PSI - Issue 46

L. Frank et al. / Procedia Structural Integrity 46 (2023) 3–9 L. Frank and S. Weihe / Structural Integrity Procedia 00 (2019) 000–000

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Based on the measured height, the cracks were assigned to two damage locations, the radius/notch area as well as the edge of contact area (EoC). The inclusion of this information resulted in the conclusion that all blades with failure between 10 5 and 10 6 cycles had cracks in the notch. The breakup of individual blades and the fractographic post examinations using light and scanning electron microscopes showed a typical fatigue fracture surface, see Fig. 3 (upper right). In contrast, the blades with failure at approx. 6 * 10 6 cycles had cracks in the EoC area. The breakup and the fractographic post-examinations also showed a typical fatigue fracture surface, but plastic deformations are visible there, see Fig. 3 (lower right). In addition, wear marks were found in the crack area. It can be stated that the occurrence of the damage location and damage mechanism depends on the level of bending stress.

3. Numerical investigations 3.1. Lifetime Assessment Concept

Numerical investigations were carried out to predict the lifetime under varying loads. This lifetime prediction is based on the concept shown in Fig. 4. The aim of the lifetime assessment concept is to determine the stresses and strains occurring in the blade root and to compare it with the corresponding material properties in order to be able to make a statement about the lifetime prediction.

Fig. 4. Lifetime assessment concept

Numerical solutions were obtained using the finite element (FE) code Abaqus. A detailed FE-model was created, which contain the exact geometry of the blade root, the clamping as well as the load introduction components, see Fig. 5 (left). The experimental values for the compressive force, the knee lever pressure and the shaker amplitude and frequency were used directly as inputs for the loads. The simulations were performed with the penalty contact model in conjunction with the coulomb friction. The model was verified with the experimentally measured groove opening as well as the measured strain gauge values. The elastic deformations of the global model are transferred to the submodel with a finer mesh. Especially the contact edges contain a fine mesh to capture the high stresses and stress gradients there, see Fig. 5. The cyclic flow curve was used to describe the material behavior for the blade root section