Experimental characterization and modeling of polycrystalline molybdenum

This dissertation concerns the experimental characterization, modeling and simulation of the plastic anisotropy and tension-compression asymmetry of polycrystalline molybdenum. In addition, extensive cross-validation of the model was done for both quasi-static and high strain-rate deformation regimes encountered in impact.;For the first time, it was established that polycrystalline molybdenum has ductility in tension for low strain rates and that the failure strain is strongly dependent on the orientation. To accurately quantify the anisotropy in plastic deformation digital image correlation techniques were used. While the current practice is to assume plastic incompressibility when evaluating the plastic strain ratios, a novel approach was taken using an orthogonal configuration of cameras to allow direct measurement of the thickness strain of the specimen. For the first time, the tension-compression asymmetry of polycrystalline molybdenum in yielding was determined (yield stress in compression larger than in tension for all orientations). Furthermore, evaluation of the ellipticity of the deformed compression specimens allowed uncovering that although the material exhibits strong strain anisotropy in tension, it has a weak strain anisotropy in uniaxial compression. For the first time, Taylor impact tests were successfully conducted on this material for impact velocities in the range 140--160 m/s.;An elastic-plastic model that accounts for all the specificities of the plastic deformation of the material was developed. Key in the formulation was the use of a yield function that simultaneously accounts for anisotropy and tension-compression asymmetry. Validation of the model was done through comparison with test results on notched specimens for the quasi-static strain-rate regime and deformed Taylor impact specimens for the high strain-rate regime. Quantitative agreement between measured and predicted response was obtained. In particular the effect of loading orientation on the response was very well described. For the Taylor impact test the model was used to gain understanding of the dynamic deformation process of this material. It was thus shown not only the predictive capabilities of the model but also its potential for use in virtual testing of complex systems composed of the material.