Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 459

Figure 29: Simulation of failure of a box girder (progressive delamination growth
in combination with buckling) [85].
traditional single skin composites. However, introducing a sandwich construction
in the fl ange was found to result in a globally more fl exible structure, making
tower clearance the critical criterion.
Overgaard and Lund [84, 85] simulated the full-scale blade test till failure [2] by
FE analysis using a cohesive zone modeling. A mixed mode bilinear cohesive law
was used. The combination of a relative coarse mesh and the use of constitutive
laws with softening leads to severe solution diffi culties, and therefore an effi cient
and robust solution strategy for dealing with large three-dimensional structures
was implemented.
This work shows that it is possible to predict structural behavior of a wind turbine
blade based on non-linear fracture mechanics in a geometrically non-linear frame-
work, i.e. account for buckling and delamination interaction (see Fig. 29). The
numerical damage predictions were compared with a fl apwise static test result. The
damage simulation displayed strong geometric and material instability interaction
which indeed caused a progressive collapse of the wind turbine blade as seen in the
full-scale experiment [2, 82]. According to the model, the critical buckling load of
the blade triggered a delamination at the corner and in middle of the fl ange at the
point of infl ection of the buckling pattern. This started a progressive chain of events
that lead to a structural collapse of the complete wind turbine blade.
8 Perspectives and concluding remarks
This chapter has presented an overview of blade materials, blade design and test-
ing methods. With more knowledge evolving, the blade design will be made more
accurately and safety factors may be reduced. However, safety factors should only