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Objectives

1. Explain the two approaches tofatigue characterization.

2. Explain the different assumptions

underlying the two approaches.3. Identify the fatigue limit, fatigue

strength, and fatigue life from an

S-N curve4. Identify the three regions of 

behavior on a da/dn – delta K

curve.

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 The Three Horsemen of Engineering

Fatigue

Corrosion Wear

Durer

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Fatigue

- Cyclic Load/ Strain

- Extended time failure process

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 Two Types of Fatigue

High Cycle

- Stress below yield strength

- Macroscopically brittle

- May be very long life

Low Cycle

- Stress exceeds yield strength

-  Very few cycles to failure

- Lots of plastic deformation

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 Terminology

Mean Stress

Range

Stress Amplitude

“R Value” 

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Two Engineering Approaches to Fatigue

1. Assume a flaw free structure

- initiation of flaw dominant

- safety factors to account for flaws

2. Assume presence of flaws of 

known type

- fracture mechanics for prediction

- measurement/ monitoring required

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High Cycle Fatigue Process

Local yielding at a defect or in a stressconcentration (filet root, scratch, bend, hole)

Dislocation pile up/ saturation

Crack formation

Crack propagation (relatively little of the life)

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S-N Approach

BCC & HCP Metals

MultipleSpecimens

DifferentStresses

Cycled to Failure

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Fatigue in FCC Alloys (Al)

7075-T6 Aluminum – notched,

constant load amplitude

Hardrath et al, NASA TN D-210

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Effect of Notches & Cold Working

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S-N Approach

 Assumes Flaw Free Structures or structures withflaws just like the test specimens.

Implies that most of the life of a structureoccurs during crack INITIATION!

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Fatigue Limit vs. Hardness

Metals Handbook, Vol 1, ASM Int.

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Relation between rotating, bending unnotched fatigue strength and

tensile strength, for alloy ( ) steels and carbon (x) steels 

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Limitations of S-N

1. Massive Scatter

2. Initiation and propagation not

separate3. Mechanisms change based on

stress

Low cycle – 

plastic deformationHigh cycle – purely elastic deformation

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The FCP Process

1. Sharp Crack  – closed

2. Stress opens crack 

3. Tip of crack blunts

4. Crack closure/ sharpening

5. Repeat

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The Fatigue Process

1. Sharp Crack  – closed

2. Stress opens crack 

3. Tip of crack blunts

4. Crack closure/ sharpening

5. Repeat

*Note: Large load = large plastic zone =

large compressive stress = difficult crack 

opening on next cycle

** Thus periodic overloads during fatigue

may actually slow the failure process

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Load Order Is Significant

l= low load cycle h = high load cycle

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Effect of Stress Level on FCP Rate

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Effect of constant Delta-K Values

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Monitor Stress Intensity and Crack Speed:

da/dN – Delta K 

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Real Data for Stainless Steel