Nanoscale in situ observation of damage formation in carbon fiber/epoxy composites under mixed-mode loading using synchrotron radiation X-ray computed tomography

Nanoscale fracture mechanism in CFRPs is still debated owing to the considerable difficulty in determining the three-dimensional mechanism of fracture using conventional techniques such as optical and/or electron microscopy relying on side-surface- and fracture-surface-based observation. In this study, microscopic damages such as fiber/matrix debonding and microcracks under mixed-mode (mode I + II) loading are characterized in situ using nondestructive nanoscopic synchrotron radiation X-ray computed tomography (nanoscopic SR X-CT). It is clearly shown that crack formation proceeds in three steps: (i) initiation at the carbon fiber/epoxy matrix interface, (ii) propagation into the epoxy, and (iii) formation of microcracks (hackles) in the resin matrix, and the resulting microstructures of cracks at the nanoscale are largely affected by the local fiber geometrical distributions. A sharp and straight interfacial crack initiates at the "thin" epoxy-resin region (thickness < half the diameter of the carbon fiber (d(CF))) and propagates along the carbon fiber/epoxy interface. The sharp cracks propagate into the epoxy at the "thick" epoxy-region (thickness > similar to 1/2 d(CF)) and hackles are formed in the resin matrix perpendicular to the local principal tensile stress direction. Nanoscopic SR X-CT provides information on three-dimensional mechanisms at the nanoscale during deformation, which is indispensable for understanding heterogenous materials such as CFRPs.