FATIGUE • Fatigue is the progressiv e and localized structur al damage that occurs when a material is subjected to cyclic loading. • The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.
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FATIGUE
• Fatigue is the progressive and localized
structural damage that occurs when a material
is subjected to cyclic loading.
• The nominal maximum stress values are less
than the ultimate tensile stress limit, and may
be below the yield stress limit of the material.
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ANALYSIS OF FATIGUE
• Stress life approach
• Strain life approach
•
Fracture mechanics approach
In this presentation we shall see about Stress life
approach.
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STRESS LIFE APPROACH
• This nominal stress (S-N) method was the first
approach developed to try to understand this
failure process
• The nominal stress approach is best suited to
that area of the fatigue process known
as high-cycle fatigue
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Stress Cycles
Typical Fatigue Stress Cycles,
(a) Fully Reversed (b) Offset, (c) Random
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The S-N Curve
• In high-cycle fatigue situations, materials
performance is commonly characterized by
an S-N curve, also known as a Wohler curve
• Most determinations of fatigue properties
have been made in completely reversed
bending (i.e., R = –1), by means of the so-
called rotating bend test
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• The stress level at the surface of the specimen is
calculated using the elastic beam equation,
S= Mc/I
S- the nominal stress acting normal to the cross-
section
M- the bending moment
c - the distance of the surface from the neutral axis
I - the moment of inertia
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• S-N data are nearly always presented in the
form of a log-log plot of alternating stress
amplitude versus cycles to failure, with the
actual Wöhler line representing the mean of
the data
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Limits of the S-N Curve
• The S-N approach is applicable to situations
where cyclic loading is essentially elastic
• This means that the S-N curve should be
confined on the life axis to numbers greater
than about 10,000 cycles in order to ensure no
significant plasticity is occurring.
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The Influence of Mean Stress
• Most basic fatigue data are collected in thelaboratory by means of testing procedureswhich employ fully reversed loading
• Most realistic service situations involvenonzero mean stresses
• Fatigue data collected from a series of tests
designed to investigate different combinationsof stress amplitude and mean stress arecharacterized by Haigh diagram
• The counting of peaks makes it possible toconstitute a histogram of the peaks of therandom stress which can then be transformed
into a stress spectrum giving the number of events for lower than a given stress value.
• The stress spectrum is thus a representationof the statistical distribution of thecharacteristic amplitudes of the random stressas a function of time