SCALE-UP OF WIND TURBINE BLADES CHANGES IN FAILURE TYPE Introduction The next generation blades for the European offshore market will be between 75 and 85m long weighting between 30 and 45 tonnes. The most public available example of this is the new Vestas V164 with a 80m blade, but also DSME/DeWind, Samsung, Alstom and Nordex are developing turbines with similar blade lengths. LM Windpower will introduce their new 74m blade mid 2013. With this new generation of wind turbine blades typical failure modes as well as their location will change. For example, the increasing length and weight makes the gravity introduced loads more important. A first, rather rudimentary summary is shown in the matrix given in Figure 1. In the four columns to the right is given a grade indicating how critical the (failure) mode is expected to be for the given blade length. 5 indicates significant importance and 0 no importance. Figure 1. Trend and changes in (failure) modes of wind turbine blades due to scaling. R is blade length. Based on scaling laws e.g. from EU-Upwind project which, see ref. [3] or Sandia report, ref. [2] different (failure) modes are relevant for the design depending of the size. E.g. the root bending moment scale-up with power of 4 to the blade length (R), caused by the increased mass. For small blades it is the tip deflection (global stiffness) which is critical and not the weight. For larger blades this changes and designers have altered their focus on decreasing the weight utilizing, at least partially, low weight/high-stiffness composites (e.g. CFRP) to cope with it. This is generally accepted in the industry when they scale-up a blade, they have a clear focus on decreasing the weight in order to decrease critical edgewise fatigue loads. Just a few years back with significantly smaller blades aerodynamic loads build-up the dominating and dimensioning load case while today with longer blades it is the gravitational forces and therefore the weight of the blades. This overview of failure modes and how they are linked to the scaling laws. Recommendations are presented on how each failure mode should be addressed either in the design phase e.g. by an experimental program or advanced non-linear numerical simulations, since not all are covered by the international standards today, ref. [4][5][7].
Typical failure modes influenced by scaling Transverse shear distortion of the cross section The combination of large edgewise loads and extreme aerodynamic forces result in load combinations which could end up into a critical transverse shear distortion failure (cf. Figure 2) for scaled-up slender blades, see ref. [1]. Figure 2. Numerical simulation shows transverse shear distortion of a cross section loaded in a combined edge- and flapwise direction. This failure mechanism becomes more important when the size (and weight) of the blades increase mainly due to larger gravity forces in edgewise direction. Today, the failure mode is not appropriately covered in nowadays certification rules, since no combination of the forces are required in the final full-scale test. Furthermore, the loads are applied the blade using clamps which support the structure in a way so it can not distort at this specific cross-sections. Failure in the cap(s) caused by Brazier loads The out-of-plane deformation (flattening) of the caps, caused by the Brazier loads, may cause failure in the caps, see ref. [8],[10]. The failure can either be a transverse tension failure in the unidirectional layers at the bottom layer or an interlaminar shear failure between the layers, see Figure 3a. A typical lay-up is not particularly well suited to reduce cap deflections since the fibres are there mainly placed in the longitudinal direction of the blade. The lack of fibres in the transverse direction causes the cap to be relatively flexible in the lateral direction. When the cap deflects there is a risk of transverse tension failure in the unidirectional laminates.in addition, it is common that manufacturing imperfections, see Figure 3c, inside the laminate further reduce the fatigue and ultimate strength of the laminate. b a Figure 3. Interlaminar shear failure of the load-carrying cap laminate caused by the non-linear Brazier forces a) Sketch of cap deformation and failure between layers b) Photo of a cap with delamination c) Photo of a cap with a manufacturing defect. When the blades scale-up the longitudinal curvature of the blade increase, which results in a larger crushing pressure, called Brazier forces. This results in larger out-of-plane deformation of the load carrying cap laminate, which then result in either transverse tension failure in the unidirectional layers or interlaminar failure. This out-of-plane deformation is a non-linear geometric phenomena, which unfortunately, is not covered by the internationally design c
standards. The test standards do not require that there is strain gauges measurement, but this strongly recommended in future full-scale test. Buckling Buckling is a structural non-linear geometric instability phenomenon which is important for design of nowadays and future wind turbine blades. The buckling capacity can either be addressed by a non-linear geometric FE-analysis or a linear eigenvalue buckling analysis. Accordingly, linear buckling analysis is a guideline for the design load, to which a suitable reduction factor is called for. However, the corresponding buckling modes need to be examined carefully, to sort out unrealistic modes at unrealistic buckling loads. Due to the up-scaling of blades the trailing edge as well a the large training edge panels will become more prone to different kind of as detailed in Figure 4. Figure 4. Buckling of the trailing edge. Buckling of the trailing edge has to validated in full-scale test but the method of applying the loads, may not give a realistic picture since the clamps can be seen as artificial boundaries, influencing the so called free length and impact the buckling loads examined during test. As an alternative to the methods Risø DTU has developed a method, using anchor plates, which give a more realistic load introduction without supporting the structure, see ref. [9]. Failure in the adhesive bondlines Out-of-plane deformations of the trailing edge panels (also noticeable growing by scaling the blade) are important since they result in peeling stresses in the bondlines, which may be the main reason for fatigue failure in trailing edge adhesive joints, see Figure 5 as well as ref. [9]. It is well-known from the literature that bondlines have a low strength when exposed to peeling stresses compared to the other loading directions, e.g. the fracture energy is approximately a factor 8-10 higher for mode 2 (shearing) than for mode 1 (peeling) loading. Figure 5. Sketch of the trailing edge shells with out-of-plane deformations. The close ups show failure at the trailing edge as well as debonding of the outer skin on the box girder. The load introduction problematic, described in the previous buckling section, is the same for this failure mode and also the anchor plate solution is recommended in future testing.
