Measurement of viscoelastic material properties of adhesives for SHM sensors under harsh environmental conditions

This paper addresses the influence of the viscoelastic material properties of adhesives on the functionality of SHM (Structural Health Monitoring) sensor applications. 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 different moisture loading conditions. Water immersion (DI) and relative humidity (RH.) at different temperature levels showed the strong temperature dependence of the absorption process. For testing under RH (Relative Humidity) conditions a novel DMA-RH equipment was used. It allows online modulus measurement at 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 occurred. The modulus decreased down to about 25 percent of the dry state modulus E' and kept constant for further conditioning. As expected a Tg reduction for "wet" samples was visible. Additional the time dependence increased proportional to the square root of conditioning time. The high filled adhesive showed much lower diffusion constants at room temperature and the modulus change was not that remarkable. For 85°C/85% r.h conditioning the modulus decrease to about 40% of the initial room temperature value.