M. Jensen, A. Hillestrøm & P. Hjuler Jensen, Risø DTU, Denmark
In recent years, alternative energy sources have taken an ever larger slice of the overall energy mix. In 2007, wind power provided some 19.7 per cent of Denmark’s electricity supply, and worldwide the capacity of wind energy has continued to accelerate. The growth of wind power however, has placed a strain on wind turbine designers. The size of blades of offshore turbines has already exceeded 60 m and the trend is for even larger blades in the future.
Due to the fact that current wind turbine blade designs are not thoroughly optimized, with regards to structural strength, large differences can be found in the safety margin against various types of failure modes.
Transverse shear distortion failure
Transverse shear distortion of a wind turbine blade cross-section is an important failure mechanism, which can ultimately lead to catastrophic collapse. In other industries, such as aeronautics and bridge building, transverse shear distortion is a well-known mechanism that must be taken into account in the design process.
However, in the existing literature relating to wind turbines, Risø DTU has been unable to find to-date any investigations relating to this failure mechanism. For some reason nobody in the wind energy sphere, to Risø DTU’s knowledge, has paid any attention to this failure mechanism in the design of wind turbine blades.
However, it is obvious that a thin walled structure without any internal reinforcement will try to distort the profile in the transverse direction. This is even more prominent when the blade is non-symmetric, both in geometry and in lay-up, since the blade will try to twist. Furthermore, the lay-up is highly orthotropic with the majority of fibres in the longitudinal directions, so the circumferential stiffness is small.
Larger blades are expected to be more susceptible to this failure mode since it is almost impossible to increase the corner stiffness in order to account for the increase in transverse shear forces. Furthermore, if future wind turbine blades are lighter due to optimization, the longitudinal curvature will increase, which will raise the crushing pressure and this could be critical for a distorted blade section.
A non-linear FE-study of a 34 m wind turbine blade showed that transverse shear distortion was likely at high combined loading, i.e. with both flapwise and edgewise loads (Figure 1).
Non-linear FE-studies and experimental investigations need to be performed for the actual blade before it can be concluded whether this failure mechanism is critical or not. It is important that the FE-studies are non-linear since both the internal crushing force and the transverse shear distortion are non-linear.
Furthermore, the FE-model must have representative stiffness in the corner regions between the shear webs and the outer shells, otherwise shear distribution is not well predicted. This often requires a detailed FE-model, either consisting of a combination of shell and solid elements or a FE-model only with solid elements.
Both modelling techniques require substantial computer power, especially if a full model of the blade is to be analyzed. At Risø DTU a very efficient cluster is used with approximately 200 computer processing units (CPUs) available.
Preventing transverse shear distortion failure
Reinforcement can be introduced to solve the distortion problem. Using either cross-reinforcement and tilted shear webs prevent transverse shear distortion.
Figure 2 shows the results of a FE-study performed with combined flapwise and edgewise loading. The combination of flap- and edgewise loads should be considered in the future, since this is a realistic load scenario.
Figure 2: FE-calculations show transverse shear distortion with combined loading. A 4-metre section is shown with deformed and non-deformed shape: top, without cross reinforcement; above, with cross reinforcement.
Transverse shear distortion failure has only been verified by FE-studies up to now, but a full-scale test is expected to demonstrate that this failure should be considered more carefully in the design (and testing) phase of wind turbine blades in the future.
Newly developed testing techniqueS
Failure mechanisms and stress distributions, different from those of traditional load cases tested, have been observed when combined loads are applied. Also, the way in which loads are applied in classical full-scale tests should be reconsidered since the contour of the typical clamps used for load application prevent the blade from distorting in a realistic manner. Risø DTU has developed new clamps called anchor plates, which allow transverse shear distortion, and are used in the full-scale tests at its new full-scale facility.
One of the main purposes of Risø DTU’s new full-scale facility is to examine the structural strength of future wind turbine blades in different load combinations. Also the way the loads are applied and the control of the loads are different from known test techniques.
Furthermore, advanced measurement techniques are used and developed in the research test facility together with traditional measurement equipment, such as strain gauges and displacement censors.
In Figures 3 and 4, the blade shown has more than 350 strain gauges and around 40 displacements transducers mounted on it. The numerous mechanical displacement sensors have also been used for validation and calibrations of the FE-models.
Figures 3: Above, a 34-metre blade is mounted with 350 strain gauges and 40 displacement transducers.
Figures 4: Right is a photograph of inside the load carrying box girder showing a small part of the measurement equipment.
Interlaminar shear failure in the load carrying cap
Interlaminar shear failure is a mechanism that occurs between the layers in the load-carrying cap laminate. The failure is caused by interlaminar shear stresses.
These can be developed by the crushing pressure causing biaxial stress distributions, or interlaminar and peeling stresses because of the curved structure being flattened out.
