Learning Forum

[anner Mayo]

I am new to Ansys, and I was going to see if I could get some help running a test of a weight being dropped on a large block of soft polyurethane foam. I am using an explicit dynamics study. Whenever I run the analysis, the dropped weight tears through the pad and bounces off of the ground. I am guessing this could be due to incorrectly defining material properties.

I defined my own material based on some known properties. If there are any obvious errors I made as far as what properties I chose to define the material, help would be appreciated. I attached screenshots of my current material properties, and a single frame of the final results.

Thank you.

[peteroznewman]

Look under the Analysis Settings. There is a category for Erosion Controls (or something like that). That controls when elements are removed from the model.  Have you tried changing those controls? By default, it will remove an element when the strain exceeds 1.5, but many elastomers can deform to much higher values of strain. Try turning that control off or use a larger value.  The benefit of having the element removed is when it gets too distorted, the time step gets really tiny and the simulation takes too long (or generates an error). Maybe if the elements stay in the simulation, the weight won't hit the floor!

[anner Mayo]

Okay I turned off erosion. The pad seems to stay intact better, but I am getting a new error "energy error too large". Do you know of a way to fix this?

[peteroznewman]

 

Did you set the Erosion Control to the On Material Failure setting?  That is where it uses the Ultimate Tensile Strength data to remove an element from the simulation.

Another setting under Analysis Settings is Energy Error threshold (or something like that). I think the default value is 1.  Try a value of 10 or 100.

I looked at the screen shot of your material.  Where did you get those values?  They seem extrodinariarly low. Both the modulus and the strength values.  Did you use the correct units?  Also, your material model is a linear elastic model.  You would be better off with a hyperelastic material model or a viscoplastic material model.

Here is a paper that has properties on a 1 lb/cu ft PU foam. http://www.pg.gda.pl/mech/kim/AMS/022006/AMS02200605.pdf

The compressive modulus is 133 psi (0.92 MPa) which is a lot bigger than 1.5 psi.  

[anner Mayo]

Thank you for your patience and assistance you've been providing me. I set the erosion controls to erode on material failure. I agree that the Young's modulus value that I used seemed extremely low. I calculated an estimate value based on some material properties I had for my foam:

Average Compression Force Deflection:

25% deflection = 0.45 psi

50% deflection = 0.65 psi

70% deflection = 0.70 psi

My goal at this point is to get any soft foam type modelto work and polish it from there. Therefore, I added a hyperelastic (Ogden model from the engineering database on Ansys). In order to run the analysis with the default Ogden material I had to add an isotropic elasticity to the model. For this I input the Young's modulus and Poisson's ratio from the article that you linked. Howevee, now I am getting a new error "time step too small". Are there any obvious reasons you know of that would be cauing this error? I have attached some screenshots of my new defined material.

[peteroznewman]

Do you need to drop a cube?  I expect the solver will have an easier time having a sphere dropped on it.

The PU foam from the literature had a Poisson's Ratio of 0.32 but that may not have a big effect compared with the 0.2 you used.