MSC Dytran & Stress Question

[VN1981]

Hello,
I am learning MSC Dytran through our MSC reseller. The problems we are going through are explicit non-linear cases (like a dropping ball tearing through a thin plate).

The stress output being referred is "Equivalent Stress". Up on further reading, it seems that Dytran is referring the First invariant of Stress as Equivalent Stress. What is the physical significance of this stress and why is this being referred to? I mean in static linear, I know the significance of Von Mises..hoping to get some similar understanding of the significance of 1st invariant of stress.

The material model we entered is a typical Isotropic (steel) with specifying yield stress and amount of strain which defines the failure criteria for element deletion.

We did ask the above questions to our trainer but we got no satisfactory answers. Hoping to get some more insights here.

 

[swimfar]

The first invariant of the stress tensor is the trace, which is sigma_xx + sigma_yy + sigma_zz, which is also three times the hydrostatic stress (i.e. -3 * Pressure). You should double check that this is really what you are getting from the software, because it seems like an odd quantity to output. I don't know anything about that software, but there has to be a way to get other stress results. Otherwise the software would be nearly useless.

Also, if your trainer can't even answer these simple questions, I'd ask for someone new. These are pre-FEA 101 questions that they should be able to easily clarify for you.

 

[VN1981]

swimfar, thanks for replying. MSC Dytran is a non-linear explicit code.

This is taken directly from the Dytran Theory Manual about using First Invariant of Stress. Please let me know if I'm interpreting the text incorrectly.

 

[SWComposites]

you need to consult a Strength of Material text to determine if "equivalent stress" is an appropriate failure criterion for the materials that you are using. for most ductile materials, Von Mises equivalent stress is used as a yield failure criterion.

 

[swimfar]

I think their parenthetical comment is an error. I'm pretty sure equivalent stress is von Mises stress here, which is the second invariant of the deviatoric stress tensor.

 

[Alex Szatmary]

To me, this looks like a typo in the manual—everything makes sense if you change "first" to "second". If you search the manual for invariant, it seems that they are using the usual definitions (with the first invariant being the trace), e.g.,

https://nexus.hexagon.com/documenta..._th/dy_th_ch03/DYMAT25_Cap_Material_Model.htm

Like FAILEST, FAILMES says it uses the first invariant:

https://nexus.hexagon.com/documenta...S_Maximum_Equivalent_Stress_Failure_Model.htm

Maybe it's a typo but if Dytran really accounts for failure using mean stress, that's fascinating and I'd like to know more.

Contact your trainer and, if they're not sure, contact support.