WARNING ZERO OR NEGATIVE SUB-VOLUME

Altair Forum User

Hi,

 

I built a type 7 contact between a complex shape (shell elements) and the same shape scaled in 2 dimensions by a factor 1.005. A force is applied perpendicular to the plane of scaling on the shell component. The scaled surface delimits a cylindrical solid at the interior, which is made up of solid tetra elements. The density and stiffness of the solid elements are low.

After computing the first time step, the error message 'zero or negative sub-volume' appears.

 

Thanks in advance

Altair Forum User

Hi,

The NEGATIVE VOLUME error happens when solid elements are very deformed and their characteristic length goes to 0. For large strain formulation the time step of an element goes to 0 when the element is compressed. In RADIOSS Starter input file (Runname_0000.rad), use Ismstr =2 in the solid property and  in RADIOSS Engine file (Runname_0001.rad) use the option/DT/BRICK/CST which will set the time step value tmin 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 Tmin, and will then switch to small strain formulation

Altair Forum User

Hi,

 

Thanks for the help

 

I have another question: in my model I have a rigid body connected to the complex shape (shell elements). The primary node of the rigid body determines my time step. The time step is increased by about a factor 10 when I set ISPHER=1. When am I allowed to use ISPHER=1? How can I further increase the nodal time step when it is determined by a rigid body?

Altair Forum User

Hi,

 Ispher is the spherical inertia flag, and if a rigid body has less number of slave nodes, using Ispher =1 is normally recommended in order to ensure the stability of the connected elements. 

In case of a rigid body and nodal time step, the time step for stability is computed at the master node.

The time step at the master node of the rigid body is computed by reporting stiffness at slave nodes to the master node.

The mass and inertia of the rigid body depends on the flag ICoG in /RBODY.

 

Altair Forum User

Thanks a lot again. 

I now have the following issue: I try to simulate the pullout of a rigid screw from a low E-modulus material (with material law2). When I apply an imposed displacement on the screw I obtain a force-displacement curve with an elastic region and maybe a plastic region. However, instead of the imposed displacement I would like to apply a concentrated force on the screw. But if I do so, the force-displacement curve has not the expected shape and the values are far too small. Do you have any any suggestion on what could be the problem? 

 

 

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Altair Forum User

Hi,

These are two different type of loading conditions (/IMPDISP and /CLOAD). Imposed displacement is like a condition that we are imposing the component to displace a particular length even if there are some obstructions. And so you will get higher values in the plot. Whereas /CLOAD  is a kind of load which is considered to act at a point. By this we mean that the length of screw over which the force acts is so small in comparison to its total length and so you will get low values.

For the pull out case /IMPDISP is the best loading condition.

Altair Forum User

Thanks for the advise.

I now apply an /IMPDISP on the screw in the pullout experiment simulation. After some displacement, the velocity of the screw starts to vary instead of being constant. This variation is smaller when I'm using /KEREL, but it still remains. Why does this happen?

Also, because I cannot simulate the experiment in real time (as it would take too long), with /IMPDISP I apply a much higher velocity on the  screw than in the real experiment. What is the effect of using /KEREL while applying such a high /IMPDISP?

Altair Forum User

I solved the problem related to the non-constant velocity in the meanwhile.

Now, since I am performing a pullout test, I am using interface type 7 as well as interface type 11 for the edge-to-edge contact. Using interface type 11 increases the pullout force (i.e. max force) considerably compared to the case with just interface type 7. I do not understand why there is such a large difference. Do you have any suggestions?

 

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Altair Forum User

Hi,

With Type 7 interface the model doesn't consider the edge to edge interface and that's why the normal force seems to less for model with only Type 7 interface.

Whereas with Type 11 it also considers the edge to edge interface along with the Type 7 general interface. 

Altair Forum User

Thanks!

I was plotting the stress against the plastic strain of some elements in order to try to partially reproduce the material's stress-strain curve (Johnson Cook material (LAW2)). Since the model has a failure criterion, I would expect the plastic strain of a material to stay constant as soon as the failure plastic strain (0.04) is reached. However, the plastic strain still increases a little after exceeding the failure strain. Why does this happen?

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Altair Forum User

Has somebody an idea about why the material behaves like this?

 

Nachiket Kadu_22143

Altair Forum User said:

Has somebody an idea about why the material behaves like this?

 

How did you define your material in LAW2??

Altair Forum User

Hi,

 

I defined the material in the following way:

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