Intro
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Your 02-21-2011 Email:
How does the concept of LCF being virtually all crack growth reconcile with claims that machining damage
leads to lower life? It doesn’t seem that we can say all the difference is involved in damage that occurs only
to a shallow depth. Unfortunately, I haven’t paid attention to the definition of damage in these claims. If
damage only occurs to a depth of a few mils it doesn’t seem that we could explain much change in life. Even
at 10 mils there would not be a big difference.
At high strain range (LCF) I would expect no change in life.
At strain ranges near a presumed HCF fatigue endurance limit, perhaps more life variation might be
observed due slow growth along the shallow slope of the S-N curve.
Robin,
You almost have it figured out. The following slides are visuals
from a lecture in my Fatigue Course in 1987; with some recent
annotations to reflect you recent questions/observations.
Mike H
Surface-Effects-for-Schwant_20110221.ppt
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MF Henry
2/21/2011 3:12 PM
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Setup
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Surface-Effects-for-Schwant_20110221.ppt
• It’s often easiest to think if you look at the
conclusion first.
• Anything you do to drive down Dep will drive up
Nf; and vice-versa.
• It still works even for surface effects, if you
look at the whole picture.
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MF Henry
2/21/2011 3:12 PM
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Cyclic Stress and Strength Effects
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• It’s a generic effect that surface roughness effects will be larger at lower cyclic stresses
and longer lives.
• But remember that the correlating effect is Dep, so you could also says that at a fixed life
the effect would be larger in a strong material than in a weaker (more ductile) material
• I know you already know this, but I’m just trying to build a complete story.
Surface-Effects-for-Schwant_20110221.ppt
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2/21/2011 3:12 PM
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Roughness Effects in Steels
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•
There’s usually lots of data for steels
Surface-Effects-for-Schwant_20110221.ppt
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Surface “notches”
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Surface-Effects-for-Schwant_20110221.ppt
• But, it’s really more than just surface roughness as a
scalar quantity
• Here are some schematics of a study Lyman Johnson
and I did on B5F5 steel in 1969 (B5F5 was a steel
used by GE Aircraft engines at that time).
• When the grinding lie was not perpendicular to the
principal cyclic stress, the cracks initiated in the
grinding lie and their early growth was in the grinding
lie. Thus, they were not Mode I cracks, and their
early growth rate was slower, and the specimen life
was longer.
• This is really the basic problem with most fillets. In
normal machining practice, the grinding or machining
lie in fillets ends up perpendicular to the subsequent
principal stress in service.
• So, it pretty obvious that the grinding (machining)
marks act like little notches, and are thus stress
strain concentrators; giving you “already existing
cracks” (defects) and local Dep values higher than on
smooth surfaces.
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Surface “notches”
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• We already saw that not only was surface roughness a factor, but also the local geometry
of the roughness and the resultant local Dep.
• But still that’s not the whole picture. Based on the schematic cross-sections shown above,
one would think that the electropolished surface would have a longer life. It turns out
that in many cases, that is not true.
• The next effect you need to consider is the properties of the surface layer in which the
grinding/machining roughness exists.
• A good machinist using good grinding practices will actually leave compressive residual
stresses on the surface, while abusive grinding practices and many (but, not all) single
point turning practices can leave a residual tensile stress on the surface. Early cracks
then could be growing in a surface layer of residual compressive or residual tensile stress.
• So in the interaction of the two effects, it’s not always obvious who wins. Sometimes it’s
the roughness, sometimes it’s the residual stress.
Surface-Effects-for-Schwant_20110221.ppt
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Surface-Effects-for-Schwant_20110221.ppt
Residual stresses and combined effects
• This table and this graph tell the interaction story.
• The table:
– Note that the grinding fatigue strength goes from 80% to 140%
of the mechanically polished fatigue strength. That’s because
the details of the grinding are important.
– Electropolishing gives a lower fatigue strength than mechanical
polishing because while both remove essentially all the surface
roughness, mechanical polishing leaves behind a slight surface
compressive stress and electropolishing leaves none.
• You can also reason out the other comparisons
• Parenthetically (relative to GE practices), it’s worth noting
here that when you measure the fatigue resistance of LSG
fatigue bars you are measuring some combination of the
materials inherent fatigue resistance plus the details of
the LSG process, but that electropolished fatigues bars
would actually let you compare the inherent fatigue
resistance of two materials or two ways of processing one
material.
• I’ve have a great GE story on this one, but it’s too long to
detail here.
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Combined effects
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Surface-Effects-for-Schwant_20110221.ppt
• A simpler example of an interaction effect.
• Pictured are micrographs of surface replicas of an
electropolished (a) and shot peened (b) surfaces
• The shot peened surface is much rougher, and even
looks like it may have some re-entrant corners that
could act like preexisting cracks.
• But shot peened specimen has a deep (~0.006”) layer
of compressive residual stress. Also there is a
reduced local Dep ahead of microcracks because of the
heavily worked material from shot peening.
• The two shot peening effects win out over the
roughness effect, and the shot peened specimen has a
longer fatigue life.
• But there can still be an additional effect. See next
page.
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Combined effects
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Surface-Effects-for-Schwant_20110221.ppt
• Shot peening sounds great, but like most good thing,
you can overdo it.
• If you peen too heavily, you can get a substantial
tensile residual stress peak below the surface
compressive residual stress. If that tensile peak is
high enough, it is possible to overwhelm the preference
for surface initiation of cracks, initiate subsurface in
the tensile peak and get a net reduction in fatigue life.
This is not uncommon in thin sheet applications.
• The data are from an early 1970’s study by Lyman
Johnson and I on IN718 sheet.
• The importance of this example is that it illustrates
that to understand fatigue you often need to drill
down another level (pun intended) to get all the
effects to make sense of some apparent fatigue
anomalies.
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HCF vs LCF
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Surface-Effects-for-Schwant_20110221.ppt
• Also need to remember that “LCF” and “HCF” are
relative terms.
• Materials can’t count. They can, however, tell if
they are being cycled predominantly elastically or
predominantly plastically. The cross-over point is
called the transition fatigue life, NT...
• A material cycled substantially above NT will
exhibit HCF characteristics, and when cycled
substantially below NT will exhibit LCF
characteristics.
• Strong materials have low NT, and ductile
materials have high NT.
• Using the graph to the left, a material with
NT=100 cycles would have all the characteristics
of HCF when cycled to give a life of 3,000 cycles,
while a material with NT=100,000 cycles would
have all the characteristics of LCF when cycled to
give a life of 3,000 cycles.
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2/21/2011 3:12 PM
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Wrap-up
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Surface-Effects-for-Schwant_20110221.ppt
• I know it sounds a little pedagogical, but this is
the actual slide I would finish the lecture with
in 1987.
• Hopefully the preceding stuff convinces you
that it depends on what the details of the
“machining damage” are.
• Let’s chat if you’d like.
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MF Henry
2/21/2011 3:12 PM
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