NDT is a general term that describes test methods that do not damage the item tested. At LM Glasfiber, we use different kinds of electronic NDT to complement visual and manual quality assurance (QA), for example during the final inspection of the adhesive joints, laminates and geometry of the blades. We have developed and patented many of these methods in-house.
All the data is stored and analysed in order to identify any areas where action may be necessary. The consistent use of NDT also guarantees documented quality for our customers.
The adhesive joints are critical points in the blade structure. That is why they are inspected with particular care. We use an IR scanner to examine the blade throughout its length, measuring exactly the same points each time. The scanner allows us to see through the laminate and check the adhesive joint. It records temperature differences in the adhesive, possibly identifying defects.
The scanner takes a series of pictures. If there are any doubts, a point can be highlighted and later analysed using electronic image processing.
If defects are found, they can almost always be repaired immediately.
The main advantage of ultrasound scanning is that it enables us to see beneath the surface and check the laminate for dry glass fibre and delamination. The scan measures the reflections of the material to the sound waves, and the output is shown as figures and curves. The same method can be used to check the thickness of the laminate, and we also use it for additional testing of adhesive joints.
The blade contour scanner automates the process of scanning the geometry of the blade profile. The tools used to produce wind turbine blades are, like the blades themselves, made of organic materials. This means that they can change over time in the context of intensive serial production. Such small changes are usually very difficult to detect, and are impossible to see with the naked eye. The geometry of the blades, in other words their precise shape, is crucially important to the energy output of the blades, so LM Glasfiber has been checking blades on a random sample basis for several years. Different points on the blade used to be measured to check that the finished blades met the specifications. This method was precise, but very slow. The blade contour scanner can now be used to carry out even more precise measurements in half the time, and all the data is collected electronically. This means we can respond more quickly and make any necessary adjustments or repairs. The collected data is also included in the documentation supplied with the blade, ensuring full traceability and transparency in our production process.
Four-point resistance measurement is a highly precise test method. It identifies faulty connections in the lightning conductor system, enabling us to carry out repairs at the right stage in the production process. Four-point testing is an ohmic resistance measurement that measures milliohms and requires four measurement cables instead of the two normally used for such resistance measurements. By using four cables, the measuring device is able to auto-compensate for the resistance in the cables – which, of course, must not be allowed to affect the result. The measurement cables are typically many times thinner than a normal lightning cable, and the resistance they provide is therefore much higher than in a lightning cable.
The plug and the blade moulds are both inspected using advanced laser scanning equipment.
The tolerances of the blade mould are very close, because even the tiniest variation in the geometry of the blade can significantly affect its performance. Laser scanning can measure variations of less than 1 millimetre.
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