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Notch factor in low cycle fatigue analysis

[ Edited ]

Hi All,

how NX includes the local yield/plasticity effect (i.e. in notched components) in fatigue life analysis?

Based on the NX documentation, the procedure is as follow:

 

Exclude the notch from the FE model;

Use linear FE solutions for stress and strain;

Input notch factor (need to be calculated, thus another model needed) to account for local yielding

 

What means by the notch factor?  is it the elastic stress concentration factor or the fatigue one (usually recommended)?

 

Does the procedure above mean that the static analysis advised for model without notch?!, i.e; for smooth specimen find the stress and strain will be corrected by the input of the notch factor which needs to be calculated and from the FE of the notched model (Kt) with a notch sensitivity factor that is required the characteristic length of the material.

 

why does the software not use the actual notched specimen where the elastic stress concentration factor is derived and do the calculation for the fatigue Kf the notch factor?

 

For notched model subjected to multiaxial loading, and exhibit a local yield, and has a positive mean stress, Does NX capable to deal with/simulate this complexity, or not?

 

Your comment is appreciated

Thanks

 

5 REPLIES

Re: Notch factor in low cycle fatigue analysis

Wazy,

 

You are correct about the approach.

 

The notch factor Kf is a ratio of the fatigue strength of the specimen with no stress concentration to the fatigue strength of a similar specimen with a notch.It is always less than the static stress concentration factor Kt. Notch sensitivity q can be used to relate Kf to Kt, as you mention. Dowling provides more on this as well as approximation when there is no yielding, local and full yielding. So from a simple model and a linear static analysis, you can predict low cycle fatigue due to stress raisers.

 

My understanding is that notch sensitivity is empirically derived, and hence the software would need additional material libraries in order to estimate Kf.

 

NX Durability supports multiaxial loading with local yielding and mean stress effects.

 

Phil

 


Philippe Tremblay
phillippe.tremblay.ext@siemens.com
www.mayahtt.com

Re: Notch factor in low cycle fatigue analysis

Thank you, Phil for confirming the procedure. 

 

I do not why the software leaves such a calculation to the user where it could be defined directly from the model and within an option to the user which correction to choose (Neuber, Peterson, etc). Hopefully, I am not missing something here, as the software should have a philosophy here.

 

Regards

Re: Notch factor in low cycle fatigue analysis

[ Edited ]

Dear Phil;

I hope you still here and can answer me (I was following your threads here, particularly that posted in 2016 as most reflect your expertise in durability-my interest topic)

 

Flowing the same topic above, it looks there is an issue with the procedure described above.

 

The power of the NX-durability is that it adopts most the advanced, and common methods for fatigue-life prediction/assessment.

 

For example, in multi axial loading case, combining the stress components, and examine the critical plane direction based on the biaxial ratio, it is almost the way (even in NX) to predict the product life and they are critical factors here. The procedure above seems to ignore all these key parameters. 

 

To clarify,

Assuming a plate within an inclined notch under axial fatigue load. This case we can say it is always there is a local biaxial load state at the notch (due to inclination). Finding the Kt, and then Kf for the notched model is suggested in the procedure discussed above 

Then to go for fatigue and durability analysis, we advised using the smooth specimens without notch which is combined with Kf to predict the life.

 

for the same example the unnotched model with axial load combined with s-N, or e-N curve will not reflect the complex case of the load and the results will be for the typical simplified case of tensile fatigue loading. 

Here there is no biaxial ratio, in contrast to the actual case and the simulation does not reflect the suggested problem,

 

Your comment is appreciated

Thanks

 

Re: Notch factor in low cycle fatigue analysis

Dear Wazy,

 

that's back on the pure fatigue theory part, where I feel sur eto answer Smiley Happy

 

When it comes to the influence of the load situation (type of load, combimation of tensile and bending etc.) the consideration of single notch factors is fastly getting to its etch as e.g. for non proportional loading the load situation changes over time.

Therefore indeed the software should do the correction based on the real biaxility ratio which may also change over time. We are also indeed planning to add a method doing this in the release - SC 13. In principle this method calculates the time dipendent biaxiality factor over time estimates a notch factor from it and applies it to the equivalent stress.

 

So not today or tomorrow but soon Smiley Happy

 

Best Regards

 

Michael

Dr.Michael Hack
Business Product Line Manager Durability
Simulation and Testing Solutions

Re: Notch factor in low cycle fatigue analysis

[ Edited ]

Dear Michael,

 

Thanks for your reply. will looking forward to seeing the coming version of SC 13

 

Regards