Radioss-Check Elem --Time

Altair Forum User

Hi,

 

I have some doubt in failure criteria .

How time step for element is calculated.

If i set 1e-8 ,,,some element failed,how to control that one.

some what i have controlled by adjusting length.

 

0 elems failed.And also i am getting (incomplete material for 1 element),how to correct that one.

Altair Forum User

Hi Mohan,

In the context of explicit analysis, time step is the time required for a shock wave (governed by the speed of sound) to propagate across the smallest distance of the element.

The elemental time step is calculated by the relation,l/c, where l=length of element and c=speed of sound. And for nodal time step the relation is sqrt 2m/k, where m=equivalent nodal mass and k is nodal stiffness.Please note that during the run interfaces can also control the time step.From the relation, it is clear that both material modulus and density control the speed of sound. The solution time step is inversely proportional to the speed of sound. So, if the material is stiffer (higher modulus of elasticity), the speed of sound increases resulting in a lower solution time step.

From the engine out file we can see that the model is having very low time step.

It can be an element, a node or an interface.

•For an element, check the related material (especially its Young modulus and density in case of an elastic-plastic material; and its viscosity in case of a visco-elastic material). There must not be an error in the units system that this data is given in.

Check the size of the element, since elemental length is proportional to time step.

•For a node, check the characteristics of connected elements. If the node is on the master side or the slave side of an interface, this interface must be verified.

•For an interface, the gap of the interface must be verified if some failure happens on the master or the slave side of the interface.

Once this is cleared you can run the simulation without time step being dropped.

You can also impose a time step in order to control the time step drop. Please watch this webinar which explains about time step control: http://www.altairuniversity.com/learning-library/considerations-about-time-step/ 

I recommend you to download and go through our free e book on RADIOSS which covers every aspects of explicit analysis where we have well covered this topic. You can download your copy from http://www.altairuniversity.com/free-ebooks-2/free-ebook-crash-analysis-with-radioss-a-study-guide/