Extensive studies on axial crushing of cylindrical shells have been carried out due to their wide applications for crashworthiness. Most of the studies have been conducted under static and low speed impact conditions to predict the deforming mechanism, resulting in typical progressive buckling and dynamic plastic buckling, from which the energy absorption capacity of the structural component can be predicted. Though the effect of the strain rate, inertia and stress wave propagation can all be considered to determine the dynamic buckling modes, the present study concentrates on a different mechanism under higher energy impact, where mushrooming, or thickening of the shell wall is observed. Both experimental study and finite element (FE) simulations were carried out. It was found that mushrooming is an important feature under high speed impact but experimental results show that the high impact energy leads to dynamic tensile fracture in the mushroomed regions and the lack of suitable models for dynamic fracture in FE codes hinders a full understanding of the problem.