On the Estimation of Short Fiber Orientation in a Filled Epoxy Adhesive and its Effect on the Tensile Strength

Short glass fiber-reinforced adhesives are state-of-the-art materials used in bond lines of wind turbine blades. Various alignments of the short fibers are emerging which depend on the adhesive flow during the application and joining process. This induces a spectrum of directiondependent mechanical properties. The tensile strength, for instance, can vary by about 20 % depending on the load direction. Therefore the adhesive performance, which is normally determined under controlled laboratory conditions and implemented in bond line design routines, can deviate from the in-situ application. This work investigates the effect of the short fiber alignment on the tensile strength distribution for an industry-standard adhesive system used in wind turbine rotor blades. The smeared short fiber orientation of the tensile specimens tested is estimated locally near the damaged surface by means of a material model. The model is fed with standard thermomechanical measurements (TMA) and calibrated via micro-CT analysis. A linear correlation between tensile strength and short fiber alignment has been identified. The material model proposed augments the identification of the upper and lower material strength limits for bond line design purposes.