Fatigue Life Prediction in Composite Wind Turbine Blades: Effects of Variable Wind Load and Nonproportional Multi-axial Stress States

摘要

Conventional fatigue analysis of composite wind turbine blades has applied a fixedrnwind load distribution that does not fully capture the effect of variable wind load onrnfatigue life prediction. Another simplification in the conventional fatigue analysis isrnthat the blades are often treated as a typical beam-like structure in which fatigue liferncalculation is limited in considering the normal stress component in the beam axisrndirection. Consequently, the effect of non-proportional multi-axial complex stressrnstates on blade fatigue life prediction has been ignored. In order to study the fatiguerneffects of variable wind load and non-proportional multi-axial stress states, arncomprehensive fatigue analysis that includes variable wind load, wind field simulationrnand aerodynamic analysis, stress analysis by finite element analysis, and fatiguerndamage evaluation based on tested fatigue data has been developed for compositernwind turbine blades. The variable wind load is represented by a joint distribution ofrn10-minute mean wind speed and 10-minute turbulence intensity. The non-proportionalrnmulti-axial complex stress states are involved when calculating 10-minute fatiguerndamage of section points through laminate thickness. The annual fatigue damage isrncalculated based on the 10-minute fatigue damage and the joint distribution.rnConsequently, the blade fatigue effects due to both the variable wind load in a largernspatiotemporal range and the non-proportional multi-axial complex stress states can berninvestigated. The case study reveals that the variable wind load has a significantrninfluence on the fatigue life of composite wind turbine blades. It is also shown thatrnneglecting transverse normal and shear stresses in the blade fatigue analysis could leadrnto substantially overestimated blade fatigue life.