Wave propagation simulation in elastic bar

Hi ,

 I'm working on simulation of wave propagation in a bar. I'm using
newmark time integration scheme to solve my problem. I have a bae
clamped at one end and under tensile at the other end. when I decrease
my time step ,at the beginning of the loading I see tiny negative
strains which is so weird. I used some numerical damping with the scheme
and I could reduce the time step but still I have this problem in
smaller time steps. Can anyone help me with this?

  Thank you very much!!!