http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Effects of moisture on composite materials
The objective of this research was to investigate various aspects of the effects of moisture on composite materials in order to obtain a physical-chemical understanding of moisture absorption and damage mechanisms in fiber/polymeric matrix composite materials. The physical and chemical processes accompanying the absorption and diffusion of moisture were assessed by examining different diffusion models for polymers and composites as well as by evaluating their effect on the degradation of properties of composite materials. A mathematical description of damage in composites was formed using solutions to two boundary value problems describing the behavior of an inclusion (fiber) embedded in an infinite matrix both with and without the presence of a circumferential crack. Solutions to these problems were used in conjunction with finite element calculations of the stress state around graphite fibers (in a square or hexagonal array) embedded in epoxy matrix and subjected to thermal and moisture loading in order to characterize fracture due to moisture. Numerical and analytical calculations of the energy available for crack propagation due to thermally- and moisture-induced residual stresses indicate that very low energies are needed to advance such cracks. Additionally, stresses along the interface were in all cases less than the matrix ultimate strength. This suggests that moisture degrades the fiber-matrix interface. Experimentally, the effects of cyclic humidity exposure at a constant temperature on the tensile, compressive, and fracture properties of AS4/3502 graphite/epoxy composite material were measured. Results show little change in of mechanical properties due to damage from moisture absorption except for the case of [90°]₈ specimens. This indicates that other than for this case, there is no macroscopically observable change in material properties for the cycling conditions considered here, although microscopic damage is present due to the humidity cycling. The experimental data do, however, appear to show effects due to chemical aging in the form of increased scatter and different energy release rates for new vs. aged specimens. Measured critical energy release rates were observed to be significantly larger than analytically and numerically calculated values. This discrepancy is attributed to the markedly different state of crack tip damage present in DCB specimens subjected to loading as compared to micro-level events in specimens subjected to thermally- and moisture-induced stresses alone.
"Major subject: Mechanical Engineering."