NUMERICAL AND ANALYTICAL VERIFICATION OF A MULTISCALE COMPUTATIONAL MODEL FOR IMPACT PROBLEMS IN HETEROGENEOUS VISCOELASTIC MATERIALS WITH EVOLVING DAMAGE

Composites are engineered materials that take advantage of the particular properties of each of its two or more constituents. They are designed to be stronger, lighter and to last longer which can lead to the creation of safer protection gear, more fuel efficient transportation methods and more affordable materials, among other examples. This thesis proposes a numerical and analytical verification of an in-house developed multiscale model for predicting the mechanical behavior of composite materials with various configurations subjected to impact loading. This verification is done by comparing the results obtained with analytical and numerical solutions with the results found when using the model. The model takes into account the heterogeneity of the materials that can only be noticed at smaller length scales, based on the fundamental structural properties of each of the composite’s constituents. This model can potentially reduce or eliminate the need of costly and time consuming experiments that are necessary for material characterization since it relies strictly upon the fundamental structural properties of each of the composite’s constituents. The results from simulations using the multiscale model were compared against results from direct simulations using over-killed meshes, which considered all heterogeneities explicitly in the global scale, indicating that the model is an accurate and fast tool to model composites under impact loads. Advisor: David H. Allen

Structural and Magnetic Properties of Neodymium - Iron - Boron Clusters

Using inert gas condensation techniques the properties of sputtered neodymium-iron-born clusters were investigated. A D.C. magnetron sputtering source created vaporous Nd-Fe-B which was then condensed into clusters and deposited onto silicon substrates. A composite target of Nd-Fe-B discs on an iron plate and a composite target of Nd-(Fe-Co)-B were utilized to create clusters. The clusters were coated with a carbon layer through R.F. sputtering to prevent oxidation. Samples were investigated in the TEM and showed a size distribution with an average particle diameter of 8.11 nm. The clusters, upon deposition, were amorphous as indicated by diffuse diffraction patterns obtained through SAD. The EDS showed compositionally a direct correlation in the ratio of rare-earth to transition metals between the target and deposited samples. The magnetic properties of the as-deposited clusters showed superparamagnetic properties at high temperatures and ferromagnetic properties at low temperatures; these properties are indicative of rare-earth transition metal amorphous clusters. Annealing of samples showed an initial increase in the coercivity. Samples were annealed in an inert gas atmosphere at 600o C for increasing amounts of time. The samples showed an initial increase in coercivity, but showed no additional increases with additional annealing time. SAD of annealed cluster samples showed the presence of Nd2Fe17 and a bcc-Nd phase. The bcc-Nd is the result of oxidation at high temperatures created during annealing and surface interface energy. The magnetic properties of the annealed samples showed weak coercivity and a saturation magnetization equivalent to that of Nd2Fe17. The annealed clusters showed a slight increase in coercivity at low temperatures. These results indicate a loss of boron during the sputtering process.