A hidden hierarchy of compressive failure in carbon fibres

Fibre reinforced polymer composites dominate demanding structural applications due to their exceptional mechanical characteristics at low weights. However, their compressive strength is much lower than their tensile strength, limiting performance and wider deployment. While composite compression failure typically involves meso-scale fibre microbuckling and kink-band formation, the underlying compressive failure mechanisms of the reinforcing carbon fibres themselves are poorly understood, yet increasingly important in high modulus applications. Here, the three-dimensional internal structure of compressive failure within individual high-modulus carbon fibres is resolved using a new correlative nanoscale resolution serial-sectioning methodology. Reconstruction of failed fibres reveals previously unobserved internal nano-kink bands within single filaments, exposing a hidden hierarchy in which a characteristic composite-scale compressive instability re-emerges within the fibre microstructure itself. These nano-kink bands challenge the conventional view of fibre compression failure as a simple bulk shear fracture event and instead indicate a localised instability coupled to the fibre skin-core microstructure. They highlight a self-similar compression failure mechanism occurring both the meso- and nanoscale, separated by two orders of magnitude. Three-dimensional imaging additionally reveals Mode I fibre-matrix debonding, crack bifurcation, and wedge formation that map onto distinct stages of the fibre failure trajectory. These previously hidden interactions establish direct structural links between fibre microstructure and compressive failure, providing new insight into the origins of compression sensitivity in high-modulus carbon fibres and identifying potential pathways towards improved compressive performance in next-generation composite materials.