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On the way from descriptor to material property

The characteristic values, so-called descriptors, extracted from the micro-samples in the short-term characterization methods of the CRC are to be mapped to material properties. Heike Sonnenberg has now been able to demonstrate the first indications of correlations and has published her findings.

Conventionally, statements about the tensile strength of a material are obtained by means of a tensile test on macro samples. The results of a test are usually presented in stress-strain diagrams. In the SFB Colored States, however, spherical micro specimens are used instead of conventional tensile specimens.

"And this poses the first challenge, because in order to infer the stress, a reference surface is required. Stress is the force per surface," explains Heike Sonnenberg. And the cross-sectional area of a sphere is not constant over its height. It is also difficult to measure the strain of a sphere that is only millimeters in size.

In subproject D01 Heike Sonnenberg has therefore developed a simplified test method, the micro compression test. This method provides different values than the classical tensile test. The micro compression test on spherical samples is used to determine so-called descriptors, from which conclusions can also be drawn about the strain, compression or even the stress in the material under mechanical stress.

Heike Sonnenberg wanted to investigate the relationships between these descriptors and the classical characteristic values of a tensile test in more detail using the example of the rolling bearing steel 100Cr6. Her initial question was: Do the descriptors from the compression test on spherical micro-samples show a comparable development as a function of tempering temperatures as the conventionally determined tensile strength?

She was able to use the now proven high-throughput method of the CRC to quickly obtain and investigate microspecimens in different, specially adjusted states (colorations). The rapid heat treatment and descriptor-based mechanical characterization within the CRC led to a very high time saving compared to conventional materials testing. Within a very short period of time, she was able to perform comparative mapping for the results of her experimental study and found that similar sensitivities could be identified for the stress descriptor from micropressure testing as described in the literature for the material property tensile strength for the selected section of heat treatment. The trend therefore points in the same direction.

"Of course, more in-depth research is needed here," says Heike Sonnenberg, "but we are pleased to see that we have made an important step forward with the transfer of the descriptors determined to the macroscopic material properties.

She has now published the results of her work in the Open Access journal Materials

Mapping der Forschungsergebnisse