Workflow of sequential rotor blade fatigue test and post-fatigue static test procedure — Full scale Fatigue tests are an important part in the certification process of new wind turbine rotor blade designs. As the blades are designed longer and closer to material limits, the fatigue tests are more time-consuming, and high overloading is more critical. By splitting the fatigue test into multiple test sequences on specific sections of the blade and accumulating the fatigue damage, the overloading can be reduced drastically, and a lot of time can be saved. Introduction Modern wind turbines have a higher rated power and longer more slender rotor blades compared to earlier designs. Thus, facilities performing full scale blade tests for certification need to be upgraded for static and fatigue testing to introduce larger loads and deal with larger tip deflections. Also, the test methods need to be adapted to avoid high overloads and reduce testing time. The most time-consuming part of the test campaign is the fatigue testing according to the current standards [1]. To introduce a damage equivalent load of a lifetime of 20-25 years rotor blades are subjected to cyclic loading with resonant excitation. Due to the lower natural frequencies of larger blades the conventional fatigue tests take even longer. Also, to sufficiently test the whole region of interest, high local overloads must often be accepted, possibly leading to premature damages, which need to be repaired mid-test. To reduce the peak loads, higher cycle numbers are required further prolonging the test. By cutting the blade into two segments (root and tip) and testing both segments simultaneously at higher natural frequencies as described by Ha et al. [2], the test duration can be reduced, but the mid-section around the cutting position cannot be tested sufficiently, which could also be overcome by using overlapping segments from two rotor blades. However, here an alternative method is proposed requiring only one rotor blade. Methodology In the proposed approach called “sequential testing”, the damage is introduced by multiple tests, each of which contributes to the resulting fatigue damage by linear damage accumulation. In the first test, the whole blade is tested with a similar setup as during a conventional test. The root and tip areas are not fully tested during the first test, which reduces the overloading in higher loaded areas. then the blade is cut into two (or more) segments, which are then tested simultaneously in separate test setups, at higher frequencies to accelerate the overall test. These setups are chosen to introduce the remaining fatigue damage in the previously undertested areas of the blade. The fatigue damage of all tests is accumulated to achieve the target damage in the whole region of interest, while drastically reducing overloads compared to conventional tests. The resulting reduction in local peak load allows for a higher load level overall reducing the required load cycles and therefore the testing time. The post-fatigue static tests also need to be executed in separate tests for each target region as shown in the workflow figure (left). The different test setups need to be attuned to one another. To find a good set of suitable test setups, including cutting position, additional masses and excitation, a numeric optimization is required. This was done based on a FE-Beam model and the simulation method by Melcher et al [3]. Furthermore, a quality measure for damage conformity was introduced. See the full paper for further details. Example As it is generally more difficult to reduce overloading in lead-lag fatigue testing than in flapwise fatigue testing, the proposed method is applied here by way of example to the lead-lag test of a current offshore rotor blade design of over 110 m in length with a region of interest between 7% and 90% blade length. A set of setups for sequential testing and a conventional test setup for comparison were designed