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Lightweight Design worldwide

, Volume 11, Issue 3, pp 52–57 | Cite as

Fatigue behavior and climatic cycle testing of rotor blade components

  • Ivo Drisga
  • Nikolai Glück
  • Tim Berend Block
  • Kai Ehrich
Production Component Testing
  • 75 Downloads

The Fraunhofer Research Institution for Large Structures in Production Engineering and the company Nordex have developed a method for combined fatigue behavior and climatic change testing of full-scale rotor blade components. The related test bench allows the testing of bonded or integrated components under realistic conditions.

Cyclic Load

The wind energy economy has a share of 30 % of the net electric consumption in Germany. Hence, it has long been considered one of the main instruments for the German energy transition. Due to high price pressure, the trend is towards bigger, intelligent and more efficient wind turbines. A main part of the plant is the rotor blade. With sensors and actuators, integrated in the blades, the wind turbines are able to adapt to changing environmental conditions. And with the aid of system monitoring, downtimes are minimized. Furthermore, aerodynamic components, such as vortex generators and serrations, which are applied to the rotor blade surface, help to increase the efficiency of the plant. Additionally, these elements reduce the noise emission, which leads to a higher public acceptance of wind turbines.

Modern wind turbines are designed for a lifetime of up to 30 years. During one rotation, the rotor blades are loaded by a reverse bending, caused by their own weight. Due to this, the extreme fibers are alternately stretched or compressed, together with their applied or integrated components, Figure 1. Integrated components, such as lightning rods, heaters and sensors, may fail because of the mechanical load. In the case of glued components, the adhesive layer is loaded mechanically, as well as exposed to climatic influences. If the adhesive layer cannot withstand this combined load, it may result in downtimes and expensive repairs.
Figure 1

Loading of attachments on rotor blades of wind turbines (adapted from Sika) (© Fraunhofer IGP)

Common methods for testing engineering strength use small test coupons. Especially when the coupon is additionally climate-conditioned, to simulate a combined load, the dimensions of the test samples are usually limited. The test results are then used to extrapolate the structural behavior of the adhesive from the material characteristic values. Indeed, the load distribution in the coupon often differs a lot from the real load distribution in the component. That is why there is a considerable risk in scaling up the test results. Full-scale blade testing on the other hand provides reliable test results for the components. Nevertheless, this test method is expensive and only viable in the prototype stage. Moreover, a dynamic climatic change test in the more than 100-m-long test bench is out of the question.

To counter this lack of convenient test methods for large attachment parts, the Fraunhofer IGP and the company Nordex cooperated in a joint project. The project partners developed a test method, together with a corresponding test bench for the combined fatigue behavior and climatic change testing of large components. The test method serves to test the engineering strength of full-scale glued mounting parts or integrated components under realistic operating conditions.

Principle of the Test Method

The aim of the new test method is to examine the engineering strength of the adhesive layer of mounting parts and integrated components in original size. For this purpose, the specimens are alternately stretched and compressed, like on the real rotor blade. This is not possible with common coupons, due to stability reasons. Further, the specimens can be climate-conditioned simultaneously, in order to simulate the aging process as a result of temperature and humidity fluctuations. The central object of the method is a specimen beam.

There is a considerable risk in scaling up the test results of coupon tests.

