Nanoscale in situ observations of crack initiation and propagation in carbon fiber/epoxy composites using synchrotron radiation X-ray computed tomography

The crack initiation and propagation under the application of opening load were analyzed in situ using nondestructive synchrotron radiation X-ray computed tomography (nanoscopic SR X-CT) with a spatial resolution of similar to 50 nm. The results show that the voids and cracks initiation are not only the result of local stresses but also are due to two competing nanoscale mechanisms, that is, fiber/plastic interface debonding and in-resin crack initiation. In the "thin" epoxy region in which the resin thickness between the adjacent carbon fibers is small, cracks propagate mainly by debonding along the carbon fiber/epoxy interface. In the "thick" epoxy region in which the resin thickness between the carbon fibers is large, cracks propagate through the extensive plastic deformation of the epoxy resin, and the propagation mechanism largely depends on the resin thickness, which affects the plastic deformation behavior of the epoxy around the crack tip. Voids (sub-mu m) often form in front of the crack tip and merge with the propagating cracks. Nanoscopic SR X-CT provides indispensable information on these nanoscale mechanisms, which cannot be obtained with traditional methods, including macroscopic observations and mechanical theory. These nanoscale mechanisms are essential for the mechanical modeling and analysis at multi scales.