Stent/Artery contact Abaqus Explicit

Hi Everybody.

I'm modelling Nitinol stent crimp and expansion
into an idealised artery in Abaqus/Explicit. I am having numerous
problems with the ratio of kinetic energy to internal energy with the
analysis. I was hoping somebody may provide me with some insight on
this.

I am modelling the crimp stage using a C3D8 elements for
the stent and have defined a surface section for the crimper surface. I
am using kinetic contact and surface to node contact (surface to surface
or genral contact do not work) Everything is in mm so my density for my
Nitinol stent is 6.9e-9. I am using semi automatic mass scaling with a
time period of 1, scaling to a target time of 1e-5 and at a frequeny of
approx every 5 increments. I am getting resonable results for the crimp
step, strains at about 8 - 10%. However on expansion of the stent is
where the difficulties lie.

Allowing the stent to undergo free
expansion into the artery I get a ballooning effect of the stent, (ends
start to curve inwards- looks like the opposite to dogboning) and when
my stent contacts my artery i get crazy stresses in my stent struts (
10,000 MPa compared to 300 - 900 MPa in the literature) I am using using
penalty contact with surface surface interaction with the stent and
artery.

Can anybody tell me why my stresses are so high in my
stent? I have looked at my time vs energy graph of both internal energy
vs kinetic energy. My energies are way off with the kinetic energies
being much higher than the internal erengy in the second step, (should
be within 5% of internal energy at all times). Is my mass scaling
strategy wrong? Anybody have any ideas.

Any help would be greatly appreciated.

thanks

Slim Jimmy