Working at the heart of test & measurement Fatigue Life Analysis Suite.
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Working at the heart of test & measurement Fatigue Life Analysis Suite
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Transcript of Working at the heart of test & measurement Fatigue Life Analysis Suite.
Working at the heart of test & measurement
Fatigue Life Analysis Suite
www.prosig.com
Working at the heartof test & measurement
History
Fatigue Analysis started in the early 1800’s
Early methods led to formulation of Whole Life Estimation– Measurement of the number of cycles Nf for
a specimen to fail at a stress level S.– Production of a set of stress-cycles curves
known as SN curves.– Allowed estimation of Stress Life.
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Stress Life
May be applied to simple specimens or entire structure
Service life is measured and decomposed into fatigue cycles (Rainflow Analysis)
SN curve is used to determine– the Number of Cycles to failure Nk for a given
stress level Sk
– Damage Increment, Dk, = 1/Nk
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Stress Life
Damage increments summed linearly give measure of total damage
If the total damage is DT then the fraction of life used is 1/DT
This is known as Miners Rule Like Rainflow counting it is
common to ‘Whole Life’ and the ‘Crack Initiation’ method
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Stress Life
Stress Life method– limited to low stress regions where structure
and materials behave elastically– need for life prediction under high stress
regions led to ‘local strain’ method– proposed 1960 by Manson & Coffin– Smith Topper Watson is modern method
Determines the duration before a crack forms
Sometimes called Crack Initiation More usually known as Strain Life
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Strain Life
Strain Life approach is similar to the Stress Life– uses Rainflow counting– uses the Miners Rule linear damage method
Principle difference is the use of a fatigue model– Instead of an SN curve Strain Life uses
fatigue related properties of material– Only necessary to analyse material and not
structure
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Strain Life
Important facet of strain life method…– relates the measured strains (nominal
strains) to local strains at a notch or other high stress point
Strain Life method– handles both the elastic and the plastic
regions– is the preferred method for highly stressed
components– predicts when crack will be initiated. Highly
stress components often fail when a crack is initiated.
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DATS Fatigue Life Analysis
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Working at the heartof test & measurement
DATS Fatigue Life Analysis
Differences between Strain Life and Stress Life
Stress Life Region
Stress and Strain
Number of Cycles
Strain Life Region
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DATS Fatigue Life Analysis
Stage One - Capture data Sufficiently long, representative signal Data Recorded from strain gauges
sited near critical point
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DATS Fatigue Life Analysis
Stage Two – Extract peaks & troughs
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DATS Fatigue Life Analysis
Stage Three - Select material
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DATS Fatigue Life Analysis
Stage Four - Calculate Fatigue Life
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Working at the heartof test & measurement
DATS Fatigue Life Analysis
Summary & General Rules of Fatigue– High cycle low stress– Low cycle high stress
More Severe
Stress and Strain
Number of Cycles
Working at the heart of test & measurement
End
