Learning Forum

[Miguel]

Hi,

Which would be the best approach to simulate a 20.000 cycles life-endurance test for an assembly with aluminium parts (main components) joined with steel bracket, bolts and nuts? I bet for a strain-life (EN) fatigue analysis but I find the nr. of cycles is a bit too low when compared to the curves....

Thanks

[Daniel Shaw]

StrainLife fatigue is applicable to low cycle fatigue, but you must have sufficient test data in the low cycle range.  If the design life is only 20 cycles, why don't you just run a cyclic non-linear analysis for 20 load steps and compare the results to a design criteria (e.g., total deformation, amount of accumulated plastic strain, etc.).  You need to know the non-linear material properties and select an appropriate non-linear material model (e.g., bilinear kinematic hardening).

[Miguel]

Thank you Daniel Shaw, but I said 20.000 (twenty thousand) cycles... I have this EN data for an aluminium 6061 alloy but 20 Kcycles is definitly too low for SN stress-life and I reckon also too few cycles for strain-life. So...

 

[Daniel Shaw]

Yes, failure at 20,000 cycles is probably due to low cycle fatigue, so the SN approach would probably not be appropriate.  However, the EN approach should be appropriate.  Why do you believe EN is not appropriate?  The low cycle curve shown in the plot extends well below 20,000 cycles.

[Miguel]

Hi again Daniel. Well, you´re absolutely right ..😬 I don´t remember where but I think I read something about a minimum of 10^5 cycles to start using the EN approach for aluminium and my mind got stuck with that figure... So you think it should work fine with this curve, right? 

[Daniel Shaw]

I cannot verify the "accuracy" of the curve (e.g. how many actual test data points are used to develop it), but if we assume that it is a valid curve, then it can be used to evaluate low-cycle fatigue in the range of 20,000 cycles.

[Miguel]

Fair enough Daniel. I have confidence in the curve reliability so...

By the way, in your first reply you recommended a "cyclic nonlinear analysis for 20 load steps and compare the results to a design criteria". In my case, besides the 20.000 cycles fatigue, I also have to assess the system behaviour (with a different loads setup) during 20 cycles but my first cycle already includes 5 steps which required several hundred iterations to converge so, if I understood correctly your proposal, repeating this over 20 times would take ages... Do you know any alternative and shorter way to evaluate the accumulated impact of these 20 cycles?

Thanks

[Miguel]

Just another doubt: as far as I understood, any fatigue analysis (both stress-life and strain-life) presupposes a linear elastic behaviour in the 1st starting cycle although, eventually, plastic local deformations might appear through the cycle (in strain-life low-cycle fatigue). Right?

But, what happens when small plastic strains already appear in the 1st cycle? Is it still applicable the fatigue analysis? The same way and with the same considerations/conclusions?

Thanks

[Daniel Shaw]

SN assumes that all stresses remain elastic throughout the loading history.  It assumes that no plastic deformation occurs.  However, it is used sometimes used with elastically calculated stresses in applications where some plastic deformation occurs in the early cycles, but the behavior “shakes down” to elastic behavior after a few cycles.

EN is applicable to elastic or plastic stresses throughout the loading history. 

[Miguel]

Thank you Daniel. What about the best way to evaluate the accumulated result of just those 20 cycles?

 

[Miguel]

Thank you Daniel. I´ll try that

[Daniel Shaw]

It may depend on the industry and the goal, but tracking accumulated plastic strain is probably a reasonable approach for most applications

[Miguel]

Thank you Daniel. I´ll try that

Thank you Daniel. I´ll try that