Micromechanics of the semi-crystalline matrix in the inter-fiber regions of thermoplastic composites
(2026)
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- Continuous fiber-reinforced polymers (FRPs) are widely used for high-performance components that are lightweight, strong, and stiff. However, accurate predictions of their damage and failure still remain a challenge. Over the past two decades, bottom-up multiscale approaches have driven composite research, emphasizing the need to understand the deformation and failure mechanisms of the constituents at the microscale. This is particularly critical for semi-crystalline thermoplastic matrices like PEEK, where crystalline phases - with a characteristic length scale comparable to the fiber diameter - and the possible formation of transcrystalline layers at the fiber/matrix interface, complicate the extrapolation of neat matrix properties to the composite level. Moreover, the local microstructure of the matrix is strongly affected by the thermal history imposed during processing. This thesis experimentally examines the influence of PEEK matrix crystallinity on its mechanical response, as well as on the deformation and damage mechanisms that govern the early stages of the PEEK/carbon fiber composite failure. First, the overall degree of crystallinity of PEEK polymers and matrices processed under three distinct processing methods was quantified. Subsequently, a range of techniques was employed to characterize the matrix microstructure in confined regions of crystallized composites. However, due to the high fiber volume fraction in the composites, which limits characterization of the inter-fiber matrix regions, model samples containing only a few fibers were manufactured. Polarized light microscopy revealed the different morphologies that develop in these model samples. These morphologies were further analyzed to assess their degree of crystallinity and microstructural features. Results show that crystalline structures of various dimensions involve different degrees of crystallinity, explained by the differences in lamellar stack densities observed between crystalline morphologies of various dimensions. Variations in lamellar stack densities were also observed within the different crystalline structures. Then, the in-situ mechanical properties of matrix pockets (i.e., matrix zones surrounded by fibers) processed under different conditions were investigated, along with those of individual crystalline structures in the model samples. Differences in indentation modulus and hardness were observed between matrices processed under different conditions, as well as between crystalline morphologies of various dimensions. Additionally, heterogeneities in modulus and hardness were detected within individual crystalline structures. Finally, the deformation and failure behavior of unidirectional PEEK/carbon fiber composites with various microstructures was examined from the specimen level down to the inter-fiber zone scale. To this end, a multiscale digital image correlation (DIC) approach combined with in-situ mechanical testing was developed to obtain precise strain field measurements. The objective was to establish a link between microstructure and strain localization mechanisms at all scales, and to provide experimental data essential for validating computational micromechanical models.
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VanpƩe, S. (2026). Micromechanics of the semi-crystalline matrix in the inter-fiber regions of thermoplastic composites. https://hdl.handle.net/2078.5/271839