Early stage fatigue damage mechanisms in composite material used for wind turbine rotor blades

One of the major design issues with new generation wind turbines is designing a rotor blade material against fatigue failure. The load­carrying components of a wind turbine rotor blade consist of composite materials, which are subjected to cyclic loads resulting from varying wind speeds. Often, these composite materials are damage tolerant in comparison with traditional materials like metals, however, the underlying fatigue damage mechanisms of composites are still not fully understood. In order to design modern rotor blades for extended service life and to develop materials with a longer fatigue life, it is necessary to have detailed understanding of fatigue damages evolution.

This PhD project contributes to the understanding of the very first mode of fatigue damage ’off­axis cracking’, and the damage modes that follows in laminates made from non­crimp fabrics. A novel modelling framework was developed for analysing off­axis tunnelling crack growth from a biaxially loaded laminate in an equivalent uniaxially loaded laminate. Thus, making it possible to perform simple uniaxial tests for creating the same cracktip stress state as in the biaxially loaded laminate. Further, an experimental campaign of tension­tension cyclic loading tests followed by a method of using a dye penetrant and X­ray CT imaging made it possible to characterize tunnelling cracks damage in 3D. Lastly, the demonstrated theoretical approach was tested in a multi­scale framework by comparing crack growth rates measured from biaxially loaded cruciform specimens with uniaxially loaded coupon specimens.

Results from the modelling framework show that the steady­state tunnelling crack growth conditions, expressed in terms of average mode mixity and average energy release rate, can be recreated from biaxially loaded laminate in a modified laminate under uniaxial loading. In the experimental analysis of off­axis tunnelling cracks evolution, X­ray CT investigations on small­sized samples cut along the direction of 60° off­axis fibre bundles, revealed variation in the height of the cracks in the thickness direction. Further, the complicated microscale architecture of the bundle based composites was found to influence largely the interface damage phenomena of crack deflection and crack penetration damage mechanisms. Cracks from the ±60° off­axis bundle extended into resin rich pockets with crack planes aligned normal to the loading direction. Depending on the orientation of backing fibres, the crack­tips either continued to penetrate between fibres along the fibre/matrix interface or deflected at near right angles.

The understanding of off­axis tunnelling cracks damage gained in the project can be used to build damage prediction models for non­crimp fabric based composites. In addition, the methodology and the presented crack deflection/penetration analysis can be applied to other off­axis angles studies, fabric architectures, and loading configurations, which may help in designing better fatigue resistance materials.