Moisture induced change of the viscoelastic material properties of adhesives for SHM sensor applications

This paper addresses the influence of the viscoelastic material properties of adhesives on the functionality of SHM (Structural Health Monitoring) sensor applications. Epoxy adhesives behave viscoelastically and show a strong temperature and time dependency of their mechanical properties. Creep processes increase the deformation under mechanical load and relaxation can decrease the stress in the adhesives (polymer) with time. These processes are most effective in the temperature range of glass transition (Tg), at which the material behavior switches between the glassy and rubbery state and all material parameters change drastically. Additional the viscoelastic material properties are influenced by environmental loading like moisture as it behaves like a plasticizer in the epoxy matrix. In this study the moisture influence on the viscoelastic behavior of structural adhesives was investigated by DMA (Dynamic Mechanical Analysis) methodology for harsh environmental loading conditions like DI water immersion and relative humidity (r.h.) environment. For testing under RH (Relative Humidity) conditions, a novel DMA-RH equipment was applied, which allows online testing at different temperatures and RH levels between 0%r.h. and 90%.r.h. Commercially available two component epoxy adhesives (unfilled, high filled) with high potential of acceptance in avionic applications were studied. For the unfilled adhesive a significant material property change due to moisture absorption occured. The modulus decreased down to about 25 percent of the dry state modulus E'. As expected the Tg was reduced for "wet" samples. Additional the change in the time dependence was seen. The high filled adhesive showed much lower diffusion constants and the modulus change was not that remarkable.