Fiedler, Bodo

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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
75 29
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Environmentally friendly flame-retardant composites
(2026)
Walter, Melissa
;
Fiedler, Bodo
This thesis investigates whether composites can be designed to be flame-retardant and eco-friendly whilst maintaining compatible thermo-mechanical properties. The first chapter demonstrates that partial modification with flame retardants conserves resources and preserves mechanical properties. The second chapter focuses on the development of eco-friendly epoxy resins to reduce environmental and health impacts. This involves the targeted use of thermokinetic methods to determine suitable material compositions and curing processes, as well as the development of validation procedures. The third chapter examines the use of bio-based flame retardants. Amino acid-cured epoxy resins demonstrate remarkable flame-retardant properties.
Publicationtype: Doctoral Thesis
53 25
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Lifetime prediction of high-performance fibre-reinforced polymer composites using Vibro-acoustic Modulation
(2025)
Willmann, Erik
;
Fiedler, Bodo
Fibre-reinforced polymer (FRP) composites are characterised by high density-specific stiffness and strength as well as high durability and corrosion resistance. However, the multi-scale and multi-phase structure of FRP also leads to a complex damage mechanism during operation. Any production deviation, such as voids or delaminations, negatively affects residual strength and service life. Monitoring the structural integrity during operation and quality assurance prior to operation is essential for reliable and safe use. Various approaches exist to detect damage at an early stage before failure-critical loads occur. This thesis investigates the applicability of vibro-acoustic modulation (VAM) as a damage detection method for carbon fibre reinforced polymer (CFRP) structures. VAM is a vibration-based non-destructive testing (NDT) method that employs high-frequency ultrasonic waves modulated as an introduced carrier signal by low-frequency vibrations with high amplitude. These modulations can be quantified and used to assess damage. In principle, the type of initiation of the low-frequency vibration can vary depending on the application, whereby the carrier signal is usually initiated by piezoceramic actuators. VAM offers great potential in large structures, especially due to its high sensitivity to cracks, simple measuring equipment, and long transmission paths. The complex – and thus well-known – material and damage behaviour of FRP is used in this work to develop a new test method using VAM. Various vibro-acoustic damage indicators are employed to predict the service life and degradation of FRP and to investigate the following research hypothesis: Damage detection and determination of CFRP structures can be ensured through vibro-acoustic in-situ measurements, even retrospectively. The VAM technique permits the monitoring of localised, concentrated damage in notched and impacted CFRP as well as statistically distributed damage in unnotched CFRP laminates. As evidenced by tensile and fatigue tests conducted under controlled conditions, the type and extent of damage could be identified from the VAM signal. Conventional measurement methods such as Acoustic Emission (AE), Ultrasonic testing (US), Digital Image Correlation (DIC) and X-ray analysis (XR) were also used to evaluate the VAM technique. In general, inter-fibre failures lead to an increase in modulation, accumulating over the service life to global delamination, which reduces modulation. Characteristic points, such as resulting damage-related VAM amplitude maxima, can be used for lifetime prediction. The sensitivity of the VAM method increases with increasing crack size. The linear correlation between the projected delamination area and the MI indicates that an estimation of the impact damage has been achieved. Artificially introduced defects such as PTFE films or contamination with release agent can not initially be detected in scarfed(Sc) joints and single-lap shear(SLS) joints using VAM. To monitor the damage progression of the adhesive film due to permittivity changes, localised capacitance measurements of the adhesive film area can be used. An analytical model for determining crack length correlates with the VAM signal. The combination of both measurement methods demonstrated that, defects in Sc specimens with homogeneous stress distribution have a significant influence on the service life due to the resulting stress concentrations. In contrast, in SLS specimens, the stress concentrations at the overlap ends dominate the development of damage compared to the damage introduced, and thus do not affect the service life. The results of this study demonstrate that the VAM method is a cost-effective in-situ technique for monitoring the structural integrity of CFRP structures. Implementation of the piezoceramic sensors required to introduce the carrier signal is simple and can be done on the surface, independent of the stress state of the structures. The influencing parameters, layer structure, and mechanical test procedures were adjusted, thereby facilitating enhanced comprehension of the measured signal and further advancement towards its implementation as a reliable NDT methodology.
