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Law 38 unloading

Hi,

I'm using Law 38 for a model using a foam specimen (see image1), an impactor and a anvil. I try to model the unloading of that foam block. I tried 2 methods following Altair helps:

1- I use a single unloading curve (see image2). It's working but, the calculation time is very high compared to the model without a unloading curve

2- I use the same unloading curve but I use it like a quasistatic curve using strain rate at 0 and the next curve at 0.0001 (see image3). I also put K_R parameter to 1. Calculation time is almost infinite, so not an option.

What I'm a doing wrong, or what should I do to make it work. Here's the attached model too.

Thanks!

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JM Desrosiers

Anybody?

Simon Križnik

Hi,

The long calculation issue is not due to the unloading curve; the time step will drop due to the element compressive deformation. This happens when elements are very deformed and their characteristic length goes to 0. In the case of large strain formulation, the time step of an element goes to 0 when the element is compressed. In a mathematical way, the element can not reverse its orientation since its stiffness increases to an infinite value; but due to numerical accuracy, the element may go to reverse its orientation.
In order to solve the problem of both the drop in cycle time step and subsequent termination of the run due to a negative volume, you might first check that the material used is well-suited to the physics which is represented. Then switch the elements to a small strain formulation. This is done as follows:
In the RADIOSS Starter input file (Runname_0000.rad), use Ismstr =2 in the solid property or in the option /DEF_SOLID; in RADIOSS Engine file (Runname_0001.rad) use the option /DT/BRICK/CST which will set the time step value symbol_tritmin at which the solid elements will switch to small strain.
This means that the solid elements using Ismstr =2 will use large strain formulation while their time step remains greater than symbol_tritmin, and will then switch to small strain formulation. Their volume will then remain constant and the element can even reverse its orientation. The drop of their time step normally stops
except for some materials, especially viscous materials.

Check the attached video for more information (relevant from 6:00)

V_h_Solid Elements in RADIOSS - Altair University