SimScale CAE Forum

What does a simulation do to a material?


#1

I am currently unhappy with the Static and Dynamic analysis reports in the simulation industry. The reports only state the maximum Stress values in a very scientific form. What I am missing in the reports is the information, what those values actually mean for the used material. Will it break? Will it bend irrevocably? Will it fail? …
I would expect a report to (automatically) contain the maximum allowed Stress values, which should be stored in the material database, and I need a short explanation (both in the Post-Processor and in the Reports), whether the simulation succeeded within the given limits or not, or if possible what it means.
So my suggestion is:
Add the fields for the maximum stress values to all materials in your material database.
In the Simulation Designer, retrieve the values from the material database (preferred) or let the user define them.
Analyze the Simulation results and provide a proper textual explanation of the Simulation result.


#2

Hi @p_guehring,

I think you can give an estimation or guideline how to handle the output information but not a guarantee that your component will fail by all means.

That is where the engineering experience comes in and you have to decide if the stresses/strains are within reason (I think ANSYS has a plugin that can tell you).

Furthermore if do a static analysis (and in almost every advanced dynamic analysis) we do not take into account the variations of loads we have in real life but making assumptions and simplifications that we as a user think are appropriate.

"What kind of material fatigue do we have?" --> We need laws to describe our material behaviour. For instance Paris’ law for fracture mechanics or other fitting laws that describe our problem in best ways possible.

Here is a video (unfortunately in German) that shows that there are tools out there to determine curves for High-Cylce Fatigue.

Best,

Jousef


#3

Hi @p_guehring,

Simulation results are only as good as the information entered in by the end user, so no software vendor will take on the liability of saying a part will not fail when they have no control of the data being entered or any understanding of the parts operating environment.

As far as material properties go, the yield stress is the ideal stress at which a part will start to fail. If you perform an analysis and the maximum stress is below the yield stress, ideally it will not fail. Since the real world is not ideal we typically apply reduction factors to the Yield stress to determine the allowable stress. For example, if you perform a static analysis on an engine mount you need to account for the vibration in the operating environment. Typically high cycle vibration will reduce the yield stress by 50%. Variability in how the part is machined or cast could reduce the yield by another 10-15%. Higher temperatures will reduce the Yield stress, as does being in a corrosive environment such as salt air.

So depending on how the part is used, its operating environment and a host of other factors, determines the allowable stress, and these are factors you cannot model. A36 Steel has a yield stress of 220 MPa. In some cases the allowable stress may be close to 220 MPa. However, it you are designing an engine mount for an ocean going vessel teh maximum allowable stress could be around 100 MPa or less.

If you are working on a specific problem right now and are trying to figure out if something will fail or not, please provide us with more detail and we will see how we can help you out.

I hope this helps.
Christopher


#4

Thanks for your feedback!
I agree that the wording has to be careful. And I understand that we are talking more about probabilities than guarantees.
A few days ago I learnt that there are ISO standards for various applications which define test scenarios with defined forces and the maximum allowed stresses. So those scenarios could also be integrated and easily selectable, and the result of the simulation would tell you whether the simulation passed those criteria or not.
As I stated before (perhaps I did not explain it well enough), the user has to define the maximum values in the Simulation designer, and the software should help the user during the definition by providing various reasonable suggestions (based on the material library) and explanations how they should be adapted for specific situations (e.g: 1. High Cycle vibration: -50%, 2. variability: -10-15%, 3. higher temperatures: -10%, 4. ocean vessel: -60%), the user can select the scenario(s) he needs and gets default values that he can override with his own targets.
Perhaps the result of the simulation could be a percentage of the safety-margin to the allowable stress?
“The simulation calculated 170 MPa stress, which is a safety margin of 12 % to the allowed stress value of 220 MPa.” in case of a successful result, and
“The simulation calculated 240 MPa stress, which is 9% above the allowed stress value of 220 MPa” in case the simulation “failedd”
I don’t have a specific problem right now, thanks for your offer for help.


#5

Hi @p_guehring,
When I was at the NAFEMS conference a few weeks ago there was a lot of discussion about Smart Apps and embedding expert knowledge into simulation tools and processes. I think you are just a visionary and a little bit ahead of the industry right now. Hopefuly this type of knowledge based tool will be coming in the not too distant future. :slight_smile:

Christopher