Publicationtype: Doctoral Thesis
62 46
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Molecular mechanisms of macroscopic ductility within brittle epoxy resins
(2025)
Mittelhaus, Janina
;
Fiedler, Bodo
Various studies show that the mechanical behavior of epoxy resins depends not only on the test temperature, the load case, and the test speed, but also on the test volume. This so-called size effect is evident in epoxy resins with an expected increase in strength. At the same time, there is also a significant increase in ductility as the test volume decreases. This means that the deformation behavior of microscale epoxy resins, such as the matrix areas between the fibers in fiberreinforced polymers (FRPs), differs significantly from the classic brittle behavior of standard bulk samples with comparatively large test volumes. However, to date, there is no comprehensive physical, mechanical-chemical, or molecular explanation for this increased ductile deformation capacity of microscale epoxy samples. For this reason, as part of this thesis, the mechanical behavior and underlying molecular mechanisms of epoxy resin films under tensile load were investigated with a greatly reduced test volume compared to standard samples. For this purpose, a suitable process was first developed and optimized to manufacture films with a thickness between 15 and 100 μm. The films produced were first examined using differential scanning calorimetry (DSC) and near-Infrared spectroscopy (NIR) with respect to their crosslinking behavior. No evidence of incomplete cross-linking was found. The samples were then shaped by punching and laser cutting the films. To clarify the relationships between the pronounced deformation capability and microstructural processes, the epoxy film samples were analyzed mechanically and spectroscopically in tensile, creep, fatigue, and relaxation tests. The loaded samples showed considerable necking and shear bands with a reduced thickness compared to the initial thickness. Shear bands were detected by photoelastic imaging. Ductility increased significantly with decreasing specimen thickness or reduced test volume. Film samples with a test volume of 0.06mm3 achieved elongations at break of up to 80% in tensile tests, which is significantly higher compared to standard bulk type 1BA samples with a test volume of 500mm3 (5-12% elongation at break) investigated in this study. In addition, the stress-strain curves of the film samples showed strain softening and hardening mechanisms during the deformation process. Spectroscopic methods were employed to analyze the molecular processes of macroscopic deformation. Polarized Raman and Infrared (IR) measurements revealed a molecular orientation of the main chains in the load direction. This explains the decrease and increase in stress after reaching the yield stress in the tensile test, as the chains “entangle” and align themselves in the load direction. After the stress has been partially relaxed by the molecular movements and macroscopically visible shear bands have formed, the stress increases again due to the strengthening effect of the molecular chains aligned in the tensile direction within the deformed sample. High-resolution microscopic IR investigations have also shown that within the deformed sample areas in the shear bands, the carbon bonds of the aromatics in the main chains are stretched, which is reflected in a peak shift towards lower wavenumbers. Following the orientation, there is therefore also a measurable elongation of the main molecular chains in the tensile direction. Digital image correlation (DIC) studies confirmed that high local strains and deformations occur particularly in the forming shear bands. In situ IR measurements also enabled a direct correlation of the decrease in the aromatic and stress-sensitive peak wavenumber with increasing macroscopic strain during mechanical tests. This allowed investigation of the stress states present in the epoxy under different external mechanical loads. For validation, the mechanically loaded and deformed samples were stored in an oven above the glass transition temperature Tg. This allowed the constricted samples to return to their original shape and the shear bands to be eliminated. After thermal annealing, the molecular main chains returned to an amorphous, undeformed state without long-range order. The shear bands and constrictions reappeared in the same way when the material was mechanically loaded again after thermal annealing. The high deformation ability of fully cross-linked epoxy can therefore be explained by reversible molecular structural changes in the form of alignment and stretching of the molecular chains, particularly in the area of the aromatic structures. The present doctoral thesis thus offers valuable insight into the molecular processes in microscopic test volumes that are favored by plane stress states and lead to an increased deformation capacity of epoxies. For even more accurate FRP modeling and design in microscale areas, the parameters of microscopic epoxy samples should be used instead of the mechanical properties of bulk epoxy samples to exploit the full potential of epoxies. In addition, the epoxy volume could be specifically reduced in critical areas of FRP, increasing the deformability locally.
Publicationtype: Doctoral Thesis
89 66
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Environmental effects on composites durability with regard to fibers, matrix, and interphase
(2024)
Gibhardt, Dennis
;
Fiedler, Bodo
The proposed work is based on aging experiments on composites and their single constituents (fibers, matrix resins, fiber sizings, and interphases). The effects of the aging temperature and the specific materials used were investigated. Prediction models for strength and lifetime were derived. The first chapter focuses on the change of the thermo-mechanical material properties of epoxies. It could be demonstrated that plasticization and physical aging co-occur and oppositional affect the major properties epoxy strength and ductility. The second part focuses on the aging effects on GFRP composites and the importance of the fiber and sizing constitution for the lifetime under environmental degradation. Therefore, a novel fiber aging methodology was developed and implemented to manufacture composites with artificially aged fiber/matrix interphases.
