V. Keryvin; G. Kermouche; S. Sao-Joao
Carbon 247 (2025) 120991
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 (110GPa 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.