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Dissertation
Bewertung von Eigenspannungen nach dem Laserstrahlhärten von komplex geformten Komponenten und additiv gefertigten Bauteilen
(2021)
Die Laserstrahlbearbeitung hat sich im Laufe der letzten Jahre zu einem wichtigen Forschungsschwerpunkt in industrieller und akademischer Hinsicht entwickelt. Neben der Erschließung neuer Anwendungsgebiete durch den Vorteil einer hohen Bearbeitungsflexibilität überzeugt die laserbasierte Materialbearbeitung vor allem durch einen lokal beeinflussbaren Energieeintrag. Jedoch sind derzeit mit zunehmender geometrischer Komplexität und Bauteilgröße nur begrenzte Tiefenwirkungen der gewünschten Druckeigenspannungen ...
Buch
Optische Verformungsmessung an heterogenen Werkstoffsystemen
(kassel university press, 2020)
Im Rahmen der vorliegenden Arbeit wird die Anwendung der optischen Verformungsmessung auf heterogene Werkstoffsysteme untersucht, wobei dieses Verfahren auf unterschiedlichen Skalenebenen eingesetzt wird. Als erstes wird die digitale Bildkorrelation zur Temperaturüberwachung bei Hochtemperaturzugversuchen bis 350 °C an 0,6 mm dünnen lotplattierten Aluminiumprüfkörpern eingesetzt. Flächige Dehnungsauswertungen innerhalb des Prüfbereichs bestätigen die homogene Temperaturverteilung, wobei die Temperatur gleichzeitig ...
Dissertation
Simulationen und Experimente im Schwellenwertbereich zur Aufklärung der Risswachstumsmechanismen in einer Aluminiumlegierung
(2020)
Lange Risse, die bei Belastungen nahe des Schwellenwerts wachsen, können Ausbreitungscharakteristika zeigen, welche denen kurzer Stadium I-Risse ähneln. Neben einer lokalen Blockade der Rissfront durch Teilchen einer zweiten Phase ist ein schubspannungsgesteuerter Rissfortschritt ein interessanter Effekt, der bei diesen Belastungsbedingungen auftritt. Das berichtete Verhalten wird im Rahmen dieser Arbeit auf das Rissspitzenfeld zurückgeführt. Zu diesem Zweck wurden Risswachstumsexperimente mit angerissenen Flachproben ...
Aufsatz
On the reliability of residual stress measurements in unidirectional carbon fibre reinforced epoxy composites
(2021-03-03)
The present work introduces a novel methodology to analyse non-uniform residual stresses in composite materials. For robust determination of stresses in complex structures made of different layers, it is first necessary to prove that residual stress analysis in a single layer structure can reliably be accomplished. In present work a unidirectional carbon fibre reinforced epoxy is investigated. For residual stress determination, the integral incremental hole drilling method is used. It is revealed that the standard ...
Aufsatz
Ti6Al4V lattice structures manufactured by electron beam powder bed fusion - Microstructural and mechanical characterization based on advanced in situ techniques
(2022-12-17)
Powder bed fusion (PBF) processes enable the manufacturing of complex components in a time- and cost-efficient manner. Especially lattice structures are currently focused since they show varying mechanical properties, including different deformation and damage behaviors, which can be used to locally tailor the mechanical behavior. However, the present process-structure-property relationships are highly complex and have to be understood in detail in order to enable an implementation of PBF manufactured lattice structures ...
Aufsatz
Effect of Grain Statistics on Micromechanical Modeling: The Example of Additively Manufactured Materials Examined by Electron Backscatter Diffraction
(2020-02-21)
Micromechanical modeling is one of the prominent numerical tools for the prediction of mechanical properties and the understanding of deformation mechanisms of metals. As input parameters, it uses data obtained from microstructure characterization techniques, among which the electron backscatter diffraction (EBSD) technique allows us to understand the nature of microstructural features, that are usually described by statistics. Because of these advantages, the EBSD dataset is widely used for synthetic microstructure ...
Aufsatz
Influence of complex geometries on the properties of laser-hardened surfaces
(2020-04-25)
Laser surface hardening provides for many advantages in terms of flexible production due to very localized and controlled energy input into the material. Laser processing offers the possibility to treat surfaces in order to locally strengthen the areas that are prone to fatigue cracking. It is well known that laser energy absorption depends on many parameters, e.g., the surface structure and the surface orientation. The incident angle of the laser beam plays a key role in this regard. When complex geometries like ...
Aufsatz
Excellent superelasticity in a Co-Ni-Ga high-temperature shape memory alloy processed by directed energy deposition
(2020-04-30)
A Co-Ni-Ga high-temperature shape memory alloy has been additively manufactured by directed energy deposition. Due to the highly anisotropic microstructure, i.e. columnar grains featuring a strong near-⟨001⟩ texture in build direction, the as-built material is characterized by a very low degree of constraints and, thus, shows excellent superelasticity without conducting a post-process heat treatment. As characterized by in situ deformation testing and post-mortem microstructural analysis, additive manufacturing ...
Aufsatz
A new method for microscale cyclic crack growth characterization from notched microcantilevers and application to single crystalline tungsten and a metallic glass
(2022-06-22)
The lifetime of most metals is limited by cyclic loads, ending in fatigue failure. The progressive growth of cracks ends up in catastrophic failure. An advanced method is presented for the determination of cyclic crack growth on the microscale using a nanoindenter, which allows the characterization of > 10,000 loading cycles. It uses focused ion beam fabricated notched microcantilevers. The method has been validated by cyclic bending metallic glass and tungsten microcantilevers. The experiments reveal a stable crack ...
Aufsatz
Mechanics of colloidal supraparticles under compression
(2021-10-13)
Colloidal supraparticles are finite, spherical assemblies of many primary particles. To take advantage of their emergent functionalities, such supraparticles must retain their structural integrity. Here, we investigate their size-dependent mechanical properties via nanoindentation. We find that the deformation resistance inversely scales with the primary particle diameter, while the work of deformation is dependent on the supraparticle diameter. We adopt the Griffith theory to such particulate systems to provide a ...