Fatigue damage due to compressive load
Fatigue damage due to compressive load
(OP)
I have a question regarding fatigue damage done by compressive-compressive load.
I believe that common approach is to consider only load cycles in which max stress is greater then zero. In other words, assumption is made that compressive-compressive cases (both max and min stresses are compressive) don’t create fatigue damage.
I’m aware that in some cases local compressive yielding could produce residual tensile stress and on that way promote failure of parts which are loaded only in compression.
But, I’m talking here about more fundamental process. If I understand correctly, crack initiation process is caused by local cyclic plastic deformation. As plastic deformation could result from both tensile and compressive cycles, it seems to me that damage due to compressive cycles should be added to.
Actually this post is caused by one remark in MSC.Fatigue manual. In section 14.3 where effect of mean stresses is discussed, there is a note that Smith-Watson-Topper mean stress correction predicts that no fatigue damage can accrue when the max stress becomes zero or negative, which is not strictly true.
Further, in numerical example given in ESDU 95006 document (“Fatigue Life Estimation Under Variable Loading Using Cumulative Damage Calculations”), no difference is made between compressive or tensile cycles. For both, absolute value of local strain range is calculated and used with Strain Life equation to obtain damage increments.
I would really like to hear your opinion on this issue. Do you consider fatigue damage from compressive-compressive load cases and how do you treat them?
I believe that common approach is to consider only load cycles in which max stress is greater then zero. In other words, assumption is made that compressive-compressive cases (both max and min stresses are compressive) don’t create fatigue damage.
I’m aware that in some cases local compressive yielding could produce residual tensile stress and on that way promote failure of parts which are loaded only in compression.
But, I’m talking here about more fundamental process. If I understand correctly, crack initiation process is caused by local cyclic plastic deformation. As plastic deformation could result from both tensile and compressive cycles, it seems to me that damage due to compressive cycles should be added to.
Actually this post is caused by one remark in MSC.Fatigue manual. In section 14.3 where effect of mean stresses is discussed, there is a note that Smith-Watson-Topper mean stress correction predicts that no fatigue damage can accrue when the max stress becomes zero or negative, which is not strictly true.
Further, in numerical example given in ESDU 95006 document (“Fatigue Life Estimation Under Variable Loading Using Cumulative Damage Calculations”), no difference is made between compressive or tensile cycles. For both, absolute value of local strain range is calculated and used with Strain Life equation to obtain damage increments.
I would really like to hear your opinion on this issue. Do you consider fatigue damage from compressive-compressive load cases and how do you treat them?





RE: Fatigue damage due to compressive load
I think it should be more important in pieces which support side buckling stresses.
Hope I´ve been useful.
RE: Fatigue damage due to compressive load
I believe that common approach is to consider only load cycles in which max stress is greater then zero. In other words, assumption is made that compressive-compressive cases (both max and min stresses are compressive) don’t create fatigue damage.
"
Never seen that.
Cheers
Greg Locock
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RE: Fatigue damage due to compressive load
I don't have the ESDU reference in front of me, but i'd be surprised if anyone takes the -ve stresses and turns them into +ve stresses. they'd also have to the zero crossing, otherwise they'd totally mess the spectrum. this sounds like a highly conservative appraoch, possibly in an application quite different to aero-structures, where the spectrum (and the impact of -ve stresses) are different.
I think the answer is that occassional small magnitude -ve stresses are negligible in a spectrum dominated with +ve stresses. Possibly they are not negligible in a spectrum dominated by -ve stresses, with a few tension peaks.
RE: Fatigue damage due to compressive load
Cheers
Greg Locock
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RE: Fatigue damage due to compressive load
Unless you have some really weird geometry, loads that vary entirely on the compression side won't cause fatigue. Probably an unforeseen tensile side of a bending load condition. Not to say you can't have breakage, striations that are indicative of fatigue would be all mashed down, but have to get there by some prior tensile loads.
One thought is you might have a Mode II or Mode III crack growth. Only seen a true Mode II once in practical aviation experience, and never a Mode III.
RE: Fatigue damage due to compressive load
der8110
I believe that you are referring to method based on fracture mechanics, in which is common to assume that some initial flaw already exists in the material.
However, my understanding is that fatigue crack could develop in “flawless” material too, at the location of high stress concentration (of whatever nature).
rb1957
Regarding calc in ESDU procedure
They didn’t convert negative stresses to positive one. Hysteresis loop response has been developed for nominal positive/negative stresses and then (absolute) values of local strain ranges are used for evaluation of fatigue life.
Ok, that makes sense in context of your replies. There is probably assumption that energy available for movement of the dislocations is proportional to the strain range.
Thing is that we are using modified stress based approach which neglects negative cycles, for the calculation of crack initiation time. And while there is apparent relation between tensile stresses and crack propagation, as pointed by der8110, I was unsure about effect of neglected compressive stresses on crack initiation.
So, as compressive cycles do have some effect on crack initiation, which is obviously lower then effect of tensile cycles, I would like to hear how you treat them.
Is it enough to apply mean stress correction (as negative mean stress will increase life) or some other factors should be applied too?
RE: Fatigue damage due to compressive load
RE: Fatigue damage due to compressive load
Bearings are an important example here. They are loaded in compression, but there is a subsurface shear stress that results. Eventually, shear strains accumulate, and cracks can initiate and propagate.
Regards,
Cory
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RE: Fatigue damage due to compressive load
RE: Fatigue damage due to compressive load
It´s the part of the beam where you apply your load and have only compressive stress. It used to be a geometrical closed curve within the center of the beam (usually), and if you applied the load in the whole surface you had compressive and tensile strains.
Apart from shear strains (that in several cases, like the one from Corypad in bearings, uses to be the highest one) I knew that appeared tensile strains when applying compressive loads to the whole surface of the beam in some parts.
Sorry but I can´t give you any further details. I´ve just moved and have my books 1200Km away. If anyone knows what I´m talking about maybe can explain it in detail.
Cheers.
RE: Fatigue damage due to compressive load
'cause landing gears see considerable tension loads due to bending, mostly from "spin-up" and "spring-back" drag loads. plus the critical sections are beams, transferring the ground loads into the wing box.
RE: Fatigue damage due to compressive load
In composites, glass filaments, and other slightly exotic substances there is no such restriction and C-C fatigue is worthy of consideration. The mechanism in composites (eg reinforced concrete) is obvious - axial compression results in a complex stress field between individual fibres and the matrix in which they are held.
Cheers
Greg Locock
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RE: Fatigue damage due to compressive load
RE: Fatigue damage due to compressive load
This also reflects Compositpro's earlier assertion that there are very few applications where variable pure compressive stresses are present during the life of an actual component.
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: Fatigue damage due to compressive load
Your case is hydrostatic stress. Deviatoric stresses are needed for plastic strain, and plastic strain is the prerequisite for fatigue.
Regards,
Cory
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RE: Fatigue damage due to compressive load
The problem was a very high and very local compressive stress resulting in very local plastic deformation that when the load was removed (ie P=0) the elastically strained material adjacent to the plastically strained material returned to its original unstrained condition and the plastic material was put in tension.
Compression stresses do not cause cracks, but plastic compressive strain can cause tension stesses when the load is returned to zero and the resulting tension stress causes cracks.