FATIGUE

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FATIGUE
Fatigue failure under cyclic stress.
Stress varies with time. --key parameters
are S and sm
Key points Fatigue... --can cause part
failure, even though smax lt sc. --causes 90
of mechanical engineering failures.
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FATIGUE DESIGN PARAMETERS
Fatigue limit, Sfat --no fatigue if S lt
Sfat
Sometimes, the fatigue limit is zero!
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FATIGUE MECHANISM
Crack grows incrementally
typ. 1 to 6
increase in crack length per loading cycle
crack origin
Failed rotating shaft --crack grew even
though Kmax lt Kc --crack grows
faster if Ds increases
crack gets longer loading freq.
increases.
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IMPROVING FATIGUE LIFE
1. Impose a compressive surface stress
(to suppress surface cracks from
growing)
--Method 1 shot peening
--Method 2 carburizing
2. Remove stress concentrators.
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PROCESSING USING DIFFUSION (1)
Case Hardening --Diffuse carbon atoms
into the host iron atoms at the surface.
--Example of interstitial diffusion is a
case hardened gear.
Result The "Case" is --hard to deform C
atoms "lock" planes from shearing.
--hard to crack C atoms put the surface
in compression.
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MEASURING ELEVATED T RESPONSE
Elevated Temperature Tensile Test (T gt 0.4
Tmelt).
Generally,
.
.
.
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CREEP
Occurs at elevated temperature, T gt 0.4
Tmelt Deformation changes with time.
Adapted from Figs. 8.26 and 8.27, Callister 6e.
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SECONDARY CREEP
Most of component life spent here. Strain
rate is constant at a given T, s --strain
hardening is balanced by recovery
stress exponent (material parameter)
.
activation energy for creep (material parameter)
strain rate
applied stress
material const.
Strain rate increases for larger T,
s
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CREEP FAILURE
Failure along grain boundaries.
Estimate rupture time S 590 Iron, T
800C, s 20 ksi
g.b. cavities
applied stress
24x103 K-log hr
Time to rupture, tr
temperature
function of applied stress
1073K
Ans tr 233hr
time to failure (rupture)
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SUMMARY
Engineering materials don't reach theoretical
strength.
Flaws produce stress concentrations that
cause premature failure.
Sharp corners produce large stress
concentrations and premature failure.
Failure type depends on T and stress
-for noncyclic s and T lt 0.4Tm, failure stress
decreases with increased maximum flaw size,
decreased T, increased rate of
loading. -for cyclic s cycles to fail
decreases as Ds increases. -for higher T (T gt
0.4Tm) time to fail decreases as s or T
increases.
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