In situ X-CT Observation of Crack Initiation and Propagation in CFRP with X-ray Microscopy

Abstract

Carbon fiber reinforced polymer (CFRP) composites are of growing use in aircrafts owing to their high specific strength and stiffness. Micro-mechanism of damages and microscopic chemical properties of the CFRPs is a key to understand the mechanical properties and durability of these materials. Especially the initiation and propagation along fiber/polymer interfaces is of great importance. However, most experimental observations have been carried out for surfaces (cross sections) of CFRP specimens typically using electron microscopes, where the crack propagation along the fibers, i.e. along the thickness direction of a specimen, cannot be observed. Recent reports pioneered non-destructive and three-dimensional (3D) observation using X-ray computed tomography (X-CT), but their special resolutions are limited down to ca. few or sub μm. Here we have succeeded in non-destructive and 3D observation of cracks initiation and propagation with a resolution of less than 50 nm under an applied stress with a new x-ray transmission microscope (XRM) using synchrotron radiation. Using the phase-contrast imaging technique, we have succeeded in non-destructive and 3D observation of the initiation and propagation of cracks with a resolution down to 50 nm under an applied stress. Fibers, matrix polymer, and cracks were clearly identified in reconstructed 3D images. This first observation has successfully shown that cracks are initiated by (a) de-bonding at the fiber/polymer interfaces and (b) void formation in the polymer, and that the two factors compete with each other and determine the propagation path.