The mechanical properties of Epon-862 crosslinked with curing agent DETDA were simulated using a molecular dynamics code named GPIUTMD fully developed on multi graphical processing units. A new algorithm was proposed to simulate the cross-linking process during the chemical interaction of epoxy with the curing agent without changing the bond data. The models were created by densely packing preconstructed oligomers of different composition and compact them into the periodic simulation box. The properties of interest predicted by the molecular dynamics method were density, glass transition temperature and bulk modulus and found to be in good agreement with available data in the literature. Also, a study on the optimal ratio of resin to cure agent was carried out, and the results showed that, with a 3.6 mix mass ratio, the composite is in its strongest mechanical and structural state.

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The microstructure of dual-phase (DP) steels is composed of a matrix of ferrite reinforced by islands of martensite and the common interphase boundaries. To study the mechanical behavior of DP steels, steel with 45% ferrite and 55% martensite was fabricated and tested in the laboratory. Two types of finite element models were then created based on SEM images. The first model directly created the grains and boundaries from the SEM images, while the second model used a Voronoi type algorithm to construct geometries which are statistically similar to the SEM images. The models consider the measured morphology of ferrite, martensite, and their phase boundaries. The Gurson damage model was then used for the ferrite and boundary regions. The obtained results correctly predicted the failure mechanisms in a DP material. The results indicate that the deformation is localized due to microstructural inhomogeneities and the nucleation of voids in the boundaries between the ferrite and martensite grains. The good correlation between the numerical and experimental observations from SEM micrographs shows the efficiency of the proposed models in predicting the failure mechanism of DP steels.

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Recent experiments on polycrystalline materials show that nanocrystalline materials have a strong dependency to the strain rate and grain size in contrast to the microcrystalline materials. In this study, mechanical properties of polycrystalline materials in micro and nanolevel were studied and a unified notation for them was presented. To completely understand the rate-dependent stress-strain behavior and size-dependency of polycrystalline materials, a dislocation density based model was presented that can predict the experimentally observed stress-strain relations for these materials. In nanocrystalline materials, crystalline and grain-boundary were considered as two separate phases. The mechanical properties of the crystalline phase were modeled using viscoplastic constitutive equations, which take dislocation density evolution and diffusion creep into account, while an elasto-viscoplastic model based on diffusion mechanism was used for the grain boundary phase. For microcrystalline materials, the surface-to-volume ratio of the grain boundaries is low enough to ignore its contribution to the plastic deformation. Therefore, the grain boundary phase was not considered in microcrystalline materials and the mechanical properties of the crystalline phase were modeled using an appropriate dislocation density based constitutive equation. Finally, the constitutive equations for polycrystalline materials were implemented into a finite-element code and the results obtained from the proposed constitutive equations were compared with the experimental data for polycrystalline copper and good agreement was observed.

In this study, a program based on finite element method is developed for rotor dynamic analysis of gas turbine rotors. In the FE model of the rotors, various minor and major parts of the rotor are modeled using the cylindrical and tapered Timoshenko beam elements and the lateral vibration behavior of the rotor is evaluated. In the paper, the lateral vibration behavior of a certain gas turbine rotor is analyzed using the developed finite element program and coupled lateral-torsional vibration behavior of the rotor is analyzed using 3D finite element model. A good agreement exists between the results obtained from two FE models. Two design models are used for the rotor one of which has 2 bearings and the other one has 4 bearings with specific locations. The effects of the number of the bearings on the critical speeds, operational deflection shapes and unbalance response of the rotor is investigated. It is found that the number of the bearings has significant effect on the first critical speed but slight effect on the second and third critical speeds. It is demonstrated that the number of the bearings can be used as one of the system design parameters.