Doctorates 2024
This page provides an overview of the successfully completed dissertations at the ILK. Interdisciplinary research and close cooperation with industry lead to groundbreaking findings. Click on a dissertation title to find out more.
Doctoral Student: Gordon Just
Supervising Professor: Prof. Dr.-Ing. habil. Maik Gude
The complex damage behavior of continuous fiber-reinforced plastics is a major obstacle to exploiting the lightweight construction potential of this group of materials. The development of material-adapted calculation models that enable a reliable prediction of composite damage is therefore an important basis for the material and component development of highly loaded fiber composite structures.
The damage models currently available are often only applicable to certain setups of fiber composite laminates, only take into account the formation of inter-fiber failures in transversely stressed layers or neglect the characteristic scattering of material properties of fiber composites, which is governed by the heterogeneous microstructure and inherent imperfections.
In the present work, therefore, a physically based modeling of the damage behavior of fiber-reinforced plastic composites (FRP) with arbitrarily oriented continuous fiber reinforcement with regard to inter-fiber failure formation under quasi-static and cyclic loading is developed. The main focus is on the development of a model approach that is characterized by the broadest possible applicability with regard to the laminate setup and the loading of the laminates.
Experimental setup for the measurement of inter fiber failure in FRP-laminates (left) and detected cracks (right).
Based on extensive experimental investigations with automated detection and quantification of inter-fiber failures (see Fig. 1), a fully analytical damage model was developed.
Model prediction and experimental results for a multiaxially reinforced FRP-laminate.
The model uses strain energy release rate approaches and thus enables the calculation of FRP laminates with different fiber orientations (see Fig. 2) and a freely selectable laminate setup. The high prediction accuracy of the model was clearly demonstrated in the work for both quasi-static and cyclic loads. Furthermore, the physically founded, energy-based approach enables the transferability of the calibrated model to laminate configurations that have not yet been investigated experimentally, without recalibration of the model or additional experiments. This ensures a broad applicability of the model with high flexibility and short calculation times.
Doctoral Student: Dr.-Ing. Moritz Kuhtz
Supervising Professor: Prof. Dr.-Ing. habil. Maik Gude
Fig. 1: Interface modification concept by perforated release foils.
Due to their high specific mechanical properties and good energy absorption capacity, fibre-reinforced plastic composites (FRP) are predestined for use in crash and impact-loaded lightweight structures. However, carbon fibre-reinforced plastics in particular exhibit brittle failure behaviour, which can lead to catastrophic structural failure and represents unexploited potential in terms of energy absorption capacity. The targeted allowance of delamination in impact-loaded FRP structures leads to a reduction in intralaminar stresses, which can prevent critical fibre fracture failure. As a result, both the energy absorption capacity can be increased and the structural integrity maintained.
Fig. 2: Test setup for the impact test on generic fan blades.
Based on a comprehensive experimental analysis of the influence of the modified interface layers (Fig. 1) on the material behaviour, a computational methodology and a method for efficient experimental-numerical parameter determination are developed. Validated computational models are used to demonstrate the influence of interface modifications on the impact behaviour of FRP structures. As a result of these investigations, design guidelines for interface-modified FRP structures are derived for the first time. Their application in experimental impact tests on generic FRP engine blades (Fig. 2) achieves higher energy absorption while maintaining structural integrity (Fig. 3).
Fig. 3: Failure behaviour generic CFRP fan blade without interface modification (a) and with interface modification (b).
Doctoral Student: Dr.-Ing. Michael Müller-Pabel
Supervising Professor: Prof. Dr.-Ing. habil. Maik Gude
Fig. 1: Schematic representation of the influence of partial cure states on the deformation behavior of epoxy resin.
Thermoset polymers offer excellent mechanical properties and are well-established as a matrix material for fiber-reinforced plastics. Due to their network-like molecular structure, they tend to exhibit brittle behavior and are not considered formable. However, if the network is only partially formed through adapted process control, thermosets exhibit strongly pronounced plastic behavior, which is interesting for the development of new types of forming-based manufacturing processes. To date, however, there has been a lack of detailed experimental data for the development of such processes, as well as material models based on this data that can correctly depict the influence of the degree of cross-linking on the mechanical properties (Fig. 1).
Fig. 2: Main components of the developed test stand for analysis of fiber-matrix-interaction.
Beside innovative test methods, the dissertation also contains suitable approaches for describing the material behavior and fiber-matrix interaction in the partially cross-linked state (Fig. 2) and thus provides the necessary basis for the technological exploitation of the cross-linking-dependent properties of thermoset matrices for the first time.