Publicationtype: Doctoral Thesis
100 117
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In-line monitoring of three-roll mill dispersion processes for functional carbon nanocomposites
(2024)
Wisniewski, Valea Kim
;
Fiedler, Bodo
This dissertation covers production, manipulation and application of carbon nanoparticle modified polymer resin systems and their nanocomposites. Carbon nanoparticles of different morphology are considered. The experimental work including results and discussion is presented in three parts beginning with the dispersion process via three-roll mill. A novel in-line monitoring method is presented und discussed with relevant material systems. The second parts reports on permanent-magnetic manipulation of carbon nanoparticle dispersions. As a possible application of nanocomposites their performance as temperature sensors under cyclic thermal loading is presented.
Publicationtype: Doctoral Thesis
124 86
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Additive manufacturing of structural and functional materials
(2023)
Drücker, Sven
;
Fiedler, Bodo
Additive manufacturing has attracted great interest in recent years as it shows several advantages over conventional manufacturing techniques. Despite its potential, many applications are still limited to rapid prototyping due to reliability issues and a limited material choice. This study shows two applications that go beyond rapid prototyping using structural and functional materials. First, a probabilistic process for lightweight design with lattice structures is proposed. Second, a new material formulation is developed that solves the anisotropy issue of fused filament fabrication and allows functional integration by electrical conductivity as well as temperature and strain sensing.
Publicationtype: Doctoral Thesis
170 95
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Thermoset state estimation using infrared spectroscopy and predictive analytics
(2023)
Doblies, Audrius
;
Fiedler, Bodo
By design fiber-reinforced plastics (FRP) show extraordinary specific strength and stiffness performance when compared to classic structural materials. Unfortunately, the extraordinary properties come at the price of complexity in design, manufacturing and operation. Most notably, manifold failure modes and complicated detectability of inherent damages represent a challenge for composite engineers until now. Throughout the life-cycle of composites, it is crucial to understand and track the material state in order to prevent premature failure. Due to the high-cost sensitivity of the composites sector implementing material health monitoring systems is usually not practical in application. Instead, non-destructive testing (NDT) methods are usually applied to assure structural integrity and damages within allowed limitations at planned maintenance events. Nonetheless, currently available NDT methods are insufficient to fully cover all possible damage scenarios. In this work a new approach to determine material properties of the epoxy resin component using Fourier-transform infrared spectroscopy (FTIR)-spectroscopy is presented. It is shown that by post-processing the acquired molecular information the material state can be determined and based on this material state inherent material properties such as strength can be predicted. First, starting with the evaluation of manufacturing parameters it is shown that by using classic chemometrics techniques FTIR measurements can be used to quantify the mixing ratio of an epoxy resin fast and reliable. Thereafter, material changes due to mechanical loading in creep are observed and described by applying advanced feature extraction methods. Finally, specimens exposed to thermal loads are analyzed and it is confirmed, that the residual strength can be determined solely by applying FTIR measurements and machine learning algorithms. Furthermore, it is demonstrated, that the method is suitable to reveal the material exposure history. It is shown that an accurate prediction of mechanical properties, as well as the processing and degradation parameters, can be derived from the measurements and significantly improved by applying data processing and machine learning methods.
Publicationtype: Doctoral Thesis
114 743
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Structurally compatible embedded sensors for damage detection in glass fibre reinforced polymers
(2022)
Buggisch, Christina
;
Fiedler, Bodo
Fibre reinforced polymers exhibit a complex failure behaviour due to their multi-scale nature. Even damage that severely influences the material integrity can often not be detected by visual inspection. Therefore, different non-destructive testing and structural health monitoring (SHM) techniques have been developed to ensure composite structures’ reliable and safe operation. Many SHM methods require skilled personnel, expensive measuring equipment and complex sensor networks that reduce the lightweight potential. Especially in carbon fibre reinforced polymers, electrical resistance measurements overcome the limitations. In conductive carbon fibre reinforced polymers, in-situ electrical resistance measurements can monitor the material state during operation without additional sensor networks. However, in unmodified glass fibre reinforced polymers (GFRPs), such resistive monitoring is not possible due to a lack of material conductivity. This dissertation provides two approaches to enable electrical SHM in GFRPs without significantly reducing the mechanical properties. The first method describes a local matrix modification with conductive, fully-integrated, pre-cured single-walled carbon nanotube (SWCNT)/epoxy thin-film sensors. The second technique outlines a local replacement of glass fibre bundles with electrically conductive fibres as conductors for capacitance measurements. The embedded SWCNT/epoxy thin-film sensors exhibit a piezoresistive effect and enable real-time, local strain monitoring. In general, tensile stresses result in a resistance increase and compressive stresses in a resistance decrease. The integrated thin-film sensors do not significantly reduce the mechanical properties for various lay-ups and load cases. Local stress concentrations due to cracks or around holes result in significant resistance increases. The sensitivity of the sensor films can be tuned by geometry variation. First durability studies prove the integrity of the sensors during cyclic fatigue tests. Buckling of a stringer component can be reliably detected during crippling tests by a resistance increase. Capacitance measurements on integrated carbon fibre bundles enable damage detection in GFRP composites due to damage-induced material permittivity changes. Developing damages with incorporated air cause an overall lower permittivity and, thus, a drop in capacitance. Matrix crack evolution and capacitance decrease clearly correlate during tensile tests. The capacitance decreases more severely during rapid crack formation and slower the closer to crack saturation. Analytical modelling of the capacitance decrease is possible with assumptions of an ideal plate capacitor. A higher distance of the bundles decreases the sensitivity towards the end of the test, where a more diffuse crack pattern is present. By integrating the bundles in different layers, crack evolution in the respective layers can precisely be monitored. Furthermore, using the measured capacitance decrease, a detection and size estimation of impact damages is possible. The two presented methods offer highly tailored SHM of GFRPs and can easily be incorporated in industrial processes – carbon fibre bundles can be integrated directly during fabric manufacturing, and pre-cured thin-film sensors can be placed in the desired locations during stacking of dry fabrics. Both methods provide cost-efficient, real-time SHM without significantly altering the material properties and therefore help to enable safer use of GFRP structures.
