The damage, repair and design of fiber composite structural materials.

Abstract

Three aspects of the repair of fiber reinforced composite materials were examined in this study. First, the in-plane compression damage of four typical fiber composite materials was studied, and different repair strategies were explored. Second, an optimum composite sheet design for least damaged by buckling and easiest to repair was studied. Third, the curing and resulting properties of the interfacial region between an anhydride-cured epoxy adhesive and E-glass fibers/vinyl ester matrix, which is critical to the quality of the repair, was investigated. For none of the four typical fiber composite materials subjected to in-plane compression do all of the load carrying fibers break, and those that do are usually in easily removable layers. Moreover, the behavior of each material was relatively consistent. As a result, a new repair strategy is proposed in which no more of the load-carrying fibers are removed from the damage zone than are broken. For the two laminated materials, a transition in failure mode from tensile to shear was found as the effective gage length of the specimen decreased. This suggests that the composite structure can be designed to relieve the applied load in desired locations and with benign failure modes. In the second part, ten composite materials with different fiber stacking sequences were subjected to in-plane compression. Three failure modes were dominant: shear, delamination buckling, and flexural failure. The volume of damage occurring in the 0$\sp\circ$-fiber plies decreased and the repair efficiency increased in the sequence: shear, flexural, delamination buckling failure. With respect to the amount of damage and repair strength recovery, it is best to have the composite structure designed to fail in delamination buckling when failure is inevitable. In the third part, model systems were used to simulate the interfacial region between adhesive and adherend. These involved curing the adhesive against a FTIR-ATR plate and against adherend material ground to a fine powder to greatly increase its surface area. The increase in reaction heat and $T\sb{\rm g}$ suggest that there is no interfacial problem for an anhydride-cured epoxy adhesive used with E-glass/vinyl ester composite materials.