crash box with imposed displacement - Why do I have a big first peak force?

Hey Altair-community,

I am really new to Radioss and crash simulation, so there are thousands of questions but mainly the tutorials and the help of the manuals are greate. There is one question which I couldn't solve for quite some time and I did a lot of try and error simulations. Maybe you can help me!

I computed a crashbox simulation and wanted to validate my model with a given paper (https://ms.copernicus.org/articles/10/133/2019/ from where I got this picture:

I simulated the crash box of the hexagon model and actually got quite good results but have some issues with the first peak force. I thought about the material parameters which I set but I don't have more informations than there is written and ther could be some buckling errors. Otherwise I did some experiments with the contact surfaces and the BC's because I thought that there is my error. The simulations have nearly all the same results (self contact throught Type7 and different other contacts like Type7, Type10 or Type2 because the model has a hard contact -see second picture). What might be the best contact and where could be a problem? 

I will add one of my simulations (which is the third graph). Eventually you got some suggestions.

Thanks a lot in advance and kind regards

Leo