Local failure mechanisms in bulk of additive manufactured steel enabled by stress tensor tomography

• Applicants: Dr.-Ing. Alexander Liehr; Professor Dr. Peter Modregger
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 570312374

In recent decades, additive manufacturing (AM) of metals has emerged as a transformative technology for automotive, aerospace, or medical applications. This innovative approach allows for the layer-by-layer construction of complex geometries that are often unattainable through traditional manufacturing methods. However, additively manufactured components often fail earlier in service life tests than conventionally manufactured components. For the latter, fatigue, crack initiation, and finally failure usually start at the surface. AM components, on the other hand, are often affected by internal defects, which are more critical for failure. Due to the complex and irregular defect geometries (e.g., voids) within the internal volume of an AM component, these defects act like internal notches. Multi-axial stress fields develop around these notches, which play an essential role in the failure mechanism. So far, no non-destructive technique has successfully characterized such stress fields in a clear manner. Recently, a novel method for the non-destructive, depth-resolved determination of all six stress tensor components has been developed. Stress tensor tomography (STT) combines synchrotron radiation-based x-ray powder diffraction with tomography, features a comparatively simple setup, and provides isotropic spatial resolution on the order of 10 micrometers, along with stress sensitivities of about 20 MPa. This project brings together the complementary expertise of its applicants to answer the question: "What is the role of stress fields around internal defects in the failure mechanism of additively manufactured steel?" For this purpose, the stress fields around internal defects of AISI 316L steel samples produced by selective laser melting will be determined. Low-cycle fatigue testing until failure and subsequent scanning electron microscopy will locate the initial point of failure. The correlation of those two results will enable the identification of either the size of internal defects or their surrounding stress fields as potential causes of failure. This will then be compared to predictions of established failure models, such as Murakami.

• DFG Programme: Research Grants
• International Connection: France, Switzerland
• Cooperation Partners: Professorin Marianne Liebi; Dr. Jonathan Wright