Radial elastic heterogeneity in a high-modulus carbon fibre assessed by nano-indentation and micro-pillar compression

Understanding the radial heterogeneity of carbon fibres is critical to optimising their performance in structural composites. In this study, we investigate the mechanical structure of a high-modulus PAN-based carbon fibre by combining instrumented nano-indentation (IIT) and micro-pillar compression (MCP). Nano-indentation mapping with 50 nm lateral resolution reveals a distinct layered skin–core structure. By combining IIT and MCP, we resolve for the first time the compressive modulus of individual radial layers within a single carbon fibre: a compliant core (110 GPa at 0.45% compressive strain), a transitional middle layer, and a stiff outer skin (520±100 GPa, at 0.45% compressive strain). Our results confirm earlier indirect estimates and show that the skin modulus approaches that of highly oriented crystallites, whereas the core remains significantly softer due to misorientation. This integrated approach provides a robust framework for characterising anisotropic heterogeneous fibres and offers new insight into fibre-scale mechanical testing.