MM211

Lecture 17introducing FATIGUE FAILUREAtta ul Haq GIK Institute-Fall 20131

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Just to have an overview where we are nowAtta ul Haq GIK Institute-Fall 20132

Major Issues to be addressed;

1What is FATIGUE

2STRESS CYCLES 4Fatigue Testing 3S-N CURVES

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What IS a FATIGUE ?

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Object --> Repeated Motion--> Weakness/tiredness --> Fatigue Example: Your legs/ muscles decrease in output due to constant running

Fatigue in materialsMetals --> Weak/fracture due to repeated loading90% service failure FatigueExamples: materials in continuous loading-unloading applications like bolts/ shafts (Brittle type fracturewhy?)

The PREMATURE failure/lowering of SRENGTH of a material due to repetitive stresses which may be below or above its yield strength Getting Started- Fatigue basicsAtta ul Haq GIK Institute-Fall 20135

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Requirements for Fatigue FailureThis type of failure occurs in Metals and Polymers, While Ceramics do not undergo this type of failure!!Atta ul Haq GIK Institute-Fall 20136

FATIGUE FAILURE Fracture Surface in a Fatigue failure:

STAGES OF FATIGUE FAILURETwo Main fracture ZONESSmooth Surface Crystalline Surface

CRACK initiationCrack PropagationCatastrophic rupture

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StriationHigh magnification SEM Image

Fatigue in Engine CrankshaftAtta ul Haq GIK Institute-Fall 20138Fracture Surface in a Fatigue failure:

Atta ul Haq GIK Institute-Fall 20139Fracture Surface in a Fatigue failure:

Macroscopic ExaminationMicroscopic Examination

Clamshell/beach markingsFiner markings-StriationsIntrusions and ExtrusionsSEM image @ high magnificationSEM image

startprogressEnd

Dislocation motion leads to PERSISTENT SLIP BANDS (PSBs)These SLIP BANDS involve INTRUSIONS and EXTRUSIONS (slip on different set of planes depending on loading cycle) Tiny Steps Stress Raisers Microcrack Initiation (along planes of high shear stress )

Schematic diagram showing Intrusions and Extrusions

CRACK Initiation and Propagation !

Crack formation and propagation is a Micromechanism in fatigueFatigue crack initiation is a surface phenomenon

SLIP BANDS Formation (Persistent Slip Bands)

Crack initiates

Extrusion and Intrusions

CRACKAtta ul Haq GIK Institute-Fall 201310

CRACK Propagation Mechanism !Atta ul Haq GIK Institute-Fall 201311

We will study Crack propagation Stages again once we get familiarize with the loading cycles

2STRESS CYCLES

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Atta ul Haq GIK Institute-Fall 201313Types Of Fluctuating Stresses

Reversed Stress cycleRepeated Stress cycleRandom/irregular stress cycles

Stress Cycles in FatigueAtta ul Haq GIK Institute-Fall 201314

Mean Stress:Amplitude Stress:Range:Radius Ratio:

Amplitude Ratio:

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CRACK Propagation Mechanism !Atta ul Haq GIK Institute-Fall 201316

Atta ul Haq GIK Institute-Fall 201317In Fatigue:Sub-critical Crack Growth Critical Crack Fracture

Let us consider three samples with same initial crack length subjected to different repeated cycles. Please note that material is also same; Higher Crack Growth rate at higher magnitude of applied stresses and vice versa

Materials life to Fracture is now highly dependent on magnitude of applied stress and final crack length (which is related to materials structural property (resistant to deformation etc..)

Fracture Mechanics for FatigueAtta ul Haq GIK Institute-Fall 201318

Fatigue Life

3S-N CurvesWhler curve/SN curves - A graph showing stress as a function of number of cycles in fatigue.

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Presenting Fatigue DataAtta ul Haq GIK Institute-Fall 201320

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Fatigue Limit/Endurance Limit:The stress below which a material will not fail by fatigue Non-ferrous ferrous Atta ul Haq GIK Institute-Fall 201321

Fatigue Life:The number of cycles permitted at a particular stress before a material fails by fatigue.The stress required to cause failure by fatigue in a given number of cycles.Fatigue Strength:Atta ul Haq GIK Institute-Fall 201322

4Fatigue Testing

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Four Point Bending Set up Cantilever loading

Modified Goodman Relation

a = e (1 m/ u)Soderberg RelationGerber RelationFailure Criteria can also be established by plotting fatigue test dataPredicts safer regions for fatiguea = e (1 m/ u)2a = e (1 m/ y)Atta ul Haq GIK Institute-Fall 201325

Miners Law and Cumulative Damage

Remaining cycles permissible at S2

N1 = lifetime at S1n1 = Number of cycles applied till S1

Fatigue life consumed at S1Atta ul Haq GIK Institute-Fall 201326Fatigue Failure will occur when

Factors Effecting Fatigue Loading and Loading CyclesSurface ConditionsDesign ConsiderationNature of EnvironmentAtta ul Haq GIK Institute-Fall 201327The nature of Surface strongly influence fatigue strength:-Surface Hardness-Surface Roughness-Residual Stresses

--Method 1: shot peening

put surface into compression

shot--Method 2: carburizing

C-rich gas

2. Remove stress concentrators.

badbad

betterbetter

1. Surface Treatments, induce compressive stressess.How to Improve Fatigue Performance

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Fatigue in PolymersFatigue behaviour and SN curves similar to that of metals except the crack initiation: There is no Concept of Persistent Slip Bands (PSBs) in Polymers , rather crack is initiated with CRAZING phenomenon.

QUIZ-5/6 on Thursday, 8th Week@5:30PM MLH

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