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Item type:Publication, Access status: with files , Vibrational fingerprints of strained polymers: a spectroscopic pathway to mechanical state prediction(IOP Publishing, 2026-06-01); ; ; ; The vibrational response of polymer networks under load provides a sensitive probe of molecular deformation and enables non-destructive diagnostics. Machine-learned force fields reproduce these spectroscopic fingerprints with quantum-level fidelity in realistic epoxy thermosets. Using MACE-OFF23 molecular dynamics, the experimentally observed redshifts of para-phenylene stretching modes under tensile load are captured, in contrast to predictions from the harmonic OPLS-AA model. The shifts correlate with molecular elongation and alignment, consistent with Badger’s rule, thereby establishing a direct link between vibrational features and local stress. Infrared intensities are predicted through a symmetry-adapted dipole moment model trained on representative epoxy fragments, enabling quantitative validation of strain-dependent responses. The combined approach yields chemically accurate and computationally efficient predictions of vibrational spectra under deformation. These results identify vibrational fingerprints as predictive markers of mechanical state in polymer networks and outline a spectroscopic route to stress mapping and structural-health diagnostics in advanced materials.Publicationtype: Journal Article48 16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Load‐Induced Shear Band Formation in Microscale Epoxy Materials(Wiley, 2025-03-31); ; ; ; Thermosetting polymer thin films exhibit distinct mechanical behaviors at the microscopic scale compared to bulk materials. Experimental results reveal significant necking and unexpected shear band formation under tensile load. This study investigates the mechanisms underlying shear band formation in epoxy resin systems composed of bisphenol‐diglycidyl‐ether and diamines. Mechanical testing methods, including creep, relaxation, and cyclic testing, as well as ex situ and in situ high‐resolution infrared (IR) spectroscopy, are used synergistically with quantum mechanical calculations to elucidate the underlying molecular mechanisms. Additionally, molecular dynamics (MD) simulations on a nanoscale model explored the (visco‐)plastic behavior and network strain in epoxies. Our findings reveal a strong correlation between shear band formation and shifts in IR spectra, specifically the redshift of para‐phenylene and the blueshift of out‐of‐plane vibrations of aromatic moieties. These shifts are attributed to load‐induced aromatic stretching in the polymer backbone. The robust agreement between experimental data and simulation results supports these observations at both the atomic and nanoscale. These insights enhance the understanding of epoxy resin mechanics, potentially informing the design of advanced composite materials.Publicationtype: Journal Article102 25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Performing Polarized Raman and Digital Image Correlation Analysis to Understand the Increased Ductility of Microscale Epoxy Materials(Wiley, 2025-03-27); ; ; ; The highly cross‐linked (epoxy) matrix material in fiber reinforced polymers has a microscopic volume between fibers and therefore exhibits different mechanical behavior in comparison to standard bulk epoxy samples. It has been found in previous studies that a decreased epoxy gauge volume leads to an increased deformation ability (necking and shear band formation). By using laser cutting to create dogbone samples from manufactured epoxy films, the gauge volume can be further reduced in comparison to previous studies, and the ductility can be enhanced even further. To understand load‐induced molecular mechanisms responsible for the increase in ductility at macroscale, this study combines digital image correlation (DIC) with tensile tests and precise force measurement. The global and local strains are calculated using the DIC data. The determined strains reach values up to 80% (global strains) and 120% (local strains), respectively. These strain values are significantly higher than those of archetypical brittle epoxy bulk samples (less than 10%). Polarized Raman spectra show that load‐bearing backbone molecules in the deformed film sample regions are oriented in the tensile load direction. This orientation might be due to the unraveling of entanglements, which can be seen as a sudden decrease followed by a subsequent rise in engineering stress values during deformation.Publicationtype: Journal Article35 18 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Molecular mechanisms of macroscopic ductility within brittle epoxy resins(2025); 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 Thesis83 58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Investigation of the ductile deformation potential of microscale epoxy materials(Elsevier, 2023-09-23); ; 80 75 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Fatigue and fatigue after impact behaviour of Thin- and Thick-Ply composites observed by computed tomography(Elsevier, 2021-07); ; ; ; This study investigates the influence of load ratio and impact damage on the fatigue behaviour of high-performance carbon fibre reinforced polymers (CFRP) with areal fibre weights between 30 gsm and 360 gsm. For undamaged samples, the ultimate tensile and compressive strength, as well as the fatigue properties, are evaluated with regard to their layer thicknesses. The fatigue tests were performed under tension-tension (R=0.1), tension-compression (R=-0.5) and compression-compression (R=10) regime. The results are illustrated as a constant-life diagram, and a piecewise linear interpolation examines a first prediction. The results show that static and fatigue performance improves with decreasing layer thickness. Particularly under tension-compression loading, significant improvements are observed, due to the suppression of matrix cracks and delaminations with thinner layers. In addition, the effect of low-energy impact on the fatigue behaviour of Thin- and Thick-Ply laminates is investigated. The tests demonstrate that although the delamination area is larger, Thin-Ply laminates can sustain higher stresses and still reach the same number of load cycles in contrast to Thick-Ply laminates. Computed tomography measurements visualize 3-dimensional the damage progression after various cycles and prove that the Thin-Ply composites show no increase in the damaged area during fatigue. The interlaminar stress at the delamination is not sufficient for expansion. In contrast, in the case of thicker layers, the damage growths progressively throughout the whole sample with increasing number of cycles.Publicationtype: Journal ArticleScopus© Citations 19 182 304