5 Werkstofftechnik
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It is widely accepted that the magnetic state of a ferromagnetic material may be irreversibly altered by mechanical loading due to magnetoelastic effects. A novel standardized nondestructive testing (NDT) technique uses weak magnetic stray fields, which are assumed to arise from inhomogeneous deformation, for structural health monitoring (i.e., for detection and assessment of damage). However, the mechanical and microstructural complexity of damage has hitherto only been insufficiently considered. The aim of this study is to discuss the phenomenon of inhomogeneous “self-magnetization” of a polycrystalline ferromagnetic material under inhomogeneous deformation experimentally and with stronger material-mechanical focus. To this end, notched specimens were elastically and plastically deformed. Surface magnetic states were measured by a three-axis giant magnetoresistant (GMR) sensor and were compared with strain field (digital image correlation) and optical topography measurements. It is demonstrated that the stray fields do not solely form due to magnetoelastic effects. Instead, inhomogeneous plastic deformation causes topography, which is one of the main origins for the magnetic stray field formation. Additionally, if not considered, topography may falsify the magnetic signals due to variable lift-off values. The correlation of magnetic vector components with mechanical tensors, particularly for multiaxial stress/strain states and inhomogeneous elastic-plastic deformations remains an issue.
Abstract While classically used to visualise the magnetic microstructure of functional materials (e.g., for magnetic applications), in this study, the Bitter technique was applied for the first time to visualise macroscopic deformation gradients in a polycrystalline low-carbon steel. Spherical indentation was chosen to produce a multiaxial elastic–plastic deformation state. After removing the residual imprint, the Bitter technique was applied, and macroscopic contrast differences were captured in optical microscopy. To verify this novel characterisation technique, characteristic “hemispherical” deformation zones evolving during indentation were identified using an analytical model from the field of contact mechanics. In addition, near-surface residual stresses were determined experimentally using synchrotron radiation diffraction. It is established that the magnetic domain distribution contrast provides deformation-related information: regions of different domain wall densities correspond to different “hemispherical” deformation zones (i.e., to hydrostatic core, plastic zone and elastic zone, respectively). Moreover, the transitions between these three zones correlate with characteristic features of the residual stress profiles (sign changes in the radial and local extrema in the hoop stress). These results indicate the potential of magnetic domain distribution imaging: visualising macroscopic deformation gradients in fine-grained ferromagnetic material with a significantly improved spatial resolution as compared to integral, mean value-based measurement methods.
Virtual-lab-based determination of a macroscopic yield function for additively manufactured parts
(2019)
This work aims for a yield function description of additively manufactured parts of S316L steel at the continuum-mechanical macro-scale by means of so-called virtual experiments using a crystal plasticity (CP) model at meso-scale. Additively manufactured parts require the consideration of the specific process-related microstructure, which prevents this material to be macroscopically treated as isotropic, because of crystallographic as well as topological textures. From virtual experiments, yield loci under various loading conditions are simulated. The scale bridging from meso- to macro-scale is realised by the identification of the simulated yield loci as a modified anisotropic Barlat-type yield model representation.
Charakterisierung von Hochtemperaturwerkstoffen durch Zug- und Ermüdungsversuche an Kleinproben
(2019)
Die Verwendung von miniaturisierten Probengeometrien in der mechanischen Prüfung ermöglicht die Entnahme des Probenmaterials direkt aus kritischen Bereichen wie Fügeverbindungen und ermöglicht die Prüfung von kleinen Bauteilen wie additiv gefertigten Strukturen. In der vorliegenden Arbeit werden exemplarisch die Ergebnisse von vergleichenden Zug- und niederzyklischen Schwing-versuchen (LCF) an dem austenitischen Stahl AISI 316L und der Nickelbasislegierung IN718 vorgestellt. Die Prüfergebnisse der Kleinproben aus AISI 316L weisen die charakteristischen Eigenschaften des Werkstoffs auf, und die Analyse der Zug- und Ermüdungsdaten führt zu Werten, die den Literaturdaten weitgehend entsprechen. Der direkte Vergleich mit Standardprobendaten zeigt jedoch systematische Abweichungen bei Zugfestigkeit, Dehngrenze und Gleichmaßdehnung, die in diesem Beitrag diskutiert werden.
