As wind turbines grow ever larger and more powerful, the demands placed on their load-bearing cast-iron components used to make their mainframe and rotor hubs also increases. Now Fraunhofer researchers are working on a new way to detect and analyse material defects in these components.
Wind turbines’ mainframe and motor hubs are made from a special kind of cast iron called spheroidal graphite. These types of cast iron components are susceptible to defects arising from material inclusions such as dross, which greatly reduce their load bearing capacity.
To ensure the reliability of wind turbines, which must function reliably for at least 20 years, component manufacturers and foundries make sure that they liberally remove all the dross and release only dross-free products for use. Since dross is often unavoidable while casting, and these types of defects are usually found on the surface of the cast part or a few centimeters below the casting skin, the raw cast part is grinded laboriously by hand.
“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 Institute for Structural Durability and System Reliability LBF in Darmstadt.
Dr Bleicher is the consortium leader of the “unverDROSSen” project, which aims to address the time-consuming post-production work needed to deal with dross.
“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,” he explained.
“Together with the Fraunhofer Institute for Nondestructive Testing IZFP in Saarbrücken, we’re developing an experimentally proven dross strength classification system.”
The team of experts have succeeded in developing test methods that foundries can use to detect, display and characterise dross. With mechanized ultrasound, they can display and measure the distribution of dross in a 3D format.
The researchers are also able to test processed component surfaces using magnetic and electromagnetic methods. With these non-destructive methods, they scan not just the cast underside, but also the dross-afflicted topside.
The team tested their technology on cuboids and 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.
This large variation means components will have to be examined on an individual basis. However the test technologies provides the information needed by manufacturers to minimise necessary rework.
In the second step of the project, data from the tests will be used to assess structural durability. For this purpose, researchers extract fatigue specimens from the test pieces, then pull them apart and press each sample back together up to 10 million times. The three-year long project will see around 500 fatigue tests carried out.
By the end of 2017, the researchers want to find out if and to what extent dross-afflicted samples weaken when put under a load such that they might fail when at maximum load. While it is known that dross leads to crack formation, which greatly reduces the component’s cyclic load bearing capacity, such components may be completely adequate for other purposes.
“In the future, we will offer a concept for reliably handling material defects in component design, manufacture and release of large cast components made of cast iron with spheroidal graphite. This will apply not just to wind farms but to all plant and mechanical engineering sector,” Dr Bleicher said.