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Where does fatigue failure occur?The most common locations for fatigue failure includethe joint interface (i.e., first "loaded" thread), the fIllet,the threads, and the thread expiration or "run-out." Asthe industry has developedbetter materials and produc-tion methodsto improve fatigue strength, the threadshave becomethe weakestpoint of the fastenerand cur-rently account for the highest number of fatigue failures.

How is fatigue strength measured?The number of variables involved and their inter-dependencein fastenerperformance havemade settingstandardsfor fatigue strength a difficult task. Currently,the number of "cycles to failure" is used to determine arelative strengthfor a seriesof fasteners.This complexmeasurementoffers a standardof performance thatencompassesall the variables of the fastener,which willfmally fail at its weakestlink.Modified Goodman DiagramSocket Head Cap Screws, Rolled Threads.Stress based on the area at the basic thread minor diametel:

The failure of fastenersin industrial and aerospaceap-plications costs u.s. industry-and the public-billionsof dollars every year in downtime and lost production.Injuries and even deathshave resulted. More than 60aircraft accidentshavebeen attributed to fastenerfailure.While recent legislation has mandatedthat suppliersmeet industry standardsfor thesecritical components,no existing regulation for industrial fastenersspecificallyaddressesthe causeof an estimated85% of all fastenerfailures- fatigue.

What is fastener fatigue?A typical industrial fastener,say,a socketheadcap screw,looks absolutely rigid, but in fact it is-it must be-quite flexible. Due to such factors as design, material,method of manufactureand heat treatment, a cap screwwill "stretch" when subjectedto mechanical and/orthermal pressure.Such cap screwsconstantly stretchand return to their original shape.(If they are subject toexcessivestress,of course, they permanently deformand eventuallydestruct.) Thesestretch-and-returnactionsare called cycles. A socket head cap screw can be sub-ject to perhaps 240 cycles a day (e.g., in an 800-tonpress) all the way up to 1 million cycles a day (e.g., inan ultrasonic horn).As this peak-to-peakcycling occurs, the fasteneris subjectto stress.Eventually a crack will occur, just as it doeswhen you rapidly flex a paper clip back and forth. Thecrack occurs at the fastener'smost vulnerable point, re-ferred to by engineersas the "maximum stressconcen-tration area." The crack spreadsand fastenerfatiguefailure has occurred.The art of manufacturing industrial fastenersis a constantsearchfor these various Achilles' heels and an ongoingdevelopmentof design and manufacturing methods forovercomingthem. The paradox of that quest is that, onceyou've "cured" one area of vulnerability, you have, intruth, createdanother. If not replaced, most dynamicallyloaded fastenerswill suffer fatigue failure eventually;the only question is when they will fail. The fastenerdesigner'sobjective becomesone of extending thenumber of cyclesto failure at a given dynamic load.

MEAN STRESS, KSI (APPLIED PRELOAD)

Modified Goodman diagrams help designerspredictfastener perfonrzance. The broken diagonal line depictsthe mean of the alternating load for a screw with a00% probability of enduring 10 million cycles. Thediagonal solid lines show that the maximum deviationof dynamic load from the mean stress is :t12 ksi whenthe screw is preloaded to 100 ksi.

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Threads: A generousradius in the root of the threadreducesthe concentrationof stresscausedby a "flatroot" profIle. Equally important is the proper radius inthe thread run-out. Again, this lessensstressby reduc-ing sharp comers and improves fatigue strength. Notethat this radiusedrun-out is not mandatedby commonsocket screw specifications.ASTM standardsrequire that threadsbe formed by roll-ing rather than cutting or grinding. Threads formed byrolling will ensurethat the grain flow follows the threadcontour. If the rolling is done after heat treatment thefatigue life can be increasedby severalhundred percent,due to the residual compressivestressesinduced by theprocess.Rolled threadsprovide a smooth finish, reduc-ing the susceptibility to a fatigue failure that could prop-agatefrom a surface imperfection. ASTM standardsdefine acceptancecriteria for thread laps that can in-itiate a fatigue crack. These standardsappearto be themost commonly violated. Although often overlooked,they are critical to the fatigue life of the fastener.Heat Treating: While heat treatment is used to producestronger parts, improper treatment can result in condi-tions that will greatly reduce the fatigue strengthof thefastener.Carburization (increasein surfacecarbon mak-ing the surfaceharder than the core) and decarburization(surface softer than the core) will reduce fatigue per-formance. Microstructural changesand cracks can becausedby insufficient temperaturecontrol. The wrongquenching media or proceduremay not produce partshardenedthroughout and can also causecracking.Surface Finish: ASTM standardsspecify surfacefinishesfor different parts of a fastener.A rough surface frnishon the screw threads or body or even a slight deforrna-tion in the fillet area representpotential initiation sitesfor a fatigue failure.

Guarding against failureClearly, design, purchasing and other industrial specifiersconcerned with fasteners must take their own precau-tions to guard against fatigue failure. In this endeavor,responsible manufacturers of fasteners are constantlyseeking out and shoring up the points of vulnerabilityreferred to earlier.The ultimate goal is to increase the number of "cycles tofailure." Here are some stages along the trail to that goal.Head Construction: ASTM standards require heads tobe forged rather than machined. This precludes the planesof weakness caused by machining and increases the headfatigue strength. In addition, head height, socket depthand width, and wall thickness must all fall within stricttolerances in order to ensure proper key engagement. Thisallows the socket head cap screw to be tightened to ahigh preload, thereby minimizing the cyclic loads felt bythe fastener. It should be noted that limited hex key en-gagement and/or oversize sockets can lead to screwfailure at low wrenching torques.Fillet Design: A smooth fIllet with the correct radiusfor the application will help to reduce fatigue failure byblending the sharp profIle where the head meets theshank. An elliptical radius will provide better distribu-tion of stress and decrease the possibility of fatiguecrack initiation.

By Paul R. FrncoManager.ProductEngineeringSPSTechnologies,Unbrako N.A.

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Some conclusionsThe FastenerQuality Act, as well as a full range ofASTM, ANSI, and military specifications,offers limitedprotection from fatigue failure by providing guidelinesfor individual fastenerparameters.In order to protectthe end user and the public, designersand specifiersmust go beyondtheseregulationsto ensurethe synergisticmatch of all facetsof fastenerproduction and supply.Preventing fatigue failure includes starting with properdesign, working with a qualified supplier, taking advan-tage of the most modem materials and production capa-bilities, and drawing on the manufacturer'sextensiveapplication and engineeringexperience.The manufactureof fastenersmust be implemented in acarefully controlled process,taking into account all thephysical, mechanical, and chemical issuesraised above.Perhapsmost important is a thorough processcontrol andassuranceprogram, designedwith the end user's appli-cation and tolerancesin mind, to guaranteeadequatetesting, both in-processand after completion.Considering the high incidence of fatigue failure and thepossible associatedcatastrophiccosts, end users shouldconsider using fatigue requirementsin their specifica-tions for critical fasteners.A qualified manufacturer canbuild thesecriteria into the production processand con-duct tests for processverification.As the future brings evenlighter parts, evenfewer fastenersin critical applications, and even more exactingpartspecifications,the designersand specifiers of thesepartsmust be evenmore awareof the role of fatigue in fastenerfailure. By working closely with an experienced,quali-fied supplier, designersand specifiers can help toreduce the incidence-and the cost-of fastenerfatiguefailure.