AFRL Engineers Develop Anisotropic Material Modeling Published March 13, 2008 By Michael Nixon AFRL/RW EGLIN AIR FORCE BASE, Fla. -- AFRL engineers developed and implemented new material models that accurately predict the behavior of highly anisotropic materials. The models are formulated for implementation across continuum-level finite element codes but can also accommodate evolution of texture and deformation mechanisms occurring on a much smaller-length scale, such as that associated with crystallographic slip and mechanical twinning. These models can also account for the strength differential existing in some materials, wherein tension is much stronger than compression (or vice versa). The new models enable engineers to leverage the anisotropy resulting from mechanical processing, as well as the anisotropy inherent to hexagonal close-packed (hcp) materials such as zirconium and titanium. This modeling capability will lead to smart munitions that are tunable to a wider range of targets identified in real time. Whereas current anisotropic models cannot accurately account for a strength differential between tension and compression, the new models can. Further, they accurately and efficiently model anisotropic materials exhibiting no strength differential. The two models that have been implemented and tested describe the orthotropic behavior of hcp materials. The theoretical yielding description is modifiable via linear transformation to an orthotropic description. The model parameters available from these transformations are quantifiable via conventional unit-level testing procedures, such as uniaxial stress measurement, for various orientations of the material. Rate effects are easily incorporated using data gathered from Hopkinson bar-type tests. In order to account for the anisotropic hardening observed in many materials, researchers developed an interpolative hardening procedure that is easily implemented. The models are first parameterized for various levels of plastic strain, and the procedure subsequently interpolates for effective strain levels occurring between any two levels. When experimental data is not readily available, data from polycrystalline simulations can provide data for parameterization of the models. To this effect, researchers from AFRL and Los Alamos National Laboratory collaborated to use VPSC (Visco Plastic Self-Consistent Code) to provide data (supplementing experimental data) in order to parameterize the models, which they then validated against beam bend tests for quasi-static rates and against cylinder impact tests for loading higher rates.