• 7/27/2008

Debate between theorists and engineers resolved:

Physicists at the TU München prove existence of internal stresses in large compressors

Compressor impellers in turbines have to withstand a lot. They not only increase the pressure of vast amounts of gases and liquids, but are themselves subject to extreme centrifugal forces. If a compressor impeller fails due to material fatigue, it can destroy an entire turbine. Subsequently, the ability to calculate the load limits of components is of extreme importance to manufacturers. The results of these calculations, however, also need to be tested in the real world. Physicists at the TU München’s FRM II neutron source have now developed an appropriate test procedure that does just this. The STRESS-SPEC instrument at FRM II enables scientists to detect stresses hidden deep within large components.

FRM II in Garching. (Foto: Wenzel Schuermann / TUM)
FRM II in Garching. (Foto: Wenzel Schuermann / TUM)

In the search for economically viable and energy-efficient substances, designs and processes, science and technology researchers are always coming up with new materials and components offering composition and performance enhancements. But modern science is also pushing the performance limits of existing materials. Direct, non-destructive analysis of internal structures created during the production and processing of materials plays a central role here. “Many methods deployed in materials research only scratch the surface,” explains Winfried Petry, scientific director of the TU München’s FRM II neutron source. “Neutron diffraction allows us to probe deep inside materials. And the results we are getting are of immense value to industry.”

The investigations into compressors were triggered by a debate between theorists and engineers. A turbine manufacturer commissioned scientists to create a mathematical model of the manufacturing process for large compressor impellers milled from heavy metal plates weighing almost 300 kg. The resulting simulation revealed that the manufacturing process generated considerable mechanical stresses in the block’s interior. These kinds of internal stresses can lead to material fatigue and cracks – in other words, premature component failure. The engineers, however, did not believe that these stresses were really present. Unfortunately, there did not seem to be any method capable of settling the argument – until, that is, the company’s came across STRESS-SPEC at FRM II.

This task proved a particular challenge for the scientists at FRM II, as an object of this size had never before been measured at the neutron source. The measurement site had to be modified to enable such a large component to be positioned exactly on the micrometer in front of the neutron beam and aligned for measuring. But once tests got underway, the team of scientists headed by Michael Hofmann, physicist at the TU München and head of the STRESS-SPEC group, was soon able to resolve the debate. The measurements clearly showed considerable mechanical stresses inside the component. The theorists were proven right. And the engineers now have the proof they need to modify the manufacturing process and ensure that no, or only minimum, stresses are generated during production. This not only extends the component’s service life, but also saves money, as expensive maintenance work can now be carried out at less frequent intervals.

Generally speaking these neutron diffraction experiments have a much wider impact: The physicists’ measurements confirm the models created by the materials scientists, thus enabling them to further hone their methods and transfer them to other parts and materials. “We are working at the cutting-edge of basic research,” confirms Michael Hofmann. “If the models in question are realistic, computer-aided materials science can cut development costs considerably. After all, large numbers of simulations can be run using computer models within a short space of time. And we can verify results using our neutron diffractometer STRESS-SPEC.”

Deploying neutrons is the only way of identifying stresses deep within solid components. Since neutrons are not electrically charged, they can pass unhindered through a wide range of materials in much the same way as light particles move through glass. Yet occasionally, a neutron collides with an atomic nucleus of the material under investigation. The neutron then loses some of its energy and is deflected from its path. This change in direction and loss of energy reveal a great deal about the obstacle and immediate collision environment. Even the smallest displacement of an atom relative to its original position is indicative of stress in the material. “When it comes to taking measurements, we are able to work with picometer accuracy, in other words 1,000th of a nanometer,” concludes Prof. Winfried Petry, scientific director of the neutron source.


Prof. Dr. Winfried Petry
Technische Universität München
Research Neutron Source Heinz Meier-Leibnitz
Lichtenbergstr. 1
D-85747 Garching
Tel.: +49 89 289 14965
Fax: +49 89 289 14995
E-Mail: winfried.petryspam prevention@frm2.tum.de
Web: www.frm2.tum.de

Dr. Michael Hofmann
Technische Universität München
Research Neutron Source Heinz Meier-Leibnitz
Instrument STRESS-SPEC
Lichtenbergstr. 1
D-85747 Garching
Tel.: +49 89 289 14744
Fax: +49 89 289 14666
E-Mail: michael.hofmannspam prevention@frm2.tum.de

Technical University of Munich

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