The beam serves as a socket for the components to be tested. The required strain and compression in the extreme fibers is generated through a periodic four-point bending of the beam. In the four-point bending principle, a beam is placed on two outer supporting pins at a set distance from each other. Two inner loading pins, placed in an equal distance around the center, load the beam in one direction. This creates an area between the loading pins with constant strain and compression respectively. This is an approximation of the real conditions at the rotor blade. To generate a real alternating load, the principle of the four-point bending is extended. In the developed test method, the beam is deformed in positive and negative z-direction relative to its rest position. For this purpose, the test bench uses double-acting supporting and loading pins. The movement of the loading pins causes a forced displacement of the beam of the test distance s. Due to this forced bending, the maximum fiber strain between the loading pins is time-variable, but locally constant. Figure 2 illustrates this principle. Arbitrary mounting parts can be applied at the 1400 mm long test area of the beam. For the test, the extreme fiber strain of the beam is measured and recorded directly with strain gauges. The used test beam has the dimensions 2400 mm × 100 mm × 100 mm (L × W × H). The strain of the extreme fibers, caused by the bending, depends on the curvature and the thickness H of the beam. Given the fact that the curvature of rotor blades is negligibly small, the curvature should be minimized. Unfortunately, a thick beam leads to high test forces. Therefore, the dimensions of the specimen beam are a compromise between curvature and required test force. To create high strain with simultaneously great stability and low test forces, the beam is executed in a sandwich design. A top layer, composed of an epoxy-biax/UD fabric, surrounds a lightweight balsa core. For the dynamic climate conditioning of the test specimens, the whole test bench operates in the climatic chamber of the Fraunhofer IGP. During the testing, the test bench is exposed to a dynamic temperature cycle. The chamber is 60 m3 in size and supports temperatures from −50 up to +60 °C with a minimal rate of change of 1 °C/min. In addition to the temperature, the chamber also regulates humidity.
Figure 2

Principle of generating alternating extreme fiber strain by four-point bending (© Fraunhofer IGP)

Advantages of the Method

The introduced new test method has a number of advantages over common coupon testing. Designed as a component test, the method can reproduce the specific assembly situation and typical manufacturing effects. Moreover, it is possible to test the components already in the stage of development, to minimize the risk of failure in the later full-scale blade tests or at the prototype. By using the double-acting four-point bending principle, the specimens are loaded with alternating strain (stress ratio R = −1). Common test methods are usually based on the tensile test of coupons with repeated loading condition (R = 0,1). If the described combined test should be performed at a universal test machine with common methods, the specimens need to be pressure- loaded. To avoid buckling, it would be necessary to use short and thick specimens. However, this contradicts the aim to test large components in original size. Furthermore, test forces will increase due to the larger dimensions of the specimens, which is why pressure- loading of the specimens is often omitted. By contrast, there is no risk of buckling when using the four-point bending principle. As a result, great strain and compression can be generated on a large area of the test beam. This allows for testing components like impacts and transitions of add-on parts in original size. While the introduction of force often causes problems during the tensile-compression test, the forces are introduced more gently through beam bending.

Test Bench Design

For the new test method, Fraunhofer IGP developed a test bench with a number of special properties. The test bench performs a mechanical four-point bending. Furthermore, it is constructed to resist the climatic conditions of the driven climate cycles. So, if required, it can be run completely in the climatic chamber. To ensure that the strain values of the real object come as close as possible to the values from the theoretical analysis, a correct bearing of the beam is decisive. In the theoretical analysis, the beam is supported only in z-direction at the supporting and loading pins, Figure 2. All other degrees of freedom remain unrestricted. This includes the rotational degree of freedom at the pins and the translational degree of freedom in x-direction. To avoid inner tensions, this way of bearing must be realized at the test bench. For this purpose, a floating bearing of the beam in the x-direction was implemented at the test bench. The beam is clamped between two rollers. This clamping from two sides allows bending in two directions. The ball bearings of the rollers permit small displacements in x-direction. This is necessary because the beam is deformed a little in the x-direction during the bending. To prevent the beam from accidentally slipping out of the clamping, there are locking plates attached to one side of the beam. The rollers, in turn, are mounted in pivoted forks. As a result, the rotational degree of freedom at the clamp remains unrestricted. Figure 3 shows the complete installation of the beam. For bending the beam, the forks are attached to a guided crossbeam, and execute a linear path-controlled up and down movement around their rest position, bending the beam in both directions in the form of the four-point bend. The amplitude of the movement is the test distance s. The distance of the loading pins can be varied between 600 and 2400 mm. The distance between the supporting pins is fixed at 2400 mm. The test distance is measured with position sensors, whereas the number of load changes is counted by an inductive sensor.
Bearing of the specimen beam (© Fraunhofer IGP)

Bearing of the specimen beam (© Fraunhofer IGP)

The test bench performs a mechanical four-point bending.

