Fatigue Failure Slides

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Lecture 17

introducing

FATIGUE FAILURE

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Just to have an overview where we are now

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Major Issues to be addressed;

1What is FATIGUE

2STRESS CYCLES

4Fatigue

Testing

3S-N CURVES

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What IS a FATIGUE ?1

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 loading

90% service failure Fatigue

Examples: materials in continuous loading-unloading applications like bolts/ shafts (Brittle type fracture—why?)

“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 basics

Automobile

Turbine blades, bridges…

Aircraft

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Requirements for Fatigue Failure

• This type of failure occurs in Metals and Polymers, While Ceramics do not undergo this type of failure!!

Cyclic loading

Tensile stresses

Plastic strain

FATI

GU

E FA

ILU

RE

Fracture Surface in a Fatigue failure: STAGES OF FATIGUE FAILURE

Two Main fracture ZONES Smooth Surface Crystalline Surface

1) CRACK initiation2) Crack Propagation3) Catastrophic rupture

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StriationHigh magnification SEM Image

Fatigue in Engine Crankshaft

Fracture Surface in a Fatigue failure:

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Fracture Surface in a Fatigue failure:

Macroscopic Examination Microscopic Examination

Clamshell/beach markingsFiner markings-Striations Intrusions

and Extrusions

SEM image @ high magnificationSEM image

start

progress

End

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 )

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CRACK Initiation and Propagation !

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

SLIP BANDS Formation (Persistent Slip Bands)

Crack initiates •

Extrusion and

IntrusionsCRACK

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CRACK Propagation Mechanism !

• Cyclic Load

• Crack Initiation (microcrack)

• Crack Propagation Stage I

• Crack Propagation Stage II

• Fracture

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

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2 STRESS CYCLES

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Types Of Fluctuating StressesReversed Stress cycle Repeated Stress cycle

Random/irregular stress cycles

Stress Cycles in Fatigue

Mean Stress:

Amplitude Stress:

Range:

Radius Ratio:Amplitude Ratio:

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Fatigue Crack Growth

()

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CRACK Propagation Mechanism !

• Cyclic Load

• Crack Initiation (microcrack)

• Crack Propagation Stage I

• Crack Propagation Stage II

• Fracture

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In 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..)

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Fracture Mechanics for Fatigue

Stress Intensity Factor K

In Cyclic Loading, we define , K;

K = Kmax – Kmin = Y (Smax – Smin)

Paris- Erdogan Equation:

𝑵 𝒇= 𝟏𝑪 (𝒀 𝛥𝜎 )𝒎𝝅𝒎/𝟐(𝟏−𝒎/𝟐) (𝒂 𝒇 (𝟏−𝒎

𝟐)−𝒂𝒐 (𝟏−𝒎

𝟐))

Fatigue Life

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3 S-N Curves

Wöhler curve/SN curves - A graph showing stress as a function of number of cycles in fatigue.

Presenting Fatigue Data

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Fatigue Limit/Endurance Limit:“The stress below which a material will not fail by fatigue”

Non-ferrous

ferrous

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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:

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4 Fatigue Testing

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Four Point Bending Set up

Cantilever loading

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Modified Goodman Relation𝜎a = 𝜎e (1 – 𝜎m/ 𝜎 u)

Soderberg Relation

Gerber Relation

Failure Criteria can also be established by plotting fatigue test data

Predicts safer regions for fatigue

𝜎a = 𝜎e (1 – 𝜎m/ 𝜎 u)2

𝜎a = 𝜎e (1 – 𝜎m/ 𝜎 y)

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Miner’s Law and Cumulative Damage

Remaining cycles permissible at S2

N1 = lifetime at S1

n1 = Number of cycles applied till S1

Fatigue life consumed at

S1

Fatigue Failure will occur when

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Factors Effecting Fatigue Loading and Loading CyclesSurface ConditionsDesign ConsiderationNature of Environment

The nature of Surface strongly influence fatigue strength:-Surface Hardness-Surface Roughness-Residual Stresses

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--Method 1: shot peening

put surface

into compression

shot --Method 2: carburizing

C-rich gas

2. Remove stress concentrators.

bad

bad

better

better

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.

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QUIZ-5/6 ON THURSDAY, 8TH WEEK@5:30PM MLH