 1 Techniques of Failure Analysis In study of any failure, the analyst must consider a broad spectrum of possibilities or reasons for the occurrence. Often a large number of fac-tors, frequently interrelated, must be understood to determine the causeof the srcinal, or primary, failure. The analyst is in the position of Sherlock Holmes attempting to solve a baffling case. Like the great de-tective, the analyst must carefully examine and evaluate all evidenceavailable, then prepare a hypothesis—or possible chain of events—thatcould have caused the “crime.” The analyst may also be compared to acoroner performing an autopsy on a person who suffered an unnaturaldeath, except that the failure analyst works on parts or assemblies thathave had an unnatural or premature demise. If the failure can be dupli-cated under controlled simulated service conditions in the laboratory,much can be learned about how the failure actually occurred. If this isnot possible, there may be factors about the service of the part or assem-bly that are not well understood.Fractures, usually the most serious type of failure, will be studiedhere in some detail. Usually undesired and unexpected by the user, frac-tures can have disastrous results when a load-bearing member suddenlyloses its ability to carry its intended load. Distortion, wear, and corro-sion failures also are important, and sometimes lead to fractures. How-ever, these types of failure can be reasonably well predicted and pre-vented. Understanding How Components Fail, 2nd Edition. Author: Donald J. Wulpi. ASM, 1999.  Procedure for Failure Analysis Reference 1 is a basic guide to follow in various stages of a failureanalysis investigation. It must be emphasized that the most importantinitial step to perform in any failure analysis investigation is to doNOTHING, simply study the evidence; think about the failed part orparts; ask detailed questions about the parts, the machine itself, and cir-cumstances of the failure; and make accurate notes about the responses.When possible, it is highly desirable to use low-power magnifica-tion—uptoabout25or50 × —withcarefullycontrolledlightingtostudythe failed part or parts.For a complete evaluation, the sequence of stages in the investigationand analysis of failure, as detailed in Ref 1, is as follows:1. Collection of background data and selection of samples2. Preliminaryexaminationofthefailedpart(visualexaminationandrecordkeeping)3. Nondestructive testing4. Mechanical testing (including hardness and toughness testing)5. Selection, identification, preservation and/or cleaning of specimens (andcomparison with parts that have not failed)6. Macroscopic examination and analysis and photographic documentation(fracture surfaces, secondary cracks, and other surface phenomena)7. Microscopicexaminationandanalysis(electronmicroscopymaybenec-essary)8. Selection and preparation of metallographic sections9. Examination and analysis of metallographic specimens10. Determination of failure mechanism11. Chemical analysis (bulk, local, surface corrosion products, deposits orcoatings, and microprobe analysis)12. Analysis of fracture mechanics (see Chapter 15)13. Testing under simulated service conditions (special tests)14. Analysis of all the evidence, formulation of conclusions, and writing thereport (including recommendations). Writing a report may not be neces-saryinmanyproductlitigationcases;itisbesttofollowtheadviceoftheattorney or client with whom the analyst is working.Each of these stages is considered in greater detail in Ref 1 and willnot be repeated here. However, it must be emphasized that three princi-ples must be carefully followed: ã  Locatetheorigin(s)ofthefracture. Nolaboratoryproceduremusthinderthis effort to find the location(s) where fracture srcinated. Also, it is 2Understanding How Components Fail  most desirable, if possible, to have both fracture surfaces in an undam-aged condition. ã  Donotputthematingpiecesofafracturebacktogether,exceptwithcon-siderable care and protection.  Even in the best circumstances, fracturesurfaces are extremely delicate and fragile and are damaged easily, fromamicroscopicstandpoint.Protectionofthesurfacesisparticularlyimpor-tant if electron microscopic examination is to be part of the procedure.Many such examinations have been frustrated by careless repositioningof the parts, by careless packaging and shipping, and by inadequate pro-tectionfromcorrosion,includingcontactwithfingers.Ifpartsmustbere-positioned to determine deformation of the total part, fracture surfacesmust be protected by paper or tape that will not contaminate the surface.Also, protect fracture surfaces and other critical surfaces from damageduring shipping by using padding, such as an adhesive strip bandage forsmall parts. ã  Donodestructivetestingwithoutconsiderablethought. Alterationssuchas cutting, drilling, and grinding can ruin an investigation if performedprematurely.Donothingthatcannotbeundone.Onceapartiscut,itcan-not be uncut; once drilled, it cannot be undrilled; once ground, it cannotbe unground. In general, destructive testing must be performed—if doneat all—only after all possible information has been extracted from thepart in the srcinal condition and after all significant features have beencarefully documented by photography. Caution is particularly necessaryin product litigation cases because the details of destructive testingshould be agreed upon by all parties in the lawsuit. Consult the attorneywith whom the analyst is working.If there are several fractures from one mechanism (a “basket case”),one should determine if any of the fractures is a fatigue fracture. If defi-nite evidence of a fatigue fracture can be found, this is usually thesource of the problem—the primary fracture. Fatigue fracture is thenormal, or expected, type of fracture of a machine element after longservice. However, there are many possible reasons for fatigue fractureand many different appearances of fatigue fractures, as we shall see. Fa-tigue fractures are quite common in mechanisms unless specific actionshave been taken to prevent them during design, manufacture, and ser-vice. Investigative Techniques While not all failures require the same degree of effort needed to in-vestigate a product litigation matter, it is imperative that the investiga-tor follow a specific plan during the analysis. The use of checklists and Techniques of Failure Analysis3  flow charts to keep the investigation on track is very effective to insurethat all elements of the analysis have been performed and properly doc-umented.The initial stages of the investigation are the most critical. This is thephase where information surrounding the failure is collected and docu-mented. Without following a well developed plan, some vital piece of evidence may be overlooked. With the passage of time it may becomedifficult, if not impossible, to recall or obtain evidence that may proveto be the missing piece of the puzzle (see also Ref 2). Normal Location of Fracture The analyst must be aware of the normal, or expected, location forfracture in any type of part because any deviation from the normal loca-tion must have been caused by certain factors that must be discovered.An all-too-familiar type of fracture is that of the ordinary shoelace. Ashoelace will inevitably fail at one of the two top eyelets, adjacent to thebowknot, as shown in Fig. 1. There are several logical engineering rea-sons why this is the normal location of fracture: ã  When the knot is tied, the lace is pulled tightest at the upper eyelets;therefore, the service stress is highest at this location. ã  Most of the sliding motion during tightening occurs at the lace as it goesthrough the upper eyelets. Therefore, the metal eyelets tend to wear, orabrade, the fibers of the lace. ã  Since the shoelace presumably has uniform mechanical properties alongits length, it will eventually wear—and ultimately tear, or fracture—atthelocationwhereconditionsaremostsevere,thatis,atanuppereyelet.If the shoelace were to fracture at any other location, such as at thelower eyelets or near the free ends, one would have to suspect that, forsome reason, the shoelace had substandard mechanical properties at thelocation of failure. Or, alternatively, the lace could have been dam-aged—such as by burning from dropped cigarette ashes—thus causingit to be weakened and fractured at an abnormal location.This familiar example of the normal location of fracture is easy to un-derstand. The situation becomes considerably more complex in metalcomponents that may have been manufactured—intentionally or unin-tentionally—with different mechanical properties at different locationsin the part.Ametalpart,however,canbeexpectedtofail,orfracture,atanyloca-tion where the stress first exceeds the strength, unlike the shoelace,which is expected to have uniform strength throughout its length and 4Understanding How Components Fail

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