Abstract: The Taylor impact experiments of quartz glass bar were simulated by using a discrete element method (DEM) approach. The simulations provided detailed failure process of the glass bar: at the impact end, the bar failed in the form of compressive failure wave; at the free end, dense tensile spallation failure occurs. The analysis showed that the spallation is the result of the interaction between the chasing unloading waves caused by the rapid decrease of stress in the failure wave front, and the incoming unloading wave caused by the reflection of the elastic compression wave front at the free end. With the impact velocity increasing, the size of the compressive failure zone increases at the impact end, and the spallation failure zone decreases at the free end. Furthermore, the structural fronts and their propagation velocity of the compressive failure zone were investigated. It was found that the "failure front" in fact was a transition zone from dense crack region (the high damage region, HDZ) to sparse crack region (the low damage zone, LDZ). It was found that the propagation velocity LDZ front is basically the same as the elastic wave velocity, which is a constant. However, the HDZ front velocity decreases as it propagates. The average velocity of the HDZ front increases with the increasing of the impact velocity, and may approach the limit value of elastic wave velocity. In experiments, people usually reports the high-speed video observations of "failure waves" in glass bar after impact, which are actually the front of the HDZ, as the dense cracks formed the HDZ reflect lights to make the region bright and observable.