Monitoring of water absorption and its effects on mechanical performance of thick GFRP structures by integrated smart sensors

Abstract

In many applications, such as offshore, marine, or wind energy industry, the aging of composites is an important issue, as the structures are exposed to severe environmental conditions. Typically, moisture gradients evolve from the surfaces in contact with moisture or water into the material. The underlying diffusion process depends on the ambient temperature and the specific material selection (e.g. the chemical structure of the polymeric resins). In this context, fiber reinforced polymers (FRP) are often assumed to exhibit approximately Fickian diffusion behavior. Long term predictions are based on isothermal aging tests, which are usually accelerated by full immersion of coupons in water with higher temperatures than the service temperature [1]. Design parameters are derived from small-scale coupon tests with saturated specimens. However, for FRP structures of several millimeter thickness in real operating conditions, it takes years until the laminates are fully saturated. As it is well known that especially glass fiber reinforced polymers (GFRP) can experience significant reductions in mechanical properties due to moisture absorption, a reliable determination of the current moisture condition at various points in the structures is beneficial. Precise knowledge about the effects of moisture on the mechanical behavior of the composites can, furthermore, extend the service life and prevent failure. Consequently, a method for monitoring local moisture absorption in thick GFRP laminates is presented in this work. Therefore, an integrated sensor system based on single carbon fiber (CF) bundles and impedance measurements as developed by Buggisch et al. [2] was evolved. Additionally, the mechanical effects of single-sided moisture absorption (only through thickness) are evaluated using four-point bending (FPB) and interlaminar shear strength (ILSS) tests