The behaviors under impact condition are different from the ones under quasi static condition. However, because of the complexity of dynamic fracture problems, the analytical result of stress intensity factor of such simple sample as three point bending beam hasn’t been determined. To satisfy the request of theoretical and experimental analysis,the finite element method program LS-DYNA is used to analyse the dynamic process (including the sine,linear,anomalistic progress) of three point bending beam under impact load.The dynamic stress,strain field at crack tip and corresponding curves of force time and displacement time in the area of crack tip are obtained. The method of determining the energy release rate and dynamic stress intensity factor under impact load is obtained by virtual crack closure method,the curve of GI (energy release rate) vs.time,KId (dynamic stress intensity factor ) vs.time under different impact loads are calculated,and according to KId-time curves the corresponding dynamic stress intensity factor of the extend time which is determined by experimental method in the literature,called dynamic fracture toughness,is determined.The results got by FEM are compared with the experimental results and show good agreement with experimental data,and more it has higher precision.As a result,this method can be used to the computtion of dynamic fracture parameters.
