Intro

Surface-Effects-for-Schwant_20110221.ppt MF Henry 2/21/2011 3:12 PM 1

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Intro

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

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.




Setup

• It’s often easiest to think if you look at the conclusion first.

• Anything you do to drive down p will drive up Nf; and vice-versa.

• It still works even for surface effects, if you look at the whole picture.




Cyclic Stress and Strength Effects

• 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 p, 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.




Roughness Effects in Steels

• There’s usually lots of data for steels




Surface “notches”

• 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 p values higher than on smooth surfaces.




Surface “notches”

• We already saw that not only was surface roughness a factor, but also the local geometry of the roughness and the resultant local p.

• 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.




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.




Combined effects

• 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 p 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.




Combined effects

• 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.




HCF vs LCF

• 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.




Wrap-up

• 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.

Intro

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