Abstract: A fatigue assessment method for variable cross-section fiber reinforced plastic beam under complex loads is studied. The blade of horizontal axis wind turbine is a typical reinforced plastic beam. Blade fatigue load cases are identified and loads are calculated by GH Bladed software, which is specified at the IEC61400 international specification and GL (Germanisher Lloyd) regulations. In order to resolve the problem resulting from the changing loads with time and beam span, stress analysis is performed with a finite element method (FEM). Considering Saint-Venant's principle, uniform cross section FEM model is built at each critical zone. Stress transformation matrixes (STM) are set up by applying six unit load components on FEM model separately. The total fatigue damage of each laminate on the critical point is counted by rain-flow counting method and Miner’s damage law based on general S-N curves. A comparison of the performance of this method is made with far more costly business software FOCUS. The results show that the fatigue damage of multi-axis FRP could be assessed conveniently by this FEM-STM method. And this proposed method gives a reliable and efficient method to analyze the fatigue damage of slender composite structure with variable cross-sections under irregular loading.

Key words: fiber reinforced plastic, variable cross-section beam, wind turbine blade, fatigue, stress transformation matrix