Fatigue problem in the root transition area In the transition zone from max. chord to the cylindric and stiff root region often cause problems, see Figure 6. The reason that large edgewise forces have to be carried through large curved panels, which results in pumping out-of-plane deformations and then give peeling in bondline where the trailing edge panels are connected to the cylindrical root. Figure 6. Photo showing a classical fatigue failure in the transition area from max. chord to the root. The failure is caused by the out-of-plane deformations of the large trailing edge panels, which is sometimes called pumping. Since the root bending moment scale-up with power of 4 to the length, this fatigue failure is to be extremely critically for future wind turbine blades. Special design specific considerations/modifications need to be taken in account during the sizing process. Web failure Web failures has in some cases been observed to be the main reason for collapse, see ref.[8]. Figure 7 presents a frozen frame picture of the first, and critical, failure mode observed in a fullscale test of a load carrying box girder. In Figure 7 the white circle (light green colour) initial face debonding of the outer skin on the shear web s sandwich section are shown, leading to ultimate collapse of the box girder, see ref. [8] Figure 7. Skin debonding failure of the sandwich shear webs which was observed in a full-scale test of a load carrying box girder, see ref. [8]. This failure could be due to, too weak skin faces in the sandwich shear webs. This failure would normally, not be observed in a commercial full-scale test since it is almost impossible to access both sides of the shear web with measuring equipment, cameras etc.. In the FE-modeling which sometimes is required by the certification bodies, this failure mode is not easy to address since it would require on a fracture mechanics approach. Furthermore, a non-linear geometric analysis is required otherwise the Brazier crushing forces will not be included, and this, as mentioned earlier, is not part of a standard certification process. As mentioned earlier the
Brazier forces are expected to be more dominant when the blades scales up and therefore also this web failure mode have to be considered more carefully in the future. References [1] Jensen, F.M. Ultimate strength of a large wind turbine blade, Risø-PhD-34(EN), PhD thesis, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, (2008). [2] Griffin, D.T. and Ashwill, T.D. The Sandia 100-meter All-glass Baseline Wind Turbine Blades: SNL SNL100-00 Sandia report, June 2011 [3] Lekou, D.J. UpWind report Scaling limits & Costs Regarding WT Blade EU-Upwind project 2010 (www.upwind.eu) [4] Guideline for the Certification of Offshore Wind Turbines. Germanischer Lloyd WindEnergie GmbH. (June 2005) [5] DNV Standard DNV-OS-J102 - Design and Manufacture of Wind Turbine Blades. Det Norske Veritas (October 2010) [6] Jensen, F.M, Sørensen, J.D., Nielsen, P., Berring, P.,Flores P.. Failures in Trailing edge bondlines of wind turbine blades 32 nd Risø International Symposium on Materials Science 2011 [7] IEC 61400-1. Wind turbines - Part1: Design requirements. 3rd edition. (2005) [8] Jensen F. M., Weaver P. M., Cecchini L. S., Stang H., Nielsen R.F., The Braziers effect in wind turbine blades and its influence on design Wind Energy Journal 2011. DOI: 10.1002/we.473 [9] Jensen F. M., Sørensen J. D., Nielsen P. H., Berring P., Flores S., Failures in trailing edge bondlines of wind turbine blades. Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark. (Risø DTU Symposium September 2011) [10] Jensen F.M., Puri A., Dear J.P., Branner K., Morris J., Investigating the impact of nonlinear geometrical effects on wind turbine blades Part 1: Current design status and future challenges in design optimization, (Wind Energy Journal August 2010- DOI:10.1002/we.415). DOI: 10.1002/we.415 [11] Kling A., Tessmer J., Degenhardt R., Validation Procedure for Nonlinear Analysis of Stringer Stiffened CFRP Panels, Proceedings of the 25th Congress of International Council of the Aeronautical Sciences, Hamburg, Germany, 3-8 September, 2006