The deformation is caused by the non-linear phenomena the Brazier effect and results in internal crushing forces, which ‘ovalize’ the section1. The out-of-plane cap deflections are increased by the lay-up, since the fibres are 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.
Layered composite materials do not have large resistance against peeling and interlaminar stresses which may lead to failure. Furthermore, it is common that manufacturing defects inside the laminate can further reduce the limit for the peeling stresses.
To investigate the interlaminar failure, specimens from a 34 m blade were cut out and tested in three- and four-point bending tests conducted at Imperial College London2. The two bending tests showed that deflections at 4 mm and 8 mm develop interlaminar crack growth.
Interlaminar shear failure reinforcement solution
An innovative solution to this problem involves the reinforcement of the cap, which prevents a curved (aerodynamic shape) from flattening out.
The restriction of the out-of-plane deformation of the cap is due to the corners being restrained from sideward deformations, thus increasing the flexural stiffness of the cap. The cap reinforcement can be made of any material, including wires, fibres and dry fibre mats, and the reinforcement needs only to be able to carry tension.
A proof of concept test (Figure 5) has been performed for the cap reinforcement. Two full-scale tests were performed on a load-carrying box girder in order to validate the cap reinforcements. The first test was conducted without wires, with the second test, which is described here, performed with wires.
Figure 5: Full-scale test of a box girder with wire reinforcement as a part of a ‘proof of concept’ for the cap reinforcement invention.
Based on a FE-study, two areas were found to be critical to buckling, so cap reinforcement wires were placed in these two regions. One critical region was the 5 m section, where three wires were placed to prevent buckling. The other critical region was in the 1012 m section.
FE-studies, which were performed before the full-scale test, showed that the reinforcement should also have an effect in regions with small transverse curvatures.
In the two areas where wires were placed, the out-of-plane deformation of the cap is reduced by 1030 per cent. In the 6.58.5 m region, where there were no wires, no significant differences were observed. In the 5.26.5 m region there is a difference, even though there are no wires. This is due to the wires in the 55.2 m region and the large reinforcement effect caused by the large transverse cap curvature. As expected, it is mainly the buckling waves, which bend inwards, that have been reduced.
Sections at 10 m and 11.5 m have reduced their deformations by around 30 per cent, while sections at 10.8 m and 11.8 m only have reductions around 10 per cent.
Not only have the out-of-plane deformations changed due to the cap reinforcement, the measured strain has also been reduced (Figure 6). A difference in the linear strain response was observed, while a non-linear behaviour in the specimen without wires was observed after 70 per cent loading.
The buckling of the cap caused the non-linearity. The test with cap reinforcements did not show any tendency to buckling, even though the box girder was loaded up to 95 per cent.
NOT JUST LARGER, BUT LIGHTER
Future wind turbine blades are expected be much lighter. In principal the blades’ weight could be reduced by a factor of four or five. In practice, this is not expected to be possible, but a realistic weight reduction of 20-50 per cent is.
Some of the difficulties that could arise include that the blade would be too flexible in the flapwise direction due to the problem of tip clearance between the tower and blade tip. In order to increase the distance between the blade tip and the tower during operation, the rotor can be coned, blades can be pre-bended or the rotor plane can be tilted.
A tilt angle of about 5° between the rotor axis and the horizontal plane is common, but a bigger angle could also be considered.
In the past, on some wind turbines the rotor plane was placed in the wake behind the tower, it was called a ‘back runner’. If the blades not are allowed to be as flexible as the strength optimization allows, the ‘remaining’ strength capacity will result in extra reliability, which is also an important issue for the wind turbine manufacturers.
The failure mechanisms discussed above are expected to become even more important in future blades designs, e.g. if the cap laminates get thinner then the tip deflection and the longitudinal curvature increase. The increase in the longitudinal curvature results in a substantially larger crushing pressure, which makes the failure modes related to the Brazier load (interlaminar failure in cap and failure in the webs) more critical.
The extent of reinforcements along the blade must depend on more FE-studies with combined loading on the actual design.
The choice of reinforcement depends on many parameters such as the manufacturing process, design, materials and the size of the blade. Further, a comprehensive experimental test programme is needed because introducing reinforcements can cause other non-expected problems.
1. Jensen, F. M., Falzon, B.G., Ankersen, J., Stang, H. “Structural testing and numerical simulation of a 34 m composite wind turbine blade”. Composite Structures 76. 52-61 (August 2005).
2. Puri, A., ”Researching methods for monitoring strain in wind turbine blades as part of an operation and maintenance programme”, PhD-Thesis (in progress) Imperial College London Department of Mechanical Engineering.
Dr Mølholt Jensen is head of structural design group (email@example.com), Adam Hillestrøm is senior business developer (firstname.lastname@example.org) and Peter Hjuler Jensen is head of programme, Wind Energy (email@example.com).