Nano-mechanical characterization of the fiber-matrix interface of polymer composites

Details

Supervisors

Pardoen, Thomas ; Nysten, Bernard

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

Fiber-reinforced polymer (FRP) composites have garnered significant interest over the past decades due to their significant mechanical properties combined with their lightweight nature. This thesis investigates the mechanical behavior of the fiber/matrix (F/M) interface in unidirectional (UD) fiber-reinforced polymer (FRP) composites. The study is motivated by the importance of the F/M interface in stress transfer between fiber and matrix, which affects the overall mechanical performance of composites. Accurate characterization of F/M interface behavior at the nano and micro scale is essential for optimizing composite design, enhancing mechanical properties, and improving the predictive capabilities of computational failure models in bottom-up multiscale modeling approach. The research employs the single fiber push-out method, among other F/M interface characterization techniques, to examine two UD carbon FRPs with different matrix types: a semi-crystalline thermoplastic matrix and a thermoset matrix. Thin films of both UD FRPs were polished perpendicular to the fiber direction to a thickness below 100μm, and fiber push-out tests were conducted using a nanoindenter machine with a flat tip. Two types of supports were used for mounting the specimens: TEM grids of specific grid size and a holder with a V-shaped groove of ~70μm edge-to-edge gap. Finite Element Analysis (FEA) was employed to extract cohesive law parameters for interface modeling through reverse analysis and to provide insights into the effect of different parameters on the F/M interface response recorded during the push-out test. The findings of the study indicated that the holder with a V-shaped groove was the optimal support for mounting push-out samples, based on a simple success/fail statistical analysis and a qualitative comparison. The uniformity of the thickness was found to be crucial for the reliability of the mean interfacial shear strength (IFSS) values used for comparison. No noticeable effect of varied loading rates on IFSS values was observed for both analyzed FRPs. However, the effect of aging was evident in the FRP with a thermoplastic matrix, suggesting either a direct impact of aging on the interface or a significant contribution of matrix plasticity to IFSS values. FEA results further highlighted the role of matrix plasticity in determining the F-D response recorded in the push-out test. Moreover, the finalized cohesive parameters, obtained by comparing FE and experimental F-D curves, suggested a compliant response before damage onset and a brittle softening (damage propagation) response for the F/M interface in the FRP with a thermoplastic matrix.