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April 10th, 2009

A step closer to automated blade inspections

Published on April 10th, 2009 | By: pwray@ceramics.org
Mobile testing unit. Credit: Fraunhofer WKI

Mobile testing unit. Credit: Fraunhofer WKI

Air inclusions (yellow) in rotor blade. Credit: Fraunhofer WKI

Air inclusions (yellow) in rotor blade. Credit: Fraunhofer WKI

German researchers soon will be demonstrating a new method to test the integrity of laminates such as those used in wind turbine blades. The group, from Fraunhofer’s Wilhelm Klauditz Institute, says their infrared method is a cost-effective way of using thermography to check for defects such as trapped air.

The concern, of course, is identifying areas that, when subjected to mechanical, chemical and lightning-induced stress, might crack and eventually fail prematurely (and catastrophically). And, there is a lot of material in these blades that can be 50-70 meters in length.

WKI claims (surprisingly, to me, since this seems a little old school) that the most prevalent method currently used to find blade defects is the “percussion method,” i.e., someone taps on the material – or in this case – the rotor blade – and uses the sound and vibration to “feel” the defect. At best, that has got to be very time consuming and logistically difficult, given what must be entailed in lifting an inspector high enough to examine the blades. And, costly – especially if whoever is insuring the turbine is insisting on frequent inspections.

Understandably, the push is on to find a more precise, cheaper and easier method. That’s where WKI steps in. “Infrared thermography is well suited to this task, as it is fast, relatively cheap and doesn’t cause any damage,” explains WKI project manager Dr. Hiltrud Brocke. “The surface is briefly heated with an infrared radiator. A special camera shows how the heat front spreads inside the material. If the front hits on any air inclusions or delaminated areas, it accumulates because heat spreads less in air than in solid laminate.”

“Because the equipment – the infrared radiator, a camera and a computer – is mobile, we can carry out measurements during production, at the end of the transport route, and also on fully assembled wind energy plants,” says Brocke.

Brocke says the equipment can provide images from several centimeters deep into the material

WKI’s system is a semi-automatic process that can inspect several square meters per minute. That’s is, it can inspect that fast if the blade is off the turbine. WKI is still somewhat stumped about how to use the thermography system on a working turbine. Research is going on to develop “climbing robots” that could automatically move along the rotor blades with the equipment.

Brocke and his team of researchers will be doing demonstrations at the upcoming Hannover-Messe tradeshow, April 20-24.



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