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Janina Mittelhaus, Julius Jacobs, Suprit Bhusare, Nazanin Pournoori, Matti Isakov, Turkka Salminen, Holger Schmalz, Gaurav Mohanty, Essi Sarlin, Bodo Fiedler

Journal of Polymer Science, 2025, https://doi.org/10.1002/pol.20240971

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.