Moisture effects on the adhesive epoxy material in bonded composite sandwich panels

This work investigated the effects of moisture on the epoxy adhesive used to bond the skin to the core of aircraft sandwich panels. Moisture is known to affect the integrity of the adhesive material and, in turn, the joint bondline in three different ways: (i) plasticization, (ii) hydrolysis, and/or (iii) swelling of the adhesive material. Plasticization and hydrolysis directly affect the molecular motions within the epoxy network. In this work, the molecular mobility was studied over a range of frequencies---from lower frequency, network segment scale motions studied by quasi-static tensile tests, dynamic mechanical analysis and stress relaxation, to higher frequency, atomic scale motions studied by nuclear magnetic resonance.

It was revealed that moisture eases the relative molecular motions within FM300^® epoxy over the entire range of motions studied. This translated to reductions in the stiffness, strength and glass transition temperature of epoxy, all of which have the potential to adversely affect the durability of the adhesive joint. The observed moisture effects were reversible, strongly suggesting these effects are due to plasticization and precluding hydrolysis as a factor in bonded joint degradation. The results also showed that moisture generally increases the stress relaxation rate of epoxy, which is consistent with plasticization behaviour. However, beyond a threshold moisture level, the relaxation rate was reduced, indicating a change from plasticization to some form of ductility loss.

A novel magnetic resonance imaging (MRI) technique was employed to investigate the moisture absorption kinetics of epoxy. These results showed that moisture penetration can be rapid even though complete saturation may take a long time.

Moisture swelling affects the sandwich panel from a structural standpoint. A finite element analysis showed that adhesive material swelling, rather than changes in stiffness, increased the stress level within the bondline, which could lead to premature failure of the bonded joint.

Overall, the results have shown that moisture plasticizes FM300 ^® epoxy and that the moisture absorption process cannot be represented by models based on simple Fickian diffusion. The results also suggest that the development of in situ NMR measurement technique should focus on detecting water inside the core of sandwich panels.