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Molecular mechanisms of macroscopic ductility within brittle epoxy resins
Citation Link: https://doi.org/10.15480/882.15768
Publikationstyp
Doctoral Thesis
Date Issued
2025
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2025-06-27
Institute
TORE-DOI
First published in
Number in series
49
Citation
Technisch-Wissenschaftliche Schriftreihe 49: (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.
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.
Subjects
epoxy films
mechanical tesing
IR microscopy
polarized Raman microscopy
shear bands
strain hardening and softening
DDC Class
620.1: Engineering Mechanics and Materials Science
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