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Fatigue Failures
Metal fatigue iscaused by repeatedcycling of of the load.It is a progressivelocalized damage dueto fluctuating stressesand strains on thematerial. Metal fatiguecracks initiate andpropagate in regionswhere the strain ismost severe.
The process of fatigueconsists of threestages:
Initial crackinitiationProgressive
crack growthacross the part Final suddenfracture of theremaining crosssection
Schematic of S-N Curve, showingincrease in fatigue life with decreasingstresses.
Stress Ratio
The most commonly used stress ratio is R, the ratio of theminimum stress to the maximum stress (S min /S max ).
If the stresses are fully reversed, then R = -1.
If the stresses are partially reversed, R = a negative number less than 1.
If the stress is cycled between a maximum stress and noload, R = zero.
If the stress is cycled between two tensile stresses, R = a
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ExplosionsCastingsBoilersHeat ExchangersPressure VesselsPipelinesLifting Equipment
FastenersGearsBearingsShaftsResidual StressesChemical ProcessingPulp and Paper Food Processing
AutomotiveShips
AlloysSteelStainless Steel
AluminumCopper Titanium
positive number less than 1.
Variations in the stress ratios can significantly affect fatigue life. Thepresence of a mean stress component has a substantial effect on fatiguefailure. When a tensile mean stress is added to the alternating stresses, acomponent will fail at lower alternating stress than it does under a fullyreversed stress.
Preventing Fatigue Failure The most effective method of improving fatigue performance isimprovements in design:
Eliminate or reduce stress raisers by streamlining the part
Avoid sharp surface tears resulting from punching, stamping,shearing, or other processes
Prevent the development of surface discontinuities duringprocessing.
Reduce or eliminate tensile residual stresses caused bymanufacturing.
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Improve the details of fabrication and fastening procedures
Fatigue Failure Analysis
Metal fatigue is a significant problem because it can occur due torepeated loads below the static yield strength. This can result inan unexpected and catastrophic failure in use.
Because most engineering materials contain discontinuities mostmetal fatigue cracks initiate from discontinuities in highly stressedregions of the component. The failure may be due thediscontinuity, design, improper maintenance or other causes. Afailure analysis can determine the cause of the failure.
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Send mail to [email protected] with questions or comments about this web site.Copyright 1999 Metallurgical ConsultantsLast modified: September 11, 2007
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