# Optically Pumped Magnetometer Measuring Fatigue-Induced Damage in Steel
## 5 Conclusions and Outlook  
This paper presents the first results from a novel experimental setup combining the generation of fatigue damage in a mesoscale specimen and retrieval of its piezomagnetic response through sensitive optically pumped magnetometers. The main advantage of this setup is the monitoring of the mechanical and especially the piezomagnetic signals with high precision and low environmental perturbations throughout the whole lifetime of the specimen under test. The prerequisite to measuring small specimens is high-sensitivity quantum magnetometers. For a ferritic steel specimen with a cross-section of 0.22 mm\({}^{2}\) and a gauge length of 400 \(\upmu\)m, the resulting magnetic field amplitude under plastic deformation was in the range of 5 nT. For a robust extraction of features, such as the magnetic field of the Villari reversals, environmental magnetic noise should be on the level of 1 pT. This was achieved by using a zero-field OPM in combination with a magnetic shield and a tailored non-magnetic loading unit. The stiffness of the loading unit permits testing of high-strength steels under compressive and tensile loads. Figure 4: Potential signatures of early fatigue damage processes with an increasing number of cycles. Top: Evolution of the areas of the mechanical (\(F-\epsilon\)) and piezomagnetic (\(B-\epsilon\) ) hysteresis curves. Bottom: Evolution of the magnetic field at the Villary reversals (arrows on Figure 3) with an increasing number of cycles. The reduced specimen volume could enable a better correlation between magnetic signals and localized damage events, such as plasticity or cracking, using complementary characterization techniques. For example, full-field DIC might be used for quantitative measurement of macroscopic crack initiation and electron backscatter diffraction can reveal internal microstructural features across the whole specimen. As the grain size of certain materials is on the order of a few hundred micrometers, investigations of individual grain boundaries become possible using the miniaturized specimen. Prospectively, even sub-surface damage, such as crack initiation at non-magnetic inclusions causing the so-called fisheye in high-strength steels, might become measurable in situ [30]. We compared the mechanical and piezomagnetic hysteresis over the entire lifetime of a ferritic steel specimen. The piezomagnetic curves showed more features than the mechanical curves. Therefore, we suggested two potential damage indicators, i.e., the area of the piezomagnetic hysteresis, such as in [5], and the magnetic field of the Villari reversals. Both responded earlier than the area of the mechanical hysteresis. In particular, the Villari reversals showed a significant increase before macroscopic crack initiation. Therefore, both might be used as early damage indicators. However, these indicators must be verified by complementary characterization and modeling [6; 27; 31]. On the other hand, these models also require validation, for which the setup proposed here opens new opportunities. Thereby, a broadened mechanistic understanding could be derived. [PERSON]. ([PERSON]), [PERSON] and [PERSON] conceived the initial idea for the experimental setup. [PERSON] and [PERSON]. ([PERSON]) wrote the control software for the apparatus. [PERSON], [PERSON] and [PERSON]. ([PERSON]) analyzed the data of the experiment. [PERSON] and [PERSON] outlined and wrote the draft version of the manuscript. [PERSON]. ([PERSON]), [PERSON] and [PERSON] supervised this work. All authors revised the final version of the manuscript. All authors have read and agreed to the published version of the manuscript. This work was supported as a Fraunhofer LIGHTHOUSE PROJECT (QMag). We acknowledge financial support from the Ministry of Economic Affairs, Labor and Housing of the State of Baden-Wurttemberg, Germany. Not applicable. The data is available from the authors upon reasonable request. The authors would like to acknowledge the support from [PERSON] for the mechanical design of the apparatus.