Two way FSI simulation of coronary artery element distortion error
 

Currently i am simulating Two way FSI simulation of a bifurcating coronary artery.i used blood properties for fluid and 5 parameter mooney rivlin model for artery.Started with coarse mesh and took time step of 0.05 with under relaxation factor in system coupling to be 0.3 and i got my self a solution.But as i decrease the time step to say 0.01 or 0.005 i get element distortion error as couple of time steps.I tried using restart points before to decrease the timestep before the error but didnt get any solution.
my first question is that how is it possible that my solution converges for large time step but not for small one
and my second question is asking for help with my element distortion error as i cant do time step validation without solving that error. Thanks