Charakterisierung von Hochtemperaturwerkstoffen durch Zug- und Ermüdungsversuche an Kleinproben
(2019)
Die Verwendung von miniaturisierten Probengeometrien in der mechanischen Prüfung ermöglicht die Entnahme des Probenmaterials direkt aus kritischen Bereichen wie Fügeverbindungen und ermöglicht die Prüfung von kleinen Bauteilen wie additiv gefertigten Strukturen. In der vorliegenden Arbeit werden exemplarisch die Ergebnisse von vergleichenden Zug- und niederzyklischen Schwingversuchen (LCF) an dem austenitischen Stahl AISI 316L und der Nickelbasislegierung IN718 vorgestellt. Die Prüfergebnisse der Kleinproben aus AISI 316L weisen die charakteristischen Eigenschaften des Werkstoffs auf, und die Analyse der Zug- und Ermüdungsdaten führt zu Werten, die den Literaturdaten weitgehend entsprechen. Der direkte Vergleich mit Standardprobendaten zeigt jedoch systematische Abweichungen bei Zugfestigkeit, Dehngrenze und Gleichmaßdehnung, die in diesem Beitrag diskutiert werden.
Virtual-lab-based determination of a macroscopic yield function for additively manufactured parts
(2019)
This work aims for an yield function description of additively manufactured (AM) parts of S316L steel at the continuum-mechanical macro-scale by means of so-called virtual experiments using a crystal plasticity (CP) model at meso-scale. Additively manufactured parts require the consideration of the specific process-related microstructure, which prevents this material to be macroscopically treated as isotropic, because of crystallographic as well as topological textures.
EBSD/CT-Scans from in-house additively manufactured specimen extract the unique microstructural topology which is converted to a representative volume element (RVE) with grain structure and crystal orientations.
Crystal plasticity model parameters on this RVE are calibrated and validated by means of mechanical testing under different texture angles. From virtual experiments on this RVE, yield loci under various loading conditions are simulated. The scale bridging from meso- to macro-scale is realised by the identification of the simulated yield loci as a modified anisotropic Barlat-type yield model representation.
Materialprüfung
(2019)
Die Materialprüfung dient der Analyse der Eigenschaften, der Qualität und der Sicherheit vonMaterialien undWerkstoffen. Die häufigsten Verfahren umfassen die Analyse der chemischen Zusammensetzung und der Mikrostruktur sowie die Ermittlung von Werkstoffkennwerten. Dazu zählt auch die Bestimmung des Materialverhaltens unter verschiedenen Beanspruchungen bis hin zu komplexen Beanspruchungen. Die Verwendung von Referenzmaterialien, Referenzorganismen und Referenzverfahren dient der Zuverlässigkeit und Richtigkeit von Messungen, Prüfungen und Analysen.
The influence of prestraining on the aging response of an Al-Cu-Li alloy is investigated by preparation of different strain states (3 %, 4 %, 6 %) of the initial aging state. The Brinell hardness of the subsequently aged samples (up to 60 h aging time) was measured and it was found that the increasing dislocation concentration in the 3 different initial states leads to faster hardness increases and slightly higher maximum hardness.
The influence of prestraining on the aging response of an Al-Cu-Li alloy is investigated by preparation of different strain states (3 %, 4 %, 6 %) of the initial aging state. The Brinell hardness of the subsequently aged samples (up to 60 h aging time) was measured and it was found that the increasing dislocation concentration in the 3 different initial states leads to faster hardness increases and slightly higher maximum hardness.