Showing results for 
Search instead for 
Do you mean 
Reply
Solved! Go to solution

Mean stress effect

Hi,

 

Is the mean stress effect included in the low cycle fatigue analysis?

and

Have we to modify the strain life equation for this purpose ? 

if so, how is that, please

 

if I want to use the shear strain life, and the critical plane method how to include the mean stress effect in this case?

 

Thanks for your help and comments 

 

5 REPLIES
Solution
Solution
Accepted by topic author Wazy
2 weeks ago

Re: Mean stress effect

Dear Wazy,

 

In strain-life approaches, the mean stress is taken into account by teh so called damage parameter.

Instead of comparing the strain amplitudes to the strai-life curve directly, one uses a mean stress corrected value.

In Morrow type approaches the idea is to split the strain value in its plasic and elestic part and apply a mean stress correction to the elastic part (as the elastic strain varies linearaly with the stress)

In Swith-Watson-Topper approaches one uses a more complex formula.

 

So you do not need to change your strain-life data - the mean stress effect is taken account for in the method, i.e. damage parameter.

 

In LMS Virtual.Lab Durability we have implemented a shear strain method that uses a damage parameter for shear (which is of course symmetric in the shear).

 

Best Regards

 

Michael

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

Re: Mean stress effect

Dear Michael,

 

Thanks for your prompt response,  

Can I use Morrow or SWT correction with Brown Miller in the critical plane method, please? 

 

Another important question in strain life approach, please 

 

The input of the fatigue notch factor (for local yield effect) Kf, Do I have to calculate it from a separate model? 

Knowing that the static analysis is available in the durability analysis.

 

Thank you so much

Re: Mean stress effect

Dear Wazy,

 

first a remark, you may know we (still) have two durability solvers at Siemens. For the current Simcenter Advanced Fatigue, I do not know all the intrisics. 

But from a theoretical point of view:

We did here in the German durability community quite some projects on multiaxiality, some findings:

  • different materials react differently to defined multiaxial behavior
  • with defined of deterministic I mean in-phase and out-of-phase loading
  • For all these deterministic loads for given materials, one can chose a matching multiaxiality model
  • For real life loading (non-proportional variable amplitude) there is no such "optimal" model
  • Therefore we prefer critical plane approaches with either pure tension or pure shear
  • The latter also make clear how to use mean stress methods

For mean stress effects for combined modes there are interpolation ideas, but not implemented in our software.

 

For Kf there are different effects, Size effects should be handled with stress gradient methods (currently only in LMS Virtual.Lab Durability, will of course also come to Simcenter in the future).

 

Best Regards

 

Michael

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

Re: Mean stress effect

Dear Michael,

 

 Again, I'm really grateful for your valuable answer,

 

Regarding the Kf input in the strain-life method, is it the only the correction factor of Kt (stress concentration factor), i.e; in terms of the notch sensitivity (q), i.e; (q)=(Kf-1)/(Kt-1), And in this case we need to get the Kf from separate analysis where Kt, calculated and Kf determined based on the formula above.

for example for mild steel assume q=0.2, for elastic analysis Kt=10, Kf= 2.8 (this will be the input in NX for strain-life analysis)

 

is that correct, please? any example to clarify will be of great help, as the help document does not show this clearly

 

Best Regards

Re: Mean stress effect

Wazy,

 

Strain life uses a modified Morrow equation. Both SWT and Strain Life criteria can be used with the critical plane 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. Dowling provides more on this as well as approximation when there is no yielding, local and full yielding.

 

Phil


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