Two of the most severe type of imperfection in structures at this length scale are unbonded areas between face sheets and the core (face/core debonding) in sandwich structures and geometric imperfections, i.e., deviation from the "idealised" geometry. Sub-structural parts, e.g. generic wind turbine blade sections, will be tested experimentally; focus will be at studying the failure mode. Based on the experiments cohesive laws, representing fracture along the core/face sheet interface, will be determined and used in finite element models.