How to eliminate the oscilation of preload in transient analysis



I am working on a numerical simulation of blasting with preload. 

I tried two methods:

1. The preload (gravity or geo-stress) is attained by implicit solution with LS-DYNA, and then the dynain file is invoked in the sequent explicit solution for blasting.

Same element formulation (-1), material model and parameters in both explicit and implicit solution.

2. Using CONTROL_DYNAMIC RELAXATION to apply preload (Gravity or Geo-stress) for a model, and checked the convergence of DR phase. Then do the dynamic analysis by invoking the drdisp.sif file in another Keywork file (idrflg set to 2).

Both of them have a problem, whicih is the curve of stress vs time always oscillates before the stress wave arrives at the monitored point (near the boundary).

Could you give me some suggestions to fix it?




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