Progressive debonding analysis of composite blade root joint of wind turbines under fatigue loading

In this paper, progressive debonding of the blade root joint of a 660 kW wind turbine under cyclic loading is studied. Progressive fatigue debonding of this adhesive joint is analyzed using cohesive zone interface modeling. The blade is subjected to various loadings such as aerodynamic forces, centrifugal force, weight, and force due to the change in wind direction. A user defined material subroutine is used in the ANSYS finite element code to predict both the crack (debonding) initiation and propagation in the adhesive joint under variable amplitude fatigue loading. The required material properties such as the critical strain energy release rate and the Paris law constants are characterized experimentally through the well-known tests for different fracture modes: double cantilever beam (DCB) for mode-I (opening mode), end notch flexure (ENF) for mode-II (shearing mode) and mixed mode bending (MMB) for mixed mode conditions. These tests are performed for composite to aluminum interfaces, which are representative of the actual material used in the manufacturing of the wind turbines blades. The experiments are performed under both static and fatigue loading. The predicted initiation and progressive damage results from numerical analyses are verified by experimental results for simple joints. It is illustrate that the maximum length of the deboned area after a typical overloading is about 8% of the total height of the root joint and during the fatigue loadings it would increase to about 40% of the total height after 1 month normal operation. The developed numerical analysis tool can then be used to predict the debonding initiation and propagation in the root joint of the wind turbine blades.