Multiscale analysis procedures for composites often involve coupling the macroscale (e.g., structural) and meso/microscale (e.g., ply, constituent) levels. These procedures are often computationally inefficient and thus are limited to coarse subscale discretizations. In this work, various computational strategies were employed to enhance the efficiency of multiscale analysis procedures. An ensemble averaging technique was applied to stochastic microscale simulation results based on the generalized method of cells (GMC) to assess the discretization required in multiscale models. The procedure was shown to be applicable for micromechanics analyses involving both elastic materials with damage and viscoplastic materials. A trade-off in macro/microscale discretizations was assessed. By appropriately discretizing the macro/microscale domains, similar predicted strengths were obtained at a significantly less computational cost. Further improvements in the computational efficiency were obtained by appropriately initiating multiscale analyses in a macroscale domain. A stress-based criterion was used to initiate lower length scale GMC calculations at macroscale finite element integration points without any a priori knowledge of the critical regions. Adaptive multiscale analyses were 30% more efficient than full-domain multiscale analyses. The GMC sacrifices some accuracy in calculated local fields by assuming a low-order displacement field. More accurate microscale behavior can be obtained by using the highidelity GMC (HFGMC) at a significant computational cost. Proper orthogonal decomposition (POD) order-reduction methods were applied to the ensuing HFGMC sets of simultaneous equations as a means of improving the efficiency of their solution. A Galerkin-based POD method was used to both accurately and efficiently represent the HFGMC micromechanics relations for a linearly elastic E-glass/epoxy composite for both standalone and multiscale composite analyses. The computational efficiency significantly improved as the repeating unit cell discretization increased (10-85% reduction in computational runtime). A Petrov-Galerkin-based POD method was then applied to the nonlinear HFGMC micromechanics relations for a linearly elastic E-glass/elastic-perfectly plastic Nylon-12 composite. The use of accurate order-reduced models resulted in a 4.8-6.3x speedup in the equation assembly/solution runtimes (21-38% reduction in total runtimes). By appropriately discretizing model domains and enhancing the efficiency of lower length scale calculations, the goal of performing highidelity multiscale analyses of composites can be more readily realized.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 181-188).

A modified Weibull cumulative distribution function, which accounts for the effect of fiber length on the probability of failure, was used to characterize the variation in fiber tensile strength in a SCS-6/ TIMETAL 21S material system and was implemented within the framework of the NASA code MAC/GMC. A parametric study investigating the effect of repeating unit cell architecture and fiber strength distribution on the RUC-averaged ultimate composite strength and failure was performed. Multiscale progressive failure analyses of a tensile dogbone specimen were performed using FEAMAC/ ABAQUS to assess the effect of local variations in fiber strength on the global response. The effect of the RUC architecture, fiber strength distribution, and microscale/ macroscale discretization on the global response was determined. The methodology developed in this work for accounting for statistical variations in microscale properties that feed into macroscale progressive failure analyses can readily be applied to other composite material systems.

The presented master´s thesis deals with lead-acid batteries and methods of their charging. First part shortly describes the problems of lead-acid batteries. There are introduced common principles, type of construction and requirements, that should be complied with lead-acid batteries. Following part discusses the basic characteristics of charging. There is given a detailed description of current pulse charging method, on which is also mainly focused this master´s thesis. For this charging method, several basic experiments were done in order to verify functionality of the measuring device, software, the tested cells and primarily to verify charging modes. The goal of this thesis is to find out suitable mode of pulse charging, which would be fast as well as tested cells-friendly. The conclusion of this master´s thesis contains a resume of acquired theoretical knowledge and an evaluation of measured results, on which was the appropriate charging mode proposed.