ME350 Static Machine Components

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ME 350 Static Machine Components
Lecture Note 10 Fluctuating Stress Fatigue
Criteria -- III Dr. Y.B. Guo Mechanical
Engineering The University of Alabama
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Todays Lecture

• Fluctuating Stresses Characterization
• Fatigue Failure Criteria _at_ Fluctuating Stress
• Examples

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Fluctuating Stresses Characterization

• Fluctuating Stress

Repeated reverse stress
w
A
A
A
Neutral Axis
A
A
. . . .
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Fluctuating Stresses Characterization

• Fluctuating Stress

Non-zero mean
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Fluctuating Stresses Characterization

• Fluctuating Stress

Nomenclature smin smax sa sm sr

minimum stress
maximum stress
stress amplitude
midrange/mean stress
stress range
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Fatigue Failure Criteria

• Modified Goodman Diagram

Derived from testing.
Tension
Modified Portion
Goodman relation considers fatigue failure
and _________________.
ultimate failure
sa
Modifications account for failure due to
__________.
sa
yielding
Points inside curve should not experience
__________ or ___________ failure.
sm
Compression
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Fatigue Failure Criteria

• Fatigue Failure

Sa ____________________
Amplitude Strength
Mean Stress
Compression Region -
___________ has little effect on Sa/Se
modified Goodman line
Tension Region -
Failure limit given by ______________________.
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Fatigue Failure Criteria

• Fatigue Failure

n safety factor
Soderberg Line
yielding

• Only the _______________ has a limit against
  ________.

Mean Strength
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Fatigue Failure Criteria

• Example
• A 1.5 dia. bar of AISI 1050 steel is to
  withstand a fluctuating tensile load varying from
  0 to 16 kip. The fatigue stress-concentration
  factor Kf is 1.85 for 106 or larger life. Find
  Sa and Sm and the safety factor against fatigue
  using (a) Berger fatigue line (b) ASME-elliptic
  line.

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Fatigue Failure Criteria

• Example (Contd)

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Fatigue Failure Criteria

• Example (Contd)