Fatigue behavior and climatic cycle testing of rotor blade components
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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.
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.
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.
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
The test bench performs a mechanical four-point bending.
Despite the large amplitude, the maximum test frequency is 1 Hz.
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