how to provide strain rate in tensile test in abaqus explicit

hi ,

How to vary strain rate in abaqus explicit in tensile test simulation.

i know the expression in tensile test that strain rate = velocity / gauge length, but how can m decide the step time.

for example if the strain rate is 100 sec-1, and gauge length is 25 mm thn the velocity is 1.66*e-5 m/s. so what is the step time in this case................

any body tell me the relation between strain rate and step time.

thanks in advance