Estimation of parameters for the Johnson-Cook failure and strength model

From engineering point of view, it is often desirable to estimate material data to be used in a penetration simulation from a uni-axial test only. Often only a conventional material data sheet is available for the material, which makes the estimation difficult. The intention with this paper is to give some practical guidelines on how to make a first estimation of some of the parameters in the Johnson-Cook (J-C) fracture and strength model. A method is described for estimation of the material parameters D1-D3 in the J-C fracture model, and the parameters A, B and n in the J-C strength model, from a tension test only. For estimation of the parameters D1-D3, Goldthorpe´s or Lemaitre´s damage models can be used to adjust the damage evolution as a function of the strain. To accomplish that, the stresses at the neck and the triaxiality factor must be calculated as accurately as possible. Subsequently, the ratio between the radius of the smallest cross section and the radius of curvature at the neck must be estimated. Comparison of results from different papers shows that it may be possible to estimate this ratio with sufficient accuracy. The analysis indicates that the ratio of the compressive fracture strain to the tensile fracture strain can be of the order of 3-7, depending on the level of ductility.