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Fatigue Test
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Objectives Recognize fatigue as a major cause of failure in
mechanical components subject to cyclic loading. Define common terms related to fatigue testing.
Describe the general procedure and equipment used
in conducting fatigue testing. Recognize the stress-life data presentation in the S-
N curve and be able to define fatigue limit, fatigue
strength, and fatigue life. Recognize the major characteristics of fatigue
fracture surface.
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Fatigue
Failure caused by cyclic loading is termed
fatigue
The number of total loading cycles applied
until fracture is called the fatigue-life.
Fatigue is responsible for more than 70% all
Engineering materials failures.
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Fatigue Stages
Fatigue failure consists of two stages
Crack initiating
Crack propagation
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Crack initiating
For machine components containing no pre-
existing cracks, the majority of fatigue life is
spent in initiating or starting fatigue cracks
and the fatigue process is described asinitiation-controlled.
Examples of these include crank shafts, gearteeth, and rotating shafts or axles
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Crack propagation Large structures or welded parts almost
always contain pre-existing cracks such as in bridges, ships, and pressure vessels. In such
structures, the majority of fatigue life is spentin growing a pre-existing crack to a criticalsize and then to final fracture. The fatigue
process in this case is described propagation-controlled.
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S-N Curve
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Fatigue Limit
For some materials, there is a stress level
below which fatigue failure will not take
place. This stress level is a property of the
material and called the endurance limit or the fatigue limit and can be observed only in the
S-N curves for steel alloys.
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Fatigue Strength
Many materials do not show some definite
limit.
For such materials we some take definite
value of stress
Most common is 107 cycles
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Important Factors
Stress concentrators
Surface finish
Size of Specimen
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Experiment In this laboratory session we will be concerned only with
fatigue testing of un-cracked specimens where most of fatiguelife is spent in the initiation stage
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