Why does the overshoot occur in Dynamic Stress Intensity Factor

I wanted to get some insight in the transient stress intensity factor (SIF).

In the time history, we notice there is an overshoot (~27%)  from the

steady state SIF for a fixed (not propagating crack). I found the

overshoot occurs at the time when the reflected wave from the opposite

crack-tip comes back to the first crack tip. I wanted to know why this

overshoot occurs, what is the physical explanation?

Sandip Haldar