Can fatigue cracks form under compression-compression stress cycles?

[bugbus]

I have heard conflicting answers to this question before, and I have to say that my background is not in materials but in structural engineering. So my experience with fatigue is less on the theoretical side and more on the code-based side.

Supposing that a steel element is subjected to purely compressive load cycles (i.e. min and max stresses are both compressive), and that there are no residual stresses of any kind in the element (such from welding, hot rolling, plastic deformation, or whatever), is it possible for fatigue cracks to form? I have heard that the answer is "yes", but that the rate is in the order of 100x slower than in tension. However, I have no way of knowing if this is right.

Is this any different, then, to a situation where the stress cycles are in tension, but due to residual compressive stress in the element (such as from peening, or some other effect), the overall stress experienced is still always compressive?

And I suppose the opposite situation is where there is a residual tensile stress in the element, but the stress cycles are purely compressive. At the point under consideration, at least part of the stress cycle occurs in tension. Is it possible for fatigue cracks to form in this situation?

 

[gtaw]

Fatigue is influenced by the change in the magnitude of the cyclic loads. In theory, even if the nature of the loading is compressive, repeated cycles of high compressive stress and low compressive stress can lead to fatigue. However, one must consider the magnitude of the change in stress and the number of cycles. If the nature of the loading is always in compression, the change in magnitude may not be sufficient to result in fatigue. The influence on the geometry of the connection must also be considered.

AWS D1.1 includes some information on fatigue. If the change in magnitude of the stress doesn't exceed some value for a particular connection geometry, fatigue probably isn't going to be a problem. Then of course, one must look at the expected number of load cycles the connection will experience, if it is below a certain number, fatigue probably won't be an issue.

Best regards - Al

 

[TugboatEng]

The applied load may be compressive but the resultant forces may not be. If you take a disk and smash it, it gets thinner in height but larger in diameter. The outer circumference will be in tension due to the increase in diameter despite the force applied being compressive.

 

[realtime2]

It also depends on the geometry. If there is an notch in a component, hole or radius,
the stress and strain at the root of the notch can go plastic, say on first loading in
compression. Then when your overall nominal load returns to Smax the local stress at
that notch will be in tension, which if sufficient can cause fatigue.
Engineers who do crack propagation tests will often initiate a crack in a notched
plate using compressive loading. After it initiates its propagation may stop
if the notch is small.

Also you may get a "barreling" effect like in a concrete cylinder test and a hard
steel sample can explode or fail in a manner similar to concrete due to shear and
internal tensile stresses, but that is pretty uncommon. The effect of a notch is
more of a common cause.

 

[dik]

About 50 years back I did some work on teeth. I met a dentist in the student union building and we got to talking... he was having problems with amalgam fillings failing when secured to steel pins in the tooth. I asked him about the properties and high compressive strength and low tensile strength. I asked him if it could be a tensile fatigue failure due to tensile stresses developed at the corners of the steel pin... and spent the next 4 months measuring and studying teeth... sure enough, it was a tensile fatigue failure... first time I'd been involved with taking measurements in the micron range and using statistics to determine if data was from the same family...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

dik,

Besides the metallurgy there are close parallels between filling a cavity and making a weld repair:

​

RT - Excavation - Cleaning - Filling - Grinding to profile - Inspection - Payment

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[dik]

Thanks... didn't know that.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[WKTaylor]

Typically I would not expect to see fatigue cracks in high quality high strength steel under compression-compression cyclic loading forces... unless/until pure elastic strain [compression, tensile, shear] becomes plastic, IE: [permanent]deformation… at any point on the structure.

THEN is when cracking might initiate, since compression now transitions to localized tensile/shear strain fields as the permanently compressive-deformed element is forced-returned to the initial low compression state.

Regards, Wil Taylor
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[Nereth1]

If we imagine a prismatic coloumn under cyclic compression with no buckling effects or similar, and we imagine that somehow a crack appears in it, then assuming the crack is closed (negligible gap - because how can it open up without a tension cycle to pull it apart?) then the thing is, the crack can still transmit compressive load because its two faces are going to be touching. So there's no additional stress intensity at the crack tip so what would drive the crack tip to grow?

If there is a gap, and thus some complex geometry, then the stress field at the crack tip will no longer be purely compression.

There are also shear stresses present on shear planes to consider.

I would say in general that any cracks that do form and propogate, are not going to be caused by pure compression. But pure compressive "loads" aren't necessarily going to make pure compressive stress fields. Maybe hydrostatic stress could do it?

 

[mrfailure]

I found a good explanation for compression fatigue cracking on ResearchGate. See below:

"I think your question is why compression-compression fatigue life is more than tension-tension or tension-compression (fully reversed) fatigue life. The possible reason is as follows.

You may be aware that fatigue is a surface phenomena in which the back and forth movement of dislocations (upon cyclic loading) along slip planes/slip directions will lead to formation of persistent slip bands-PSBs ( a kind of slip traces that are permanent). These PSBs are essentially an extra plane of atoms that upon reaching the specimen surface produces small permanent projections, called extrusions (there are theories that say complimentary intrusions....a sink-in of plane of atoms often forms). The extrusions upon continuous cyclic loading will intensify and eventually create a surface imperfection (notch effect) and a micro crack will finally initiate from the intrusion/extrusion....that's is the theory of how a fatigue crack will initiate...

If your cycle is tension-tension or tension-compression, you are forcing the dislocations to move outward, sufficient enough to create intrusions/extrusions after a reasonable number of cycles. On the other hand if your cycle is compression-compression, dislocation movement is largely inwards (barring a few dislocations that may go in the other directions). The possibility of formation of intrusion/extrusion....the primary phenomena that controls the fatigue life...is ceased or at least delayed substantially. That is the reason why you would find longer fatigue life (higher fatigue strength as you put it) in compression cycling. How would you define fatigue failure in such cases is a challenging issue..."

Sivaprasad, S.. (2014). Re: Why is the compressive fatigue strength of a material more than that of pure tension cyclic load?. Retrieved from:

https://www.researchgate.net/post/W...d/534d03c2d2fd64963d8b45c5/citation/download.

 

[bugbus]

Thanks for everyone's replies

I have read through them all very carefully and learned a lot

 

[romychaj]

fatigue/spalling in roller bearings is compressive, right?

 

[dik]

The problem is that the loading regime may generate local tensile stresses depending on the geometry and loading.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EdStainless]

The other thing that happens is that there is some area that yields in compression.
Then when you unload that area goes into tension.
These are often microscopically small bands, but they form real cracks.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[1503-44]

Mohrs circle. Compression + compression still induces shear.

 

[isoca]

I think the trick in this analysis is to think that a piece subjected to "compression" only compresses in all directions.
Pure compression is only obtained under a condition of hydrostatic stresses.

 

[Jughandle]

Agree with all above. Cyclic triaxial compression = no failure. (Except with fissionable materials taken to extreme density Anything other than purely hydrostatic would produce resolved shear stresses, maybe dislocations extruding out as mentioned, etc.