[en] In order to facilitate a realistic approximation of jet forces due to longitudinal cracks in pipes under pressure and its variation with time, exact calculations of leakage and cracks border form during opening, are required. Complex phenomena of dynamic plastification and depressurization of the system have to be taken into consideration. The opening areas of longitudinal cracks are computed by an elastic-plastic large displacement dynamic finite element analysis using in the general purpose program MARC. Results of these analyses are compared with experimental data. Plots showing the variation with time of displacements and stresses are presented. Three finite element models, from simple to complex geometries, are presented. Changing boundary conditions and time steps are explained. Fracture mechanics parameters (J-integral) are computed around the crack tip. These values are compared with experimental data in order to show the tendency pf the crack to propagate or to arrest. The longest stable crack is calculated for each Finite Element Model. Elastic-plastic dynamic phenomena of a running crack in a pipe under pressure are presented, and its computation with an elastic-plastic large displacement dynamic finite element program is explained. (Auth.)