At first glance, the rotor blade appears to be flawless, but the expert knows that outward appearances cannot be trusted. He taps the surface and listens. A full, deep sound tells him that the laminate is homogeneous, while a more flat, hollow tone indicates irregularities in the material. Delaminated and hollow sections of a certain size near the surface can also be detected by running an expert hand over the surface material. But even an experienced inspector cannot find all hidden faults in this way.
Rotor blades consist mainly of glass fibres which are processed to form mats or meshes. In order to make a 60-metre rotor blade, hundreds of these mats have to be laid flat inside a mould and impregnated with special resins in a vacuum. Even minor irregularities can cause air bubbles or other faults to form, and these often lead to mechanical stresses in the material when the blade is subjected to everyday loads. As a consequence, the laminate can rip and cause the rotor blades to fail prematurely.
Researchers at the Fraunhofer Wilhelm-Klauditz-Institut WKI in Braunschweig are able to make such faults visible. '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.' In this way, the researchers can peer several centimetres into the material. '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. The researchers will be demonstrating their technology on a rotor blade section incorporating several typical faults at the Hannover-Messe from 20 to 24 April (Hall 27, Stand G20).