Fatigue life prediction in composite wind turbine blades: Effects of variable wind load and nonproportional multi-axial stress states

Conventional fatigue analysis of composite wind turbine blades has applied a fixed wind load distribution that does not fully capture the effect of variable wind load on fatigue life prediction. Another simplification in the conventional fatigue analysis is that the blades are often treated as a typical beam-like structure in which fatigue life calculation is limited in considering the normal stress component in the beam axis direction. Consequently, the effect of non-proportional multi-axial complex stress states on blade fatigue life prediction has been ignored. In order to study the fatigue effects of variable wind load and non-proportional multi-axial stress states, a comprehensive fatigue analysis that includes variable wind load, wind field simulation and aerodynamic analysis, stress analysis by finite element analysis, and fatigue damage evaluation based on tested fatigue data has been developed for composite wind turbine blades. The variable wind load is represented by a joint distribution of 10-minute mean wind speed and 10-minute turbulence intensity. The non-proportional multi-axial complex stress states are involved when calculating 10-minute fatigue damage of section points through laminate thickness. The annual fatigue damage is calculated based on the 10-minute fatigue damage and the joint distribution. Consequently, the blade fatigue effects due to both the variable wind load in a large spatiotemporal range and the non-proportional multi-axial complex stress states can be investigated. The case study reveals that the variable wind load has a significant influence on the fatigue life of composite wind turbine blades. It is also shown that neglecting transverse normal and shear stresses in the blade fatigue analysis could lead to substantially overestimated blade fatigue life.