Recent experimental studies on spall fracture show that the impact parameters can lead to instabilities in the strain rate dependence of the dynamic strength of the material. Here we consider the effect of the dynamic impact parameters on the time dependences of the material ultimate stress. Numerical modeling of spall fracture is conducted for ultrapure aluminum. The time-stress relations are obtained from a one-dimensional wave problem using an elastic formulation. The fracture conditions are determined by the incubation time criterion for spall fracture. The obtained results show that the dynamic strength can depend not only on the strain rate, but also on other impact time parameters. The structural-temporal approach used in fracture modeling allows taking into account the loading history, which makes it a convenient tool for considering the corresponding effects. The dynamic strength is calculated with varying impact time parameters. The strain rate dependences of the ultimate stresses obtained for diffe