Cut Approach - Sensibility

Question

Hi,

I have an issue with cut approach (https://altairuniversity.com/learning-library/radioss-sub-modeling-cut-approach/).

I'm modeling an half of full vehicle with the cut approach above. From this sub-model, with hyperstudy I modify a little bit the geometry and study how the crash changes dynamic. I wonder how dynamics could vary among runs if the section-force imposes original displacements?

Many Thanks

Simon Križnik · Answer

yes to both questions

Mr.Alb · Answer

ok, let's suppose for instance that my changes were about thickness on the front rails for my vehicle. Thickness varies within few millimiters (about 3mm), but when It's thin, the car folds deeply against the wall, whereas if It's thick, the car folds a little and then has a rebound. With the cut approach I have imposed the displacement to the section force so that the car's dynamic is fixed, isn't it? In this way, I cant't appreciate differences due to the thickness changes, can I?

(Total mass isn't very affected from thickness changes (less than 1%), but stiffness might be)

Many thanks again for your reply

Simon Križnik · Answer

Hi,

if the geometry changes significantly it is possible the crash dynamic also varies from the original in:

*  		mass, affecting the kinetic energy  	
 	*  		stiffness, because of different load paths/crushing behavior

It is difficult to estimate how much submodel affects the global model. This is why the reanalysis of the optimized global model is recommended.

RD-E: 5400 Cut Methodology Conclusion (from Radioss help):

The cut methodology allows you to reduce the computation time by running smaller models once a first full model has been run. Its main limitation lies from the fact that the interactions between the selected parts (for sub-model) and the unselected parts must be small enough so that the behavior of the unselected parts is not significantly affected by unselected parts. Therefore, the interactions between the unselected parts and the selected parts (for sub-model) could be well defined by the section (no extra contact for instance).

To assess those hypothesis, it is also important to compare the section force (or moment) between full model and sub-model.

In industrial applications, like in dummy analysis, it is advised to keep only the dummy and its direct environment as sub-model. Then about 90% of the CPU cost could be saved compared to the full car model. In low speed impact or beginning of impact with high speed, it is advised to keep only the deformed area as sub-model. Then about 30% of the CPU cost could be saved compared to the full car model.