Lecture 15 Fatigue Mechanical Behavior of Materials Sec. 11.1-11.3 Jiangyu Li

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Lecture 15Fatigue

Mechanical Behavior of Materials Sec. 11.1-11.3

Jiangyu LiUniversity of Washington

Mechanics of Materials Lab

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S-N Diagram

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Safety Factor

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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 a=500 Mpa. Suggestion is made that parts of this type be replaced when the number of cycles applied reach 1/3 of the life.

a) What is the safety factors in life and in stress

b) Is the suggestion good?

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Facture Mechanics Method of Fatigue

aFK

aFK

I

I

minmax

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Crack Growth

> >

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Fatigue Life

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Fatigue Crack Growth Rate

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Crack Grow Rate

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Crack Growth Rate

f

i

f a

am

N

f

mI

aF

daC

dNN

KCdNda

)(

1

)(

0

2

max)(

1 F

Ka Icf

aFK I

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Effect of R Ratio

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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.

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Example

kpsicIM

4.115/

6430)(

1

0

f

i

f a

am

N

faF

daC

dNN

inab

a

inF

Ka

f

f

Icf

111.0

,07.1,254.0

127.0)(1 2

max

Initial estimate, F=1

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Assignment

Mechanical Behavior of Materials

11.3, 11.7