The features of the fracture of the aluminum alloy AMg4.5 under dynamic tension according to the Kolsky methods using Split-Hopkinson pressure bar are considered. Experimental and theoretical data of strength characteristics of the alloy in the initial coarse-grained and ultrafine-grained states are presented. It is shown that the high degree of deformation of the material obtained in the process of severe plastic deformation by torsion under high pressure has a negative effect on the dynamic strength of the material. Keywords: dynamic strength, incubation time criterion, Split-Hopkinson pressure bar, severe plastic deformation.