LM Glasfiber’s main reason for deciding to start our own testing programme in 1999 was to avoid the long waiting times for external test beds. Keeping all the test facilities in-house gives us much more flexibility and means our customers receive their blades sooner. The standard of documentation is also improved, and the data obtained is more effectively targeted for use in other development work.
We currently have five test beds for dynamic testing and one for static testing. The largest of these has recently been extended, with room for blades up to 80 metres long. Upgrading our facilities involved installing one of the strongest cast concrete structures in the world – the anchor block. This is because the bending moment of the blade root more than doubles in relation to the length of the blade.
Our technical testing laboratory has been accredited for the testing of wind turbine blades by DANAK – the Danish Accreditation and Metrology Fund. This means that our measurements and reports can be used as the basis for type approval by authorities such as Det Norske Veritas (DNV) in Norway and Germanischer Lloyd in Germany.
As part of the accreditation process, LM Glasfiber’s technical expertise and processes were studied using the ISO 9001 and DS/EN ISO/IEC 17025 standards concerning general requirements for testing and calibration laboratories. In addition, DANAK carries out annual audits in our laboratory.
Before obtaining type approval, our blades must undergo static testing, in which they are subjected to extreme loads. A traction rig is attached to the blade by steel wires fixed to carefully selected points. Over a period of 30 to 60 minutes, the blade is then drawn out to the point at which it is subjected to the prescribed maximum load. It must stay in this position for at least 10 seconds without breaking.
The tensile test is repeated twice flapwise and then the extreme load is tested in all directions: leading edge, trailing edge, suction side and pressure side.
It takes about a week to complete the static test programme, and the whole programme has to then be repeated on the same blade after the dynamic test. This is done to ensure that the blade can handle extreme loads after being exposed to high fatigue loads.
Dynamic testing takes considerably longer. The aim is to subject the blade to fatigue loads corresponding to 20 years of normal wear and tear. This is done by setting the blade into oscillation corresponding to its natural frequency. Five million oscillations edgewise are followed by five million oscillations flapwise.
The larger the blade, the lower the frequency. So the test takes around 3.5 months for a 37.3 metre blade, whereas a 61.5 metre blade is left oscillating for a whole year. During the test, an infrared camera is used to check for small breaks or cracks in the laminate, while measurements are taken from the many strain gauges fitted to the surface of the blade.
Strain gauges are used to collect data during static and dynamic testing. In static testing, the data collected primarily relates to force, strain and deflection. In dynamic testing, the strain data is of particular interest. About 2 GB of data are collected for each blade. This data can be used for LM Blades calculations (including the structure) and also for documenting and developing the blade.
We constantly publish articles about design, materials and processes, as well as about the Group’s current business activities. These are often prepared in conjunction with conferences, etc.
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Our aim is to boost the efficiency of wind turbine blades – and also reduce their price. This requires perfect interaction between the blades and all the other wind turbine components.
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