Close up - Vestas shrugs off V112 blade furore

DENMARK: Vestas' plans for its latest turbine the V112 3MW took a knock last month when it was revealed the prototype had suffered a blade failure. Windpower Monthly technical writer Eize de Vries investigates the reasons behind the problem and Vestas' response.

Vestas has confirmed its confidence in its new V112-3MW turbine, following the negative publicity that surrounded the failure of a rotor blade in a prototype in September. Vestas has already received a handful of orders for this new turbine, including a large contract to supply 140 units for a development in Australia.

The company has established that human error was to blame for the failure. Incorrect positioning of one of the tools caused a wrinkle to form during the curing process. This was not identified due to inadequate inspection and led to a seven-metre section of a rotor blade to come off, according to Vestas's technology R&D president, Finn Strøm Madsen.

The blade design has passed all certification tests, including bending cycles in the two main directions, Madsen points out. A similar failure could not have occurred during series manufacture, when a machine positions and applies carbon fibre-embedded slabs onto the spar surface.

This does not require removable plastic wrapping like the manual positioning process does, according to Madsen. "The prototype blades were not exposed to a similar testing procedure because they were manually produced; they do not represent our industrial series product," he adds.

The V112-3.0MW flagship prototype is Vestas's most important product launch since the lightweight V90-3.0MW turbine was introduced in 2003. On September 8, the V112 prototype suffered a major failure in one of its three 54.6 metre-long rotor blades. This sparked rumours that a design error may be to blame. But those fears have now proven to be unfounded, says Madsen.

Back to the future

The V112-3.0MW marks a return to traditional drive technology for Vestas. While its predecessor, the V90-3.0MW, had introduced a compact integrated drive system, with the new V112 Vestas returns to conventional wind technology, but offering a 55% larger rotor-swept area. Its technical features include a non-integrated drive system, a permanent magnet generator and a passive liquid-cooling system.

Unlike most competitor products, Vestas's rotor blades have a unique design consisting of a central spar and an upper and lower blade shell. As a main structural element, the spar expands from the blade foot towards the blade tip but ends several metres before the tip, at a narrow cross-section where the airfoil is very thin.

The spar's initial shape, at the point where it is incorporated with the blade foot, is circular. It then turns into a square and finally becomes a rectangular tapering cross-section with convex sides.

The two widest spar sides are stiffened by carbon fibre and match the inner curvature of the upper and lower blades. Through a final lamination process, the three components together form a structural strong blade. "Carbon fibres substantially contribute to blade stiffness, which in turn reduces blade deflection under load," says Madsen.

Blade deflection, or bending, is an issue especially for long blades. Suppliers have to ensure that the blade tips do not hit the tower during unfavourable circumstances such as gusty wind conditions.

Vestas designs the blades so that the carbon fibres are embedded into prepreg epoxy slabs, which are attached at predefined positions to the spar body. During the spar curing process, the prepreg melts and the carbon fibres become fused into a permanent laminate. During processing, carbon fibres are rather fragile. They can only transfer loads when fully stretched into the laminate.

The root of the problem

The blade broke about seven meters from the tip. Madsen says: "On the same day, Vestas experts conducted a root-cause analysis. They found that the blade had broken exactly at a spar cross-section. And a carbon fibre wrinkle was clearly detectable in the laminate."

According to Madsen, the failure surface showed a "clean cut", which means that the cross-section had a smooth surface. This points to a material fatigue-related failure; an impact-related failure would display an irregular surface.

Further analysis revealed that human error had caused the failure. "We were pleased to have found the root cause so quickly," says Madsen. But the company decided to hire experts from Scandinavian classification society DNV to conduct a second independent analysis. They reached the same conclusions as Vestas. "More supportive evidence was provided by the fact that all 113 strain gauges subdivided all over the prototype nacelle, tower and blades did not register any excess loads," Madsen adds.

A Finite Elements Analysis simulating what would happen in the future was also carried out, intentionally introducing the specific wrinkle imperfection that had been detected into the blade design parameters. This analysis confirmed that the blade would have failed after five or six months in operation – just as happened to the prototype.

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