Modeling of steel behavior with application to armor penetration

Problems involving interaction between armor and projectile are often studied with the aid of hydrocode softwares such as Autodyn. The numerical calculations require input data that among others describe the material response to large strains and high strain rates. One of the most frequently used models for metals subjected to the discussed conditions is the Johnson and Cook (J&C) equation. In this report the implementation of the J&C material strength model in Autodyn is considered. Further, material parameters for the J&C model for a novel high nitrogen alloyed steel (HNS) with potential as a future armor is presented. The results show that the implementation of the J&C model in Autodyn 2D 4.1.13 does not reproduce the strain rate dependency of the flow stress appropriately. A user subroutine developed at FOI is shown to improve the numerical solution. In the recent release of Autodyn 2D 4.2 the J&C model reproduces the strain rate dependency more accurate. Penetration simulations using a 5.56 mm AP projectile penetrating a HNS armor are performed, and the results are discussed and compared with experimental data. It is shown that the Lagrange formulation gives numerical problems and can not in general be used to predict the interaction between armor and projectile without experimental verification. It is also seen that HNS has ballistic properties comparable to a conventional steel armor with the same hardness.