Learning Forum

[Cri1994]

hi everyone, 

I have a question for you.

When I have to simulate an impact between two body,(for examples bolt and plate), what do I have to do? Explicit dynamics analysis or transient analysis?

Are there some differencese?

thanks a lot for your time

Best regards

Cristiano 

[jj77]

See slide 21 in this for when to use one or the other

https://support.ansys.com/staticassets/ANSYS/Conference/Santa%20Clara/downloads/Shock%20Analysis%20of%20Electronic%20Comonents%20-%20MingYao%20Ding.pdf

[Cri1994]

Hi jj77,

Thanks for your reply,

I saw that, so is transient analysis implicit ?

I don't understand very clear the difference between transient structural and explicit dynamics.

What analysis do I have to do to simulate an impact ?

Can you help me ?

thaks for your time.

[peteroznewman]

How fast is the bolt that impacts the plate? 

• If it is going so fast as to punch a hole in the plate, that would be a good model to do in Explicit Dynamics.


• If it is not going so fast and the bolt will deflect the plate and bounce off, that would be a good model to do in Transient Structural.

I described a Drop Test simulation in this Discussion, which used Transient Structural analysis (implicit equations).

[Cri1994]

Hello Mr.Peter

Thanks for your reply, the acceleration of plate is 60g. The velocity is around 9-10 m/s.

 

Thanks,

Cristiano 

[peteroznewman]

Hello Cristiano,

An object accelerating at 60g from rest will be going 9.8 m/s in 16.6 ms after traveling 81.6 mm.  Does that sound like it describes your problem?

Kind regards,
Peter

[Cri1994]

Hello Mr.Peter,

 

yes exactly, with this velocity which is analysis correct to do?

Thanks,

Cristiano 

 

[peteroznewman]

Hello Cristiano,

Start with Transient Structural. It is easy to convert to an Explicit Dynamics model as 90% of the model building effort is common.

No doubt the velocity is good, I was asking about the 60g, where does that come from? How long does that last?

Kind regards,
Peter

[j.drozdowski]

Good day Sirs.nI'm also looking for brief summary what is exact difference between implicit and explicit analysis. I read the discussion and few others articles. Below is the summary of my understanding of differences between implicit and explicit dynamic problems.nCould anyone correct me if I'm wrong ?Common:n-Both analysis are solving problems where the speed matters - so the dynamic problems involving inertia effects.nnImplicit (Ansys system: Transient structural):n-solver is looking for global equilibrium of the model in each increment and then calculate stress and strain.n-it is used for dynamic problems happening relatively slow so it involves bigger time increments than explicit solvern-the analysis take more time than explicitn-in implicit solver we are selecting size of time increment while in explicit analysis solver is doing it automaticly.nnExplicit (Ansys system: Explicit dynamics)n-solver do not need the local equilibrium before moving to the next increment.n-it is used for phenomenon happening very fast (impacts etc.)n-it calculates faster than implicit solver because it doesn't have to look for an equilibriumn-time increment in explicit analysis is very smal and it depends on the mesh quality/size and the density of the material.n-it use mass scaling to adjust time increment (solver is locally changing the density of material)n

[peteroznewman]

It's not true that implicit models take more time than explicit models to solve. It depends on the time step required by explicit, and the end time, so an explicit solution could take longer to solve than an implicit model.nExplicit doesn't use mass scaling by default, it is an option that can be turned on if the mesh size is not highly uniform. If the mesh is highly uniform, there is little benefit to mass scaling.nOne big difference is that the explicit solver automatically calculates body self-contact and does that reliably. Defining contact in implicit is a manual model building process and may trigger convergence difficulties in the solver.nAnother big difference is that the explicit solver automatically removes failed elements during the solution and the solver reliably continues. Removing failed elements in the implicit solver requires a lot more upfront work and may trigger convergence difficulties in the solver.n