Since the test bench is exposed to large temperature fluctuations, a robust mechanical design was chosen for the drive system. Two double eccentrics are driven synchronously via a central drive shaft. The double eccentrics allow a continuous adjustment of the test distance between 10 and 40 mm. The eccentrics drive the crossbeam with the attached loading pins. The engine and all drive components are designed for temperatures of at least −30 to +50 °C. Despite the large amplitude, the maximum test frequency is 1 Hz. The absence of hydraulic or pneumatic components results in a high degree of efficiency, and the test bench can be operated comparatively economical, with an average power consumption of approximately 1 kW. Due to the consistent use of electrical components, the test bench is suitable for operation in a laboratory environment, Figure 4.
Figure 4

Test bench with clamped test beam (red: exemplary degrees of freedom and test displacement) (© Fraunhofer IGP)

Test Procedure

In a first test program for wind turbine manufacturer Nordex, different adhesives were examined for their suitability for the application of aerodynamic elements (serrations, vortex generators) to rotor blades. Due to the length of the mounted serrations, Figure 5, the deformations of the underlying surface due to the expansion and compression of the extreme fiber add up to such an extent that, in addition to the longitudinal elongation, significant shear deformation occurs in the adhesive layer. This can lead to failure of the adhesive layer under cyclic stress. This effect is intensified both by the increasing adhesive stiffness at low temperatures and the higher shear stresses or lower elongation/compression at break with the same deformation. Climate-conditioning additionally allows the simulation of aging processes as well as the influence of ambient conditions on the mechanical performance. The size of the beam allows a total of eight serrations to be examined simultaneously in one test run. In addition to different adhesives, production-related influences such as surface treatment and sealing are also investigated. Based on climate change tests from the automotive industry, rapid temperature changes were combined with holding levels. Figure 6 shows two exemplary climate cycles, each with a duration of 24 h, which are repeated periodically over the course of the test.
Figure 5

Test beam with attached specimens (© Fraunhofer IGP)

Figure 6

Example for temperature cycles for the combined fatigue behavior and climatic change testing (© Fraunhofer IGP)

Despite the large amplitude, the maximum test frequency is 1 Hz.

Outlook

The example of a test of adhesive bonding presented here shows that the developed test method is suitable for testing bonded attachments in real size with realistic results. The tests showed the limits of the permissible extreme fiber strain of the rotor blade in combination with external climatic influences on the tested adhesive layer. This is not possible with conventional coupon testing and, in this case, requires empirically determined scaling factors. As a result, adhesive bonds for attachments of rotor blades can be qualified in terms of their engineering strength before use. This allows risk to be minimized at an early stage of development. Time loss and high costs resulting from subsequent repair work and design changes can be reduced. With the large specimen dimensions and the large extreme fiber strain achieved, the test method can also be applied to other attached and integrated components of fiber composite structures, such as those used in the maritime sector or in vehicles. These include, for example, integrated sensors, aerodynamic auxiliary parts, heating elements or metallic inserts.

Test Bench Data

  • ▸ Test length: 1400 mm

  • ▸ Material: GRP, CFRP sandwich (balsa, foam)

  • ▸ Extreme fiber strain: maximum ± 1 %

  • ▸ Climate: −30 to +50 °C, 0 – 85 % rel. humidity

  • ▸ Test frequency: maximum 1 Hz

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Ivo Drisga
    • 1
  • Nikolai Glück
    • 1
  • Tim Berend Block
    • 2
  • Kai Ehrich
    • 3
  1. 1.Fraunhofer Research Institution for Large Structures in Production Engineering (IGP) RostockGermany
  2. 2.Nordex Energy GmbHHamburgGermany
  3. 3.Nordex Energy GmbHRostockGermany

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