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Fraunhofer LBF engineers are heading the “unverDROSSen” project which aims to free wind turbine mainframes and rotor hubs of material defects.

Wind turbines are becoming increasingly powerful, and the demands on the components used are growing, and so is the risk of material fatigue. Material defects such as inclusions from slag, known as dross, are considered undesirable because they greatly reduce the load-bearing capacity of cast iron components with spheroidal graphite.

Spheroidal graphite is also used to make a wind turbine’s mainframe and rotor hubs. Manufacturing such components is difficult due to the build-up of dross that often occurs despite tricks in casting techniques.

“Compared to other material defects, such as cavities in the component, there is as yet no way of reliably dealing with dross,” says Dr. Christoph Bleicher from the Fraunhofer LBF in Darmstadt. Since the beginning of 2015, he has been the consortium leader of the “unverDROSSen” project, which aims to move away from the customary requirements for dross-free products and thus dispense with the time-consuming post-production work.

“To be able to do this, we have to provide manufacturers and users with a sound measurement concept so they can evaluate the degree and type of dross. That’s why, together with the Fraunhofer Institute for Nondestructive Testing IZFP in Saarbrücken, we’re developing an experimentally proven dross strength classification system,” says Bleicher.

The experts have now succeeded in developing test methods with the aid of which they can detect, display and characterize dross. They use mechanized ultrasound testing to display and measure the distribution in a three-dimensional format.

They also test processed component surfaces using magnetic and electromagnetic methods. With these methods, they scan not just the cast underside, as is customary, but also the dross-afflicted topside.

“Using our nondestructive testing technology, we measured cuboids each measuring 500 x 500 x 200 millimeters. We found that the dross distribution in the test pieces varied extremely. Sometimes the material defect extends across a very large surface area, and it can range in depth from a few millimeters to several centimeters,” reports Fabian Weber from Fraunhofer IZFP. “Our results have not allowed us to derive any kind of regularity.”

This means components will have to be examined individually in the future, too. However, with the valuable information that nondestructive IZFP test technologies provide, manufacturers can at least minimize necessary rework.

In the second step of the project, Fraunhofer IZFP data will be used to assess structural durability. For this purpose, researchers from Fraunhofer LBF extract fatigue specimens, each measuring exactly 150 x 35 millimeters, from the delivered test pieces.

A total of around 500 fatigue tests are planned during the three-year long project.

Image and content: Fraunhofer Institute for Structural Durability and System Reliability LBF

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