Publicationtype: Doctoral Thesis
185 143
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Damage tolerance of high-performance composites
(2022)
Körbelin, Johann
;
Fiedler, Bodo
High-performance composite laminates are structural materials that combine low density with high stiffness and strength. High safety and reliability requirements for structural parts require the consideration and detailed knowledge about damage tolerance of materials and structures. The multi-scale nature of composites results in the occurrence of different failure modes and a complex failure behaviour where matrix failure at the micro-level influences the failure process at all higher observation levels. Consequently, the occurring matrix damage has a decisive influence on the damage tolerance. The research hypothesis of this thesis is: Understanding and adapting the matrix's damage behaviour is fundamental to improving the damage tolerance of high-performance composites. To adapt the matrix's damage behaviour, carbon-fibre reinforced plastics (CFRP) with a few-layer graphene (FLG) modified matrix with layer thicknesses varying from ultra-thin-ply (28 μm / 30 g/m2) to thick-ply (220 μm / 240 g/m2) were investigated. The FLG modification introduces additional damage modes into the matrix and significantly increases the composites mode I and mode II inter-laminar energy release rates (ERR). The modification increases the damage initiation stress of the notched tensile specimen and the usable design space and damage tolerance. For ultra-thin-ply laminates, the notched strength is increased due to crack blunting because the FLGs initiate distributed damage on the microscale in highly stressed areas. The damage resulting from low-velocity impact (LVI) decreases with the FLG modification for all layer thickness, but only the residual compressive strength of thick-layer laminates improves. Additionally, the FLG modification leads to a considerable improvement of the compressive strength. As another approach to influence the matrix damage, bio-inspired laminates with a helicoidal layup with a pitch angle of 2.07° were realised. Due to the small angle between the ply, the laminates suppress the formation of delamination damage and only exhibit subcritical matrix cracking before final failure. As a result, the compressive residual strength after LVI is significantly increased compared over 45°-quasi-isotropic (QI) layup, despite a lower proportion of 0}{\degree} fibres in the load direction. Due to the low portion of load oriented fibres, the unnotched tensile strength is lower than 45°-QI layups. However, the formation of helicoidal matrix damage leads to crack blunting, and as a result, almost no notch sensitivity occurs, and similar notched strengths as 45°-QI layups are achieved, despite the low portion of load oriented fibres. Additionally, this thesis investigated the influence of temperature on LVI and the residual strength to further understand the influencing factors of damage tolerance. Because the matrix's mechanical performance decreases with increasing environmental temperature, the composite's damage behaviour is temperature-sensitive. LVIs with impaction energies between 8 J and 25 J and temperature ranging from -20 °C to 80 °C were investigated. A change in temperature leads to a substantial change in damage behaviour. With increasing temperature, the delamination area reduces and fibre-failure occurs on the impacted side, which reduces the residual tensile strength considerably. The compressive residual strength was determined at 20 °C and 80 °C. The results point out that an elevated temperature during compressive loading has a more decisive influence than the impaction energy. To further understand the influence of temperature on the impact damage and the residual strength, temperature- dependent material properties were determined and implemented into a continuum damage model (CDM) to describe CFRP's progressive failure under temperature influence. LVI simulations validated that the material model could predict the damage resulting from different temperatures and impact energies and following residual compressive strength. The results reveal that not an increase of the inter-laminar ERR cause a reduction in delamination size but the change in intralaminar damage and overall structural response.
Publicationtype: Doctoral Thesis
409 277

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