Abstract: Recent experimental studies on spallfracture show that the impact parameters can lead toinstabilities in the strain rate dependence of the dynamicstrength of the material. Here we consider the effect of thedynamic impact parameters on the time dependences of thematerial ultimate stress. Numerical modeling of spall fractureis conducted for ultrapure aluminum. The stress-time relationsare obtained from a one-dimensional wave problem using anelastic formulation. The fracture conditions are determined bythe incubation time criterion for spall fracture. The obtainedresults show that the dynamic strength can depend not only onthe strain rate, but also on other impact time parameters. Thestructural-temporal approach used in fracture modeling allowstaking into account the loading history, which makes it aconvenient tool for considering the corresponding effects. Thedynamic strength is calculated with varying impact timeparameters. The strain rate dependences of the ultimate stressesobtained for different types of impact are found to bedifferent, which is also consistent with experimental data.