Residual stresses determine the service life and application characteristics of metallic components. They arise from the interaction of mechanical and thermal loads during the (chipping) production of the component in a way that is hardly predictable. They occur as tensile or compressive residual stresses: the former promote the formation of cracks, the latter reduce the formation of cracks, i. e. are "good" residual stresses. The question of whether the so-called edge zone, which extends to several hundred micrometers below the surface of the component, has tensile or compressive residual stresses, is therefore decisive for the quality of the finished component.
At the Institute of Production Engineering and Machine Tools (IFW) at the PZH, Bernd Breidenstein, a natural scientist and X-ray expert with a doctorate, is working on the subject of residual stresses. He is the head of the Institute''s analytical department and has introduced a procedure at the IFW that enables residual stresses to be measured in previously unattainable peripheral zone depths in a non-destructive, efficient and "normal" laboratory environment.
Breidenstein sketches the previous limitations using a common example: "In steel components, residual stresses can only be measured to a depth of about 5.5 micrometers using the usual X-ray method - far too little to be able to make a statement about the peripheral zone and thus the properties of the component". In order to obtain information from the depth of the edge zone, layer by layer material is removed - since mechanical and thermal processes would distort the result, electrolytic polishing is chosen. This takes time: you can schedule a working day for measurements in eight depth steps.
The solution to this problem is possible thanks to the further development of available detectors and the resulting change in the X-ray method: instead of using the usual monochrome, characteristic X-ray radiation, Breidenstein uses the entire spectrum of Bremsstrahlung up to 50 kV with a tailor-made X-ray unit. A special detector cooled to minus 20 degrees Celsius also reduces background noise so massively that even the very small signals generated according to the Bragg equation are recorded and evaluated when this radiation is diffracted at the lattice planes characteristic of the material.
With the input of known correlations and material constants, such a single measurement over a large wavelength range finally delivers the distance "d" - the lattice plane distance of the crystalline component - in different depths of the edge zone. This provides information as to whether this distance d is smaller (residual compressive stress) or greater (residual tensile stress) than the characteristic value of the - unstressed - material.
"With this process, we are entering completely new areas of the peripheral zone - the limitation is around 600 microns for aluminum and 50 microns for steel", reports Breidenstein. "We achieve even greater depths by combining this new process with the electrolytic processes, but at which the component is then destroyed." Breidenstein and other IFW scientists are currently working on the further development and validation of the new analytical methods within the scope of the Collaborative Research Centre "Tailored Forming". Throughout Germany, there are only two other laboratories that deal with questions of residual stress analysis in a similar way - the exchange is correspondingly close.
Companies that see their own research needs in terms of internal stress are welcome with their questions and topics. In addition to basic research, the Hannover Centre for Production Technology also focuses on practical exchange with users from industry.
Note to the editor:
For further information, please contact Dr. rer. nat. habil. Bernd Breidenstein, Head of Analytics at the Institute for Production Engineering and Machine Tools, on telephone +49 511 762 5206 or by e-mail at breidenstein@ifw.uni-hannover.de.
