A novel approach is presented and used in a generic tunneling crack tool for the prediction of crack growth rates for tunneling cracks propagating across a bond-line in a wind turbine blade under high cyclic loadings. In order to test and demonstrate the applicability of the tool, model predictions are compared with measured crack growth rates from a full scale blade fatigue test. The crack growth rates, measured for a several metre long section along the blade trailing-edge joint during the fatigue test, are found to be in-between the upper-and lower-bound predictions.

Future sizes of wind turbine rotor blades will exceed 50 m. For transportation, it is favourable to make them in two parts and connect them in a suitable way at the operational site. With the co-operation of industry and research institutes in three countries, a spar beam-connection principle was selected as a possible solution. The pre-design, including finite element analysis, for the structural details were carried out at DLR, Stuttgart (Germany). LM Glasfiber A/S (Denmark) realised the concept in a 13.4 m GRP blade with a wound tube as a transverse load and bending moment transferring spar stump. The sectional blade was investigated for static and fatigue integrity at DLR up to 5 million load cycles in a sinusoidal one-step test. Strain gauges were applied at those locations shown by the finite element analysis to be critical. During the fatigue tests, the blade was observed by means of a thermoelastic stress analysis camera (TSA) by CLRC (United Kingdom) with the aim to observe the stress distributions. These measurements located stress concentrations not otherwise predicted by the finite element analysis. At those locations more strain gauges were applied and found to show relatively high stresses. The strain measurements can be used to calibrate the signals shown by the TSA camera. Thus, on the basis of the combined use of different design and measurement methods, a promising way is shown to find stress “hot spots” in complex composite components and to inform directly and immediately the manufacturers of those articles about possible or necessary modifications.

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