Most micro-mechanical analyses for composites are based on repeated unit cell models (RUCs) by assuming a periodical distribution of the reinforcing phase. In this paper, the uniqueness of solution by applying unified displacementdifference periodic boundary conditions on the RUCs has been proved. Further it is deduced that (1) selection of the RUCs for a fixed periodic array may not be unique, however, the solution is independent on the choice of the different RUCs; (2) boundary traction continuity conditions can be guaranteed by the application of the proposed unified displacement-difference periodic boundary conditions. Illustrative examples are presented and advantages of applying this type of unified periodic boundary conditions are discussed.

Filament winding (FW) is one of the most common techniques for manufacturing composite pipes. The material properties and failure mechanism of composite pipes depend largely on winding pattern. In this study a micro mechanical approach for filament wound composites (FWCs) is pursued. A diamond-shaped repeated unit cell (RUC) is first constructed which characterizes the micro architecture of FWC pipe, such as winding angel, shift between successive circuits and the area of local undulation region. The micro mechanical model is embedded into commercial FEM code of ABAQUS as user-defined subroutine thus the link between the analyses in macro engineering structural scale and in micro material structural scale is established. By averaging micro stiffness constants over the cell macro ones needed for engineering structural analysis can be obtained. On the other hand, the macro structural analysis provides average stresses/strains of the cell locating at any concerned region of the macro structure for local stress and damage analysis. Effects of tow undulation caused by tow crossover on micro stresses are taken into accounted. The model is applied to glass/epoxy wound pipes with various winding angles and winding shifts. Mechanical properties are predicted and damage evolutions are simulated. The effects of delamination damage, usually introduced by lateral low velocity impact, on stiffness and ultimate strength of FWC pipe are also investigated.

No abstract