Objectives

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Objectives

• Explain the two approaches to fatigue
  characterization.
• Explain the different assumptions underlying the
  two approaches.
• Identify the fatigue limit, fatigue strength, and
  fatigue life from an S-N curve
• 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 stress
  concentration (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
Multiple Specimens Different Stresses Cycled to
Failure
BCC HCP Metals
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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 with
  flaws just like the test specimens.
• Implies that most of the life of a structure
  occurs 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

• Massive Scatter
• Initiation and propagation not separate
• Mechanisms change based on stress
• Low cycle plastic deformation
• High cycle purely elastic deformation

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The FCP Process

• Sharp Crack closed
• Stress opens crack
• Tip of crack blunts
• Crack closure/ sharpening
• Repeat

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The Fatigue Process

• Sharp Crack closed
• Stress opens crack
• Tip of crack blunts
• Crack closure/ sharpening
• 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
• hhhhhhhhlllllllllllllllllllllllllllll best
• lllhlllhlllhlllhlllhlllhlllhlllhlllh
• hhlllllllhhlllllllhhlllllllhhllllllh
• lllllllllllllllllllllllllllllhhhhhhhh worst

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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