introduction to fatigue

8/7/2019 Introduction to Fatigue

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Introduction to Fatigue

By

V. Hudson Paul

2010214002


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Overview of FatigueMany different mechanical failure modes exist in all fields of

engineering.

These failures can occur in simple, complex, inexpensive, or

expensive components or structures.

Failure due to fatigue, i.e., repeated loading, is multidisciplinary and

is the most common cause of mechanical failure.

Even though the number of mechanical failures compared to

successes is minimal, the cost in lives, injuries, and dollars is too large.

Proper fatigue design can reduce these undesirable losses.

Proper fatigue design includes synthesis, analysis, and testing.

The closer the simulated analysis and testing are to the real product

and its usage, the greater confidence in the engineering results.


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y The pri ciples f fatig e behavi r a fatig e esig have been

evel ped, sed, and tested b engineers and scientists in all

disciplines and in many c ntries.

y The c rrent capability f c mp ters and sim lated testing has a

pr nounced influenceon theefficiency and qualityof today's fatigue

design procedures.

y owever, in proper fatiguedesign, both computer synthesis and

analysis must be integratedwith proper simulated and field testing,

along with continuedevaluationof product usage and maintenance,

including non-destructive inspection.

FATIGUE ANALYSIS NEEDS


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Tips in Design for Fatigue1.Do recognize that fatigue failures are the most common cause of mechanical

failure in components, vehicles, and structures and that these failures occur in allfields ofengineering.

.Do recognize that proper fatigue design methods exist and must he incorporated

into the overall designprocess when cyclic loadings are involved.

3.Do not rely on safety factors in attempting to overcome poor design

proce-dures.

.Do consider that good fatigue design, with or without computer-aided design,

incorporates synthesis, analysis, and testing.

5.Do consider that fatigue durability testing should be used as a design

verification tool rather than as a design development tool.


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STRATEGIESIN FATIGUE DESIGN

Fatigue design methods have many similarities but alsodifferences.

The differences exist because a component, structure, or vehiclemay be safety critical or non-safety critical, simple or complex,expensive or inexpensive, and failures may be a nuisance orcatastrophic.

The product may be a modification of a current model or a new

product. Significant computer-aided engineering (CAE) andcomputer-aided manufacturing, CAM) capabilities may or maynot be available to the design engineer.


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Choosing the fatigue life model

Choosin the fati ue life odel is a si nificant decision.

Currently four such odels e ist for desi n en ineers. These are:

1. The no inal str ess-life (S-N) odel.

2. The local strain-life (I-N) model.

. The fati uecrack rowth(da/dN-(K)model, first formulated in the 1960s.

4. The two-sta e model, whichconsists ofcombinin models 2 and

to incor oratebothmacrosco ic fati uecrackformation (nucleation)

and fati uecrack rowth.


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Purposes of Design

1. Designing a device, perhaps a special bending tool or a test rig, to beused in the plant where it was designed. It is called by an "in-house tool."

. Changing anexisting product by making it largeror smaller thanpreviously,using a different material ordifferent shapes, perhaps alinkage and coil spring in placeof a leaf spring. It is called by a "new

model."

3. Setting up a major project that is quitedifferent from past practice. Aspacecraft or anoceandrilling rig or a new typeof treeharvester isexample. It is called by a "new product."

. Designing a highway bridgeor a steam boiler. Theexpected loads,acceptable methods of analysis, and permissible stresses are specified bythe customeror by a code authority. It is called by "design to code."


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Tips in Design Related to Crack Initiation

1. Do recognize that fatigue is a localized, progressive, andpermanent behaviour involving thenucleation and growthof cracks to

final,usually sudden fracture.

. Do recognize that fatigue cracks nucleate primarilyon planes of

maximum shear andusually growon the planeof maximum tensile

stress.3. Doexamine fracture surfaces as part of a post-failure analysis, since

substantial information concerning the causeof the fracture can be

gained. Theexamination can involve a small magnifying glass or

greater magnificationup to that of theelectron microscope.

.Donot put fracture surfaces back together again to see if they fit or

allow corrosiveenvironments (including rain and moisture from

fingers) to reach the fracture surface.


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5. Do consider that stress-strainbehaviour at notches or cracks

under repeated loading maynot be the same as that observed

under monotonic tensileor compressive loading.

. Do take into consideration that your product will very likely

contain cracks during its design lifetime.7. Do recognize that most fatigue cracks nucleate at the surface,

and therefore that surface and manufacturing effects are

extremely important.

8. Donot assume that a metal that has good resistance to cracknucleation alsohas good resistance to crack growth and vice

versa.


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Fatigue Loading


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Tips in Design for Fatigue Test and the Stress-Life (S-N)

Approach

1. Do consider thewide rangeof test systems and specimens availablefor fatigue testing. Tests can range from those performedon small,highly polished specimens for material characterization to full-scaledurability tests of large structures.

. Donot neglect to refer to ASTM, ISO,or similar standards on fatiguetesting anddata reduction techniques.

3. Do consider that the fully reversed fatigue strength, Sp at 106 to108cycles for components can vary from about 1 to 70 percent of theultimate tensile strength and that theengineer can substantiallyinfluence this value by properdesign and manufacturing decisions.

. Donote that cleaner metals, and generally smaller grain size forambient temperature,have better fatigue resistance.


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5. o reco nize that frequency effects ar e generally small only whencorrosion, temperature, or other aggressi e en ironmental effects ar eabsent.

6. o consider that surface finish can ha e a substantial influence onfatigue resistance, particularly at longer li es.

. o not neglect the ad antages of compressi e mean or compressi e residual stresses in impro ing fatigue life and thedetrimental effect of tensile mean or tensile residual stresses indecreasing fatigue life, and that models ar e a ailable to account forthese effects.

. o attempt to use actual fatigue data in design; however, if this is

not possible or reasonable, approximate estimates ofmedian fatiguebehaviourcan be made.


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Thank You

introduction to fatigue

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