Construction and characterization of stainless steel/polyurea/E-glass composite joints

This work was motivated by the desire to improve the long-term durability of E-glass/Hysol 9394 epoxy/Al-6XN stainless steel joints whose fracture energy in a previous study was shown to fall from ∼950 J/m² (ambient) to a mere ∼88 J/m² after only two days of exposure to a 90%RH, 50 °C environment. This paper reports a new polyurea chemistry to bond the E-glass and steel sections with very promising results. The fracture energy under ambient condition was measured to be 1232 ± 15 J/m² using a double cantilever beam experiment. This value degraded by only 13% to 1070 ± 35 J/m² in samples that were conditioned for 30 days at 90%RH and 50 °C. The failure in all samples was cohesive, within the first ply of the E-glass composite that neighbored the polyurea interface. The intrinsic fracture energy measurements, devoid of inelastic effects, were also carried out by submerging samples in a liquid nitrogen bath. Values of 590 ± 25 J/m² were obtained, which can be used by the designers to set the local failure condition in design simulations of large-scale structures.