Fracture and Fatigue Tests and Analysis of Composite Sandwich Structure

A composite sandwich system is investigated in this research. Quasi-static fracture toughness and fatigue crack growth experimental and analytical approaches are the focus. The particular system studied is comprised of a Nomex (aramid fiber) honeycomb core with graphite/epoxy facesheets (skins).

A modified version of the double cantilever beam (DCB) specimen geometry is used for experimentation. The critical strain energy release rate, G_c, is used to characterize the fracture toughness of the facesheet-core joint. Fatigue crack growth testing is also performed. Novel analytical and experimental techniques are coupled and utilized to address challenges presented by the material system, especially difficult crack visualization. Crack length and growth can be estimated with an empirical approach, employing a compliance calibration. Experiments can also be simulated once several constants are estimated, aiding design. Many of these techniques can be generalized to other adhesive DCB experimentation.

Results show that cold tests result in higher fracture toughnesses and slightly slower fatigue crack growth rates than room temperature tests. The hot temperature has less significant impact. Although only a limited amount of very slow growth data (<10⁻⁶ mm/cycle) is measured, the material appears to behave with a fatigue threshold of ≈7% of fracture toughness; and a Paris crack growth model is successfully fit with an exponent of ≈3.2. Results also show the failure of this system is always in the aramid paper core material.