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Mechanics of Materials Lab. Lecture 15 Fatigue Mechanical Behavior of Materials Sec. 11.1-11.3 Jiangyu Li University of Washington. S-N Diagram. Safety Factor. Example.

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Mechanics of Materials Lab

Lecture 15

Fatigue

Mechanical Behavior of Materials Sec. 11.1-11.3

Jiangyu Li

University of Washington

Jiangyu Li, University of Washington


S n diagram
S-N Diagram

Jiangyu Li, University of Washington


Safety factor
Safety Factor

Jiangyu Li, University of Washington


Example
Example

9.10 For AISI 4340 steel in Table 9.1, a life of 1.94x105 cycles to failure is calculated for the stress amplitude of sa=500 Mpa. Suggestion is made that parts of this type be replaced when the number of cycles applied reach 1/3 of the life.

  • What is the safety factors in life and in stress

  • Is the suggestion good?

Jiangyu Li, University of Washington


Facture mechanics method of fatigue
Facture Mechanics Method of Fatigue

Jiangyu Li, University of Washington


Crack growth
Crack Growth

>

>

Jiangyu Li, University of Washington


Fatigue life
Fatigue Life

Jiangyu Li, University of Washington


Fatigue crack growth rate
Fatigue Crack Growth Rate

Jiangyu Li, University of Washington


Crack grow rate
Crack Grow Rate

Jiangyu Li, University of Washington


Crack growth rate
Crack Growth Rate

Jiangyu Li, University of Washington


Effect of r ratio
Effect of R Ratio

Jiangyu Li, University of Washington


Example1
Example

  • For a bar subjected to 0<M<1200lb · in, Sut=185kpsi, Sy=170kpsi, Kic=74kpsi·in1/2, C=3.8 10-11 (in/cycle)/(kpsi·in1/2)m, m=3.0, a=0.004in. Estimate Nf.

Jiangyu Li, University of Washington


Example2
Example

Initial estimate, F=1

Jiangyu Li, University of Washington


Assignment
Assignment

Mechanical Behavior of Materials

11.3, 11.7

Jiangyu Li, University of Washington


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