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    Anisotropic thermal and electrical properties of CFRP
    (2026)
    Karsten, Julian
    ;
    Fiedler, Bodo
    Carbon fiber-reinforced polymers (CFRPs) exhibit strongly anisotropic in-plane thermal and electrical properties, yet efficient identification of laminate-scale transport parameters remains challenging, since the measured response reflects both material architecture and experimental boundary conditions. This thesis establishes non-destructive, model-based methods for single-specimen identification of orthotropic in-plane thermal and electrical transport properties in CFRP laminates and combines them with a non-destructive characterization of laminate lay-up. A pulse-echo ultrasonic workflow evaluates C-scan slices in the Fourier domain and reconstructs the depth-dependent ply-orientation profile. Reliable orientation detection is demonstrated for unidirectional, quasi-isotropic, and Bouligand laminates across varying thicknesses and ply counts. Near-surface regions, porosity, and pronounced geometric features define the methodological boundaries within which the method can be applied and interpreted reliably. Anisotropic thermal transport is quantified by heat-spot infrared thermography combined with an analytical orthotropic diffusion model. Under prescribed heat-flux boundary conditions, the directional response follows the inverse resistivity to diffusion, whose principal-axis values correspond to the longitudinal and transverse thermal diffusivities. This relation enables identification of the directional diffusivities from a single unidirectional specimen through Gaussian-profile broadening and isotherm-evolution analyzes. In addition, a rule-of-mixture approach relates laminate behavior to constituent diffusivities and fiber volume content. Anisotropic electrical transport is characterized using regular n-gonal prismatic specimens, where perimeter-based resistance measurements under full-face electrodes are evaluated by means of effective electrode distance and effective width to identify the principal longitudinal and transverse resistivities. Additional contact- and shunting-dominated configurations clarify the conditions for reliable resistivity measurement.
    Publicationtype: Doctoral Thesis
      74  23
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    Laserschweißbarkeit von laseradditiv gefertigten Aluminiumbauteilen
    (Springer, 2024)
    Beckmann, Frank
    ;
    Emmelmann, Claus
    Publicationtype: Doctoral Thesis
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    Patientenorientierte Implantation zementfreier Hüftendoprothesen
    (Shaker Verlag, 2020)
    Bätz, Johanna
    ;
    Morlock, Michael
    Publicationtype: Doctoral Thesis
      93
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      58
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    Near-infrared laser surface treatment of milled CFRP aircraft structures for bonded repair applications
    (2020)
    Harder, Sergej
    ;
    Fiedler, Bodo
    Carbon fibre reinforced polymers (CFRP) are widely used as a material for aircraft structures. During their lifetime, aircraft structures can be subject to damages that have to be repaired to maintain aircraft safety. For a lightweight and material-efficient repair, the scarf bonded joint is the preferred solution. The scarf bonded joint is produced by removing the damaged material by milling and the application of a repair patch through adhesive bonding. A critical step during the adhesive bonding process is surface treatment since it significantly influences the bond strength. This work investigates the effects of near-infrared (N-IR) surface treatment on the adhesive properties of milled CFRP. The laser treatment is able to remove disruptive milling debris from the surface. As a result, a significant increase in bond strength is achieved. Furthermore, the laser treatment is able to strip the fibres. The fibre stripping improves the bond strength compared to the already high standard of the reference process. The increase of the roughness and surface area is seen as the driving mechanism behind the further improvement. Opposed to the reference process, laser treatment is additionally capable of removing contaminations. The removal of contaminations is a considerable advantage of laser treatment, given the severe risk that is posed by contaminations. Though no clear evidence of a functionalisation was found, the chemical composition shows a high functionality without any contaminations and is therefore suitable for adhesive bonding. The main downside of the process is the physically induced interaction between the epoxy resin matrix and the N-IR laser and the consequential formation of cavities under matrix-rich areas. The cavities, though comparably small, can only be partially filled and could induce failure in the bond. Nevertheless, for all tested scarf ratios and lay-ups, an increase of the bond strength compared to the reference process was achieved. This shows the high potential of N-IR laser treatment for composite repair applications.
    Publicationtype: Doctoral Thesis
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