Blades are crucial components of wind turbines. The requirements e.g. on aerodynamic performance and mass lead to light weight structures with a high material utilization. Thus improvements in material and structural reliability research can be utilized. In spite of the large effort of the designers and manufacturers, damages are observed in blade life. Among the most often observed defects in wind turbine blades are bond line damages and bond line cracks and defects. The analysis of full scale blade tests in this work gave data on typical bond line damages and the corresponding mechanical strains. A non-conservative damage prediction was observed, as the fatigue performance of the adhesive was below the material level performance. This work contributes to an improved design of adhesive bond lines in wind turbine blades by the development of a sub-component specimen and experimental tests. The specimen enabled a model validation with realistic boundary conditions and allowed for a better, and physically based understanding of the influencing variables.