8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 1/12

Fatigue Test

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 2/12

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.

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 3/12

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.

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 4/12

Fatigue Stages

Fatigue failure consists of two stages

Crack initiating

Crack propagation

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 5/12

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

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 6/12

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.

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 7/12

S-N Curve

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 8/12

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.

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 9/12

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

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 10/12

Important Factors

Stress concentrators

Surface finish

Size of Specimen

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 11/12

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

8/12/2019 Fatigue Test.pdf

http://slidepdf.com/reader/full/fatigue-